I’ve been interested in terminal ballistics for a long time. There are lots of opinions out there but finding hard scientific fact is not easy, though there is a lot of stuff that pretends to be.
Luckily I’ve been able to talk to a few people doing weapons R&D for the military, and have been able to combine some of the things they’ve told me with known mechanisms of physiology and anatomy to produce the model explained below.
Some of what you see will fly in the face of current fashions in Gun Magazines, but my reasons for these should hopeful be clear.
This is a pretty long article I’m afraid. The first part deals general principles of incapacitation, with some emphasis on combat pistols since this is the usual field of interest. Later there is a section on high penetration pistol rounds for hunting, animal defence and other purposes, and a discussion of rifle rounds, including some notes on the 5.56
Most of this article deals with incapacitation. Wounds that incapacitate are not necessarily fatal, while fatal wounds do not necessarily incapacitate. With the exception of hits to the brain few wounds are instantaneously fatal, so an incapacitating hit is important in preventing a target being a further threat. From a hunting perspective an incapacitating round cuts down on the tracking.
There have been many theories of Incapacitation a.k.a. Stopping power. “Stopping power” is a somewhat more catchy term than “Incapacitation”, other commonly used terms including “Knockdown energy”, “Knockdown potential” etc. Certain writers will pedantically argue that a bullet doesn’t have the power to knock someone over or recite stories about people shot in the front falling forward etc. This is rather a waste of time. These are convenient terms for the property under discussion if it is borne in mind that they should not be taken literally. The fact that the term “Stopping Power” is not an accurate literal description does not mean that property we are examining does not exist. Malaria has nothing to do with “Bad-Air” but it can still kill you.
Discussion of Incapacitation has been known to get quite heated. In fact it often degenerates into personal attacks and name calling.
There are two bodies of data you are most likely to encounter these days, the Marshall and Sanow study and the IWBA.
The M&S study was a bold effort but poorly designed, so little credibility can be given to many of the results. Less forgiveable is that some of its proponents refuse to admit that there are errors.
I’ve included several links to the International Wound and Ballistics Association since it is a good source of basic data. If reading their articles on the site be aware that it is a matter of dogma with many of them that temporary cavity has no contribution to stopping power unless it causes physical damage. This seems to be an over-reaction against those that say all you need to stop someone is to dump lots of energy. The IWBA model fails to explain incidences where a target has been incapacitated without being mortally wounded and regained consciousness a few minutes later. In fact the IWBA model fails to explain how a blow from a fist or nightstick can disable an individual with no more permanent damage than some minor brusing.
When it comes to close range shooting the main consideration is incapacitation. i.e. quickly stopping the target doing whatever it was than made you shoot him in the first place. Whether or not such a wound proves fatal is a secondary consideration. Incapacitation is dependent on several factors. These are what I call the “Five Ps”:
Firstly you have to hit your target, and you have to hit him in the correct place. One of the main faults of the Marshall and Sanow studies was that it failed to consider hits to different regions of the torso. No differentiation was made between a shot that pierced the heart and shattered the spinal column and one that simply hit a lung. This procedure had some interesting effects.
For one thing it biased results in favour of rounds already in widespread use with police departments. This is most noticable for the 125gr .357 magnum. Trained personnel such as police and military can be expected to shoot better under stress than civilians. Most officers carry .357, 9mm or .40S&W pistols since they are lighter than larger bore weapons. There is therefore lots of data for “one shot stops” for these weapons and for the bullet weights in most common use. By the same mechanism, data from rounds that are not so popular such as .45 Long Colt, 44 Spl and 44 magnum would have come from less practised personnel and show a lower effectiveness. This also explains why the most popular load in .32ACP shows such a high level of one shot stops (better than .45ACP hardball!). Most of the data for this round was probably from backup guns and was fired at very close range, so placement was better.
Marshall and Sanow classed different rounds by a percentage “one shot stop” value. The Fuller Index is a formulae that estimates this figure from bullet performance. Because of the flaws in experimental design these figures have little real relevance.
The primary target for incapacitation against humans is the Central Nervous System (CNS) the brain and spinal cord. Any hit to this area will have a decisive effect as long as the bullet has sufficient penetration. In combat we cannot guarantee perfect placement, which is why we use ammo that increases the effects of other mechanisms too.
The location of the CNS as a target can be visualized as a “Lethal T”, the cross bar being between the two temples and the vertical down the centreline. This is a good enough image if the target is facing you, but if not you must be aware that the vertical runs down the middle of the back and the cross bar is really a ring around the head. Certain organizations train their personnel to shoot “Centre of Mass”, but if the target is not facing you such targeting may miss the CNS.
To reach the CNS a bullet must have enough penetration to pass through the skull or the flesh in front of the spine. This is the reason that low energy bullets such as .25 and .22LR are best selected in a non-expanding configuration for self defence.(1) Mushrooming may limit penetration to such an extent that the CNS is not reached, removing the primary incapacitation mechanism.
After damage to the CNS, the main mechanism of incapacitation is a sudden drop in blood pressure, usually caused by a drop in blood volume resulting from physical damage. The effects of this pressure drop are not permanent, but often the related blood loss can prove fatal before the target recovers. Penetration contributes to this mechanism too, since the deeper the bullet goes the more likely it is to pass through a vital organ or major blood vessel.
How much penetration is a topic of some debate. You want the bullet to reach the vitals, but in a police situation it is undesirable to have the bullet exit the body and endanger innocents. Also it can be argued that a bullet has only a finite amount of energy and that a bullet that stays in the targets body places all of it into a target.
The average depth of the human torso is 9.4". In combat a subject’s arms may obscure a shot to the torso so for combat rounds a penetration of 12-15" is probably sensible. The far side skin of a torso can absorb the equivalent of 4" of flesh penetration, so there is some leeway. For a close range defensive weapon penetration as low as 10" may be acceptable. Some authorities have claimed that 18" is preferable, but rounds that have been designed to meet these criteria have poor stopping records and often over-penetrate.
Rounds such as the .44 magnum will usually penetrate to greater depths and will exit the body. Such bullets have sufficient size, weight and energy to still do decisive damage even though only a fraction of the potential energy is used. Exit wounds tend to increase the rate of blood loss anyway.
As well as penetrating deep enough, an effective bullet will also make as wide a wound channel as is possible. This facilitates rapid blood loss. Wound channel size is increased by bullet size, tumbling, mushrooming and bullet shape a round nosed bullet tends to push flesh aside while a flat faced one crushes what is ahead of it, creating a more open channel. This channel caused by the bullet’s path is called the permanent cavity.
The final bullet characteristic that contributes to incapacitation is energy, and more importantly how a particular bullet uses it.
A bullet may use its energy to cause the bullet to mushroom. This increases the diameter of the wound channel, and also increases the rate of energy transfer to the surrounding tissue. This energy is used as a wave of movement that causes the a temporary “stretch” cavity. The stretch cavity can contribute to the permanent cavity if it passes through a tissue that has an elasticity exceeded by the speed of expansion. This is usually seen in hits to the bones, spleen, pancreas, kidneys or liver. Assault rifle bullets that have passed close to bones have been observed to sometimes cause damage to the bone, even though the bullet and bone did not make direct contact. Muscle tissue is fairly elastic so less likely to be permanently damaged by a stretch cavity. Most pistol rounds have insufficient energy to cause permanent damage resulting from temporary cavitation.
The other effect of the temporary cavity occurs if it passes through a nerve plexus. Stretching of the nerve membranes disrupts their membrane potentials causing nerve function to either be inhibited or overstimulated. This may cause a temporary stunning effect rather like a punch from a boxer. In some locations this nerve stimulation may confuse the body’s blood pressure regulation systems, causing a drop in blood pressure. In other locations this may cause pain, shortness of breath and/or nausea. Spinal vertebrae have been observed to focus pressure waves resulting in either physical injury or temporary effect. Many hunters have found a hit near the spine or neck will drop an animal.
Another potential effect of the temporary cavity if it passes through a major blood vessel is that it may cause a surge of blood. Autopsies of some individuals killed by chest wounds from bullets show haemorrhaging of blood vessel in the brain. Mechanism is believed to be compression of thoracic blood vessels causing a steep rise in blood pressure. Such a rise in blood pressure may cause the body to try and compensate, causing a drop in blood pressure and loss of consciousness.
These varying possible effects of the temporary cavity are more variable and therefore less reliable mechanism than the effects of the permanent cavity. Many of these effects will only occur if the temporary cavity occurs in a specific location in the body. Some authorities dispute that such an effect exists, but such a model that they propose does not account for the effects of stun bags, baton rounds or simply being punched. Martial artists know that even a light blow can stun a nerve, so it seems unlikely that a stretch wave passing through a nerve plexus will have no effect. Interestingly, several of these major plexi of the thorax have nerves associated with factors that will effect blood pressure such as heart rate and vasodilation.
Certain bullet designs have tried to maximize energy dump at the expense of penetration not surprisingly they’ve had variable success.
The mental state of the target definitely has an effect on the reaction of a bullet hit. The same strike may make some people give up while make others berserk. This is a factor in incapacitation, but one nearly impossible to predict and unrelated to bullet characteristics.
The lighter something is, the easier it is to accelerate it to a high speed. By shaving a few grains off a bullet’s weight, muzzle velocity can be increased, and this gives a big increase in muzzle energy, since
This looks good on paper but it is not how much kinetic energy a bullet has but how it puts it to use that is important.
Also, it is not how much energy the bullet has at the muzzle but how much it has at the target that is important. The same property that lets a light bullet be accelerated more readily (low inertia) also means that it can be more easily slowed by the air it is passing through. Most handgun fights take place at less than 6 metres, and light high velocity rounds are intended to give the best performance within this range. However, shots at longer ranges are by no means exceptional, and at these ranges lightweights often lack sufficient target effect.
A heavy bullet may have less energy at the muzzle, but will have a greater proportion of this energy retained by the time that it reaches the target.
This can be visualized by imagining a graph of energy plotted against distance. The lighter bullet will have a zero point much higher on the axis than the heavier one. However, the line plotted for the lighter bullet will have a steeper downward gradient than for the heavier one.
Muzzle energy can be deceptive, and is not really a good indicator of incapacitation capability. For example, a .38 Spl +P 115gr bullet at 1,250fps has 399 ftlbs of energy, while a 158gr at 890fps has only 278 ftlbs. Penetration of gelatin for both rounds is effectively the same (14.8-15.4"), and in actual shootings the 158gr has proven a more consistent manstopper. An interesting thing about these two rounds is they also have very similar momentum. Momentum is calculated by Velocity x Mass. Mass is not the same as weight and some websites fail to make this clear or seem unaware of this distinction. I am grateful to Chad Eastridge for pointing out the errors on this page and I will be correcting the other affected Scrapboard pages as time permits. Weight includes the force of gravity (32.174 ft/sec^2) so Weight must be divided by this value of gravity to get mass. For bullets this gives us a formula of :
You seldom see momentum mentioned in the Gun press, and when it is it is often misunderstood. As this page on bullet physics nicely explains
“One can think of energy absorption (of a target) as Force x Distance, and momentum absorption as Force x Time. Hence, the heavier but slower bullet with the same energy will travel the same distance in the absorbing material, but because of larger momentum, will take a longer time doing it. It will therefore also impart a greater “kick” to the absorber object.”
When talking about firearms, Kinetic energy is expressed in terms of “Foot-pounds”. 200 ftlbs is theoretically the energy needed to move a one pound weight 200ft vertically off the ground, or a 200lb weight one foot, or a 100lb weight two feet, etc. However, the KE is a scalar quantity, with magnitude but no direction.
To illustrate this, lets consider a 150gr bullet impacting at 2700fps, giving a terminal energy of 2,428 ftlbs. If fired against a 400lb object it should move it 6ft off the ground, or a similar distance if hit from the side. A 200lb object should be thrown nearly 12ft. Obviously we don’t see anything like this in the real world. Even if we allow for the friction of the ground and elasticity of tissue, a man or deer hit by such a bullet doesn’t move anything like this distance. This is because the movement of an object hit by a projectile is determined by the momentum, not the kinetic energy.
The 150gr bullet has a terminal momentum of 1.79 ftlb/sec, which will move our 400lb target back at a speed of no more than 0.05" per sec. This correlates with what we see in the real world. A deer hit by a bullet flinches rather than being thrown several yards.
Now let us compare that bullet to another projectile, a 1lb cannon ball with the same 2,428 ftlbs of terminal energy. This ball would be moving at 395fps, which does not sound much in firearms terms, but is about 269mph. Since the ball weighs a pound, momentum will be 12.28 ftlbs/sec. That should move our 400lb target back at a speed of 0.36" per sec, which sounds credible for a projectile of this weight and speed.
The difference between a 1lb cannon ball and 150gr bullet is obviously extreme. Is what we have seen significant with smaller differences in projectile weights? If we calculate the momentum for two more bullets with the same energy, we get a 200gr at 2338fps giving 2 ftlbs/sec and a 250gr at 2091fps giving 2.32 ftlbs/sec.
What we have just illustrated is that:
Momentum is, of course, a product of both velocity and weight. At a very high velocity a lighter round may have more momentum than a heavier one, and therefore be more effective. However, velocity decays in flight while mass remains constant. For most handgun bullets this does not seem to be a significant factor out to beyond 50yds, even though the rounds of lower sectional density lose a greater percentage of their initial velocity.
Both .38 rounds described earlier have a muzzle momentum of 0.62-0.64 ftlbs/sec. When both rounds have a terminal energy of 250 ftlb the 115gr at 989fps has 0.50 ftlb/sec of momentum while the 158gr at 844fps has 0.59 ftlb/sec. From these figures it should become apparent why throughout history slow heavy pistol bullets have proven so effective.
Some simple calculations show that the most effective loads in a given calibre have high terminal momentums, all other factors such as bullet construction and efficiency of hollow-point being equal.
Most 9mm/.38 combat rounds have a muzzle value of about 0.62 ftlb/sec, while one of the reputed best performers in the calibre, the 125gr 357 at 1450fps has a value of 0.8 ftlb/sec. The .380ACP has between 0.39 and 0.41 ftlbs/sec. Most .40S&W loads average 0.77 ftlb/sec while .45 ACP start at 0.84 ftlb/sec. Momentum seems a good way to compare loads of the same calibre. If there is a choice of two rounds of similar momentum, the larger calibre is preferable due to the larger diameter wound channel.
An important point worth repeating is that it is terminal momentum, not initial momentum that is important. Lightweight rounds with a high muzzle energy and momentum may not have this when they reach the target. The website below has velocity figures for handgun rounds at 25 and 50yds. This includes rounds fired from carbines so some of the tables are at much higher velocities than are possible from pistols.
To get some idea of the effective range of round, calculate the terminal momentum at several ranges and plot these on a graph. This is probably one of these jobs were it is quicker to use paper than a computer. Also include a “standard” round of the same calibre, such as 124gr 9mm at 1200fps or 230gr .45ACP at 850fps. It should be obvious if and at which ranges the round has more momentum.
You’ll also notice from these tables that the percentage of retained velocity at 50yds seems to correlate with the round’s sectional density, formula for SD being.
A 135gr .40 with SD of 0.12 has only 86% of its original velocity at 50yds, a 9mm 115gr (0.126) has 91%, .357 125gr (0.14) has 94% and a 230gr .45 (0.162) has 98%
Another advantage of using a heavy bullet is that it is far less likely to be deflected. There is no point in having good shot placement if the bullet takes a random path as soon as it encounters a rib or pocket of change.
As a rule of thumb, choose the heaviest expanding round available for that particular calibre. Most pistol bullets are loaded with rounds way below their optimum weight for the charge and calibre, so unless you start using very exotic handloads you are unlikely to get a bullet weight “too heavy” for the charge.
Once you have the heaviest bullet, find the highest velocity loading in that weight that you can handle. A slow-heavy bullet is better than a fast-light one, but a fast-heavy one even better. Impact velocity is more important than muzzle velocity. If a round is traveling too slow when it hits the target mushrooming is unlikely.
There are a couple of possible exceptions to the above. One is for 9mm Luger ammo. Some of the 147gr subsonic Hollow point rounds have been designed with too much emphasis on penetration, so have a very pointed nose that often fails to mushroom. Some large bore magnum rounds such as the 44 magnum can be found with very heavy hollow points intended for hunting that may not be as effective as lighter rounds in the human torso. On the other hand, these still make a big deep hole and it is not the percentage of energy the bullet deposits that is as important as the quantity and rate of transfer.
With very low energy bullets such as 22s and .25s, hollow-points should not be used for defensive fire since they may limit penetration to such an extent the bullet does not reach the vital structures.
Great White Hunter John “Pondoro” Taylor suggested the Taylor Knockdown formula (TKO), sometimes called “Taylor Index”, which integrates calibre and momentum to generate a relative value that is a guide to the potential of a round to incapacite a target.
This obviously does not take into account any factors such a bullet shape, construction, design or tendency to tumble, mushroom or fragment. In this respect the basic TKO offers an indication of the minimal performance one could expect from a round. It is, however, still a useful tool for comparing loads and gaining some idea how a round may perform if it fails to mushroom. I don’t think the TKO is exact enough to let you say that, for example, a round with a TKO of 15 has twice the likelihood of stopping someone as a round with a TKO of 7, but a load with a higher TKO will usually be a better choice for defensive applications.
Jane’s Infantry Weapons and Jane’s Ammunition Handbook give all data in metric. TKO can be worked out directly with the following formulae:
Most pistol hollow-point rounds are designed to expand to 150% of their original diameter, so one can multiply the calibre by 1.5 to get an idea of how the round will perform if it mushrooms. Since we don’t know the likelihood of mushrooming we must express the bullet’s TKO value as a range rather than an average. Therefore a 230gr hollow-point .45 at 850fps has a TKO of 12.57-18.85 and a 124gr 9mm hollow-point at 1200fps has a TKO of 7.55-11.32.
Some argue that TKO is only useful for comparing pistol bullets to pistol bullets or rifle to rifle. Some hunters that have used both handguns and rifles assert that TKO is relevant for comparison. A 44 magnum 240gr at 1400fps with a TKO=20.6 is more likely to drop a deer more often than a .270 Win 130gr at 3100fps with a TKO=15.9.
If there is a discrepancy, it is in comparing bullets with a high tendency to tumble with those that do not. One factor I don’t think TKO figures in is the tumbling of modern spitzer bullets. I suspect that many of the hunting weapons Taylor used used round nose ammo. This would certainly be true of the large calibre big game weapons.
If a bullet tumbles then at some time during its rotation it will present its lateral areas, which will be larger than its frontal area. Suppose we multiply a bullet’s length by its calibre, and use a correction factor of, say, 0.75 to allow for the shape not being a rectangle. This gives us an approximate area for the side of bullet. If we divide that area by p, take the square root and multiply by 2 then we have the equivalent bullet diameter that would have the same area. For a 62gr 5.56mm round of 0.224 x 0.906 area is equivalent to a 0.44 bullet, and for a 150gr 7.62mm bullet of 0.30 x 1.28 area is equivalent to 0.60 calibre. As a “Quick and Dirty” calculation we can simply double the calibre.
TKO of a 5.56mm 62gr at 3100fps will therefore range from 6.15-12.08 and for a 7.62mm NATO 150gr at 2750fps as 17.68-35.36.
This would seem to agree with the observation that the 5.56mm often displays very variable stopping power. For the 5.56mm I think the true TKO is in fact much higher, since the round causes extra damage if it fragments. I’ve not idea how to quantify this, however.
As I’ve stated earlier, comparison of a bullet’s momentum is only really relevant when calibers are similar, and the TKO illustrates this. If two rounds have the same TKO, then by definition they will have the similar performance when it comes to incapacitating a target. The smaller calibre round would have a higher momentum to give it the same final TKO. A .45 with a TKO of 12.57 has a momentum of 0.86 ftlb/sec. A 180gr .357 at 1369fps has the same TKO and a momentum of 1.09 ftlb/sec. The smaller, higher momentum round should move an object it hits at a higher speed than the larger, but the TKO is the same, illustrating the effect the larger calibre has. A higher momentum is therefore only an indication of a better round if the round is of the same or larger calibre.
The 1899 Hague convention prohibits the use of expanding ammunition for conventional conflict between military forces. In fact the Hague convention is only binding to the signatory nations. (2) Not only does the Hague Convention not apply to non-signatories, but it no longer applies to the signatories should a non-signatory be involved in the conflict.
“The Contracting Parties agree to abstain from the use of bullets which expand or flatten easily in the human body, such as bullets with a hard envelope which does not entirely cover the core, or is pierced with incisions.
The present Declaration is only binding for the Contracting Powers in the case of a war between two or more of them.
It shall cease to be binding from the time when, in a war between the Contracting Parties, one of the belligerents is joined by a non-Contracting Power. ”
Many non-signatories such as the US and UK voluntarily observe certain parts of the Hague convention so use FMJ pistol ammo. This may seem a little odd given that the St Petersburg Declaration on explosive ammo is freely ignored for HMG and cannon rounds.
“The Contracting Parties engage mutually to renounce, in case of war among themselves, the employment by their military or naval troops of any projectile of a weight below 400 grammes, which is either explosive or charged with fulminating or inflammable substances”
For most western military forces the choice is between the .45ACP round and the 9mm Luger/ Parabellum round. From the principles described above, it should come as no surprise that the .45ACP is the more effective incapacitation round in FMJ configuration. Quite simply, the .45ACP makes a bigger hole to let the blood out quicker.
Tests of the 9mm FMJ and the .45 FMJ prove to be interesting. Both have a similar level of muzzle energy and both have adequate levels of penetration. The .45 creates a large diameter wound channel and a good sized temporary cavity. The faster 9mm round creates a narrower channel and a larger temporary cavity. A large permanent cavity is a more reliable incapacitation mechanism than temporary cavitation, and this is borne out by combat reports of 9mm and .45 FMJ performance.
Suppose for purposes of illustration we assume that both rounds penetrate 12" and the only tissue they destroy is that directly in their path. A 9mm round would therefore create a cavity with a 13.4 square inch surface area from which blood would be lost and the .45 would create a cavity with a 17 square inch surface area which is 27% more surface for blood loss. Due to the elasticity of the tissue the wound cavity will in fact contract after the bullet has passed through. For a non-expanding, non-tumbling bullet wound channel diameter will reduce to 66% of the bullet’s calibre but the bullet will have damaged a surface area of tissue equal to its full circumference multiplied by depth.
In fact the superior stopping power of large bore rounds over smaller faster ones has been well known since the days of the Indian Mutiny,(3) if not earlier. We can make modern medium calibre rounds travel at much higher velocities, but this only seems to increase the temporary cavity, not the permanent one, at least for non-expanding ammunition.
For military operations where non-expanding ammunition must be used a large calibre automatic pistol is to be preferred. If possible, a bullet with a flat area of nose should be used. This provides better energy transfer and also cuts a more efficient wound channel. Round nose and ogival bullets tend to push tissue to one side, while a flat nosed bullets destroy tissue in front of it. Suitable rounds include TMJ and semi-wadcutters -although some automatics will need tuning to feed these reliably.
If in a situation where you can use expanding ammunition, the question presents itself:
Medium calibre guns can be more compact or have a higher magazine capacity, although many modern .45 autos are available in both compact and high capacity models.(4) If you look at a table of data from tests against calibrated gelatin you’ll see that many medium rounds will penetrate to a desirable depth and expand to the same diameter.
Some Sample penetration figures:
|185gr .45ACP JHP||10"||158gr .38spl (“FBI load”)||12.6"|
|125gr .357 JHP||14"||147gr 9mm Luger JHP||13.2"td>|
|230gr .45ACP JHP||14.2"|
The quick answer to the question is “No”.
The first reason for this is that hollow-point and softpoint ammo is not 100% reliable.
Imagine two bullets of the same weight and velocity impacting flesh. The first bullet is of a smaller calibre and pointed. It will penetrate quite easily since it is concentrating all of its force on a small area. The second bullet is wider and flatter. Its force is spread over a wider area so the relative resistance the flesh offers to its penetration is larger. The bullet will be slowed down at a greater rate than the first. To slow down a bullet has to lose energy so some of that energy will be lost to the surrounding tissue as friction and motion. This second flatter, wider bullet will also be creating a wider wound channel and more of the tissue ahead of it will be crushed rather than pushed aside. A larger wound channel means a greater surface area for blood lost. The more rapid the blood loss the greater the drop in blood volume and an increased chance of a sufficient drop in blood pressure to cause a loss of consciousness.
For a combat pistol bullet we need a balance of penetration and expansion. We need it to penetrate deep enough to reach vital organs or the CNS. If our round can do this then it is no bad thing if it dumps lots of energy into the target and makes a big hole while doing so.
Most combat pistol rounds are hollow-points. They are fired as full calibre rounds of a reasonable aerodynamic shape and on impact with flesh they are supposed to expand into a wider and flatter shape. The mechanism for this expansion is the tissue acting hydraulically on the inner surface of the hollow-point cavity. The larger this surface area the greater the total amount of force applied to change the bullet’s shape.
Let us consider the performance in gelatin of three pistol rounds that penetrate to the same depth. The following data was taken from the July 1996 edition of Handguns magazine, pages 51 and 53. I have added the calculated momentum and an estimate of MWA ratio. All three rounds produced 12" of penetration in gelatin.
|Velocity (fps)||Energy (ftlbs)||Gel Penetration||Expanded diameter (inches)||Crush Cavity Volume (Cubic In)||Stretch Cavity Volume (Cubic In)||Momentum (ftlb/sec)||Estimated Minimum Wound Area Ratio|
|9mm Luger 124gr Gold Saber JHP||1,180||384||12"||0.65||4.0||38.5||0.65||2.4|
|.357 Mag 125gr Federal Classic JHP||1,450||584||12"||0.65||4.0||79.8||0.8||2.4|
|.45 ACP 230gr Hydra-Shok JHP||850||369||12"||0.78||5.7||28.4||0.87||2.87|
Comparing the 9mm and the .357 both rounds are effectively the same weight and expanded to the same diameter, creating the same volume permanent cavity. They differ in velocity and energy and this difference can be seen in the larger stretch cavity/temporary cavity created by the 357.
The 9mm and the .45 have very similar energy levels. Since the .45 expanded to a greater diameter one might have expected this to transfer more of this energy and create a bigger stretch cavity. The .45 round is considerably heavier than the 9mm and the increased inertia of the bullet helps overcome the increased resistance to penetration that the larger diameter might experience. Rather than losing energy to create a temporary cavity the .45 is using it to force the bullet forward and create a bigger wound channel.
Comparing the .357 and the .45, one creates a larger permanent wound cavity and the other dumps more energy and creates a larger temporary wound cavity. The creation of a permanent cavity seems to be the more reliable and consistent Incapacitation mechanism.
The above data is for hollow-point ammunition that has expanded. The performance of hollow-point ammunition should never be assumed to be 100% reliable. A certain amount of force will be needed to deform the hollow-point and if the bullet is travelling too slow there will be insufficient energy available to do this. This article estimates an impact velocity of at least 800fps is needed. Cavities can get plugged by clothing or debris or there may be inconsistencies in manufacture. The more reliable hollow-point is likely to be that with the greater cavity surface area. The greater the surface area, the higher the total force applied to the round to expand it. For the automatic rounds the larger cavity will most probably be on the larger 45. Revolver rounds do not need to feed through an automatic’s mechanism so can use softer leads and a wider variety of bullet shapes designed to improve mushrooming.
If a revolver or automatic round fails to expand then the round that had the bigger original calibre is likely to be the more effective since it will create a bigger permanent wound channel.
Jacketed soft point and hollow-point was developed in the 1960s. The rounds had to be robust enough to feed though automatic pistols, and to ensure that there was sufficient energy to mushroom the round the bullet weight was decreased to increase velocity. This worked, but energy decreased rapidly with range, and beyond a certain range the bullet was less effective than a FMJ round of the same calibre. Improvements in bullet design mean that we can now have heavier hollow point ammo, but beyond a certain range the bullets will still not mushroom.
For any hollow point round of a given calibre, weight and design there is obviously a threshold level of energy or momentum under which it will not mushroom. I’ve seen references about there also being a upper threshold for hunting rifle bullets, but this is unlikely to be a factor at handgun velocities. I’ve not seen any published data on thresholds for pistol hollow-points. These would be of great use to shooters, but for commercial reasons we are unlikely to see them.
Reliability is another factor. Even at shorter ranges, expanding ammo does not work every time. I’ve seen a test where four brands of HP ammo each had four rounds fired into water tank. Only one brand expanded 4/4, another 2/4 and the other two all the rounds deformed but did not mushroom. The sample group is not big enough to say “brand A works every time and brand D never does”, but it does illustrate that there is considerable variation in performance. These rounds were fired under ideal conditions: in combat hollow-point cavities can often be plugged by clothing and building materials.
Against humans hollow-points must deal with clothing of differing thicknesses and construction plus belts, zippers, buttons, cigarette packs, and varying layers of fat. Ribs are a solid barrier but brittle, capable of partially fragmenting some bullets and itself becoming secondary fragments. There is nothing consistent in what the bullet is hitting so results are unlikely to be consistent too. In some animals such as pigs hollow-points have filled with plugs of hide and acted as solids.
Bullet design also has an effect on the reliability to mushroom. Soft lead rounds that can be used in revolvers or derringers will expand more easily that the semi-jacketed bullet needed by automatics and high velocity revolver loads. Automatics may have problems feeding rounds with a proportionally large diameter hollow-point.
I suspect that the larger cross section of a large bore bullet and the larger capacity of the hollow-point cavity also improve the tendency to expand.
Any combat round must have sufficient weight and diameter to deliver and effective wound even if it fails to expand.
Many medium calibre rounds have a good penetration depth. If they mushroom they create a good diameter wound channel, but when this doesn’t happen the channel created is much smaller. It is also worth observing that some of these rounds mushroomed have a diameter not much more than an unexpanded .45. The statement should not be taken to mean that I am suggesting that a .45 FMJ is more effective than an expanded 9mm JHP but that the difference between an expanded 9mm and an unexpanded .45 may be less significant than one might think. To prevent underpenetration many hollow-points are designed to expand to 150% of their original calibre which for a 9mm gives a diameter of 0.53". Therefore many 9mm loads that expand do not create a wound channel that much bigger than a .45 JHP that fails to expand. Potentially the 9mm JHP that can expand to 150% can create an initial wound channel of between 0.355" and 0.53" while a .45 JHP with the same expansion characteristics creates a channel of between 0.45" and 0.675" diameter. After the bullet has passed through the wound channel will close to 66% of the projectile’s diameter if it was a non-expanded pistol round and to 82% if the projectile was mushroomed. I believe this tendency has more relevance to those that have to treat bullet wounds than to incapacitation. The surface area of the cavity from which blood will be lost will be a product of the full diameter of the projectile.
Most medium calibre expanding rounds are too light. Tests indicate that bullets lighter than 200gr are far more likely to be deflected by an impact and veer off path, missing the internal structures that they were aimed at. The only medium calibre round that did not show this tendency in tests was the 200gr 38 Spl LRN round. Soft lead versions of this round had even more marked wounding. 200gr bullets are available for the 10mm Auto, but these are of FMJ configuration. (5)
Most large bore rounds use bullets of at least 200gr. Rounds such as .45ACP 230gr, .44 Special 246gr, .45 LC 255gr are widely available and noted for their effectiveness.
The most effective hollow-point ammunition is that which has sufficient weight to perform as normal FMJ should it fail to expand. Large calibre rounds are more effective in such an instance than medium ones. Quite simply bigger hole, faster blood loss.
Some gun magazines have taken to publishing gelatin performance when discussing new loads. Unfortunately, some writers discard the data from rounds that fail to expand, when how often this happens would be one of the most useful things to know! Ballistic gelatin needs to be quite carefully prepared if it is to be a realistic tissue analogue and you have no guarantee that this has been done correctly. Many writers fail to mention the range that the shots were made at too.
The gelatin usually used is 10% gelatin chilled to 4 °C. As a calibration a .177 caliber steel BB fired at the block at 590fps (± 30) should penetrate 8.5cm ± 0.8 (for a 10% calibration tolerance) or ± 1.7cm for a 20% calibration tolerance. This formula was developed by Dr. Martin L. Fackler at the U.S. Army’s Wound Ballistics Laboratory to give the most reproducible simulation of muscle tissue or soft tissue. Gelatin seems to be a useful medium for examining penetration, primary cavity, tumbling and mushrooming performance. Gelatin has a much lower elastic limit than most living tissues so the temporary cavitation produced may be greater than that exhibited in an animal target. Skin has been demonstrated to have a contributory effect to temporary cavitation size. A joint of meat with the skin removed will display a greater temporary cavity when shot than one unskinned. The volume of the body part or animal shot may also be expected to have some affect since the larger the mass around the wound channel the greater the resistance to movement due to inertia. Incidentally the NATO standard for gelatin testing is 20% gelatin, a concentration that appears to have been arrived at arbitrarily and seems to have little physiological basis.
Can you test ammo yourself? A bullet fired into a body of water takes twice the distance to mushroom and lose its energy than it would in flesh. Some clever souls have built troughs filled with water filled plastic bags or rows of paper cartons. From these they have been able to study mushrooming, penetration depth and even if the bullet stays on course. The paper or plastic may change the conversion factor slightly, but this can be catered for by firing a round with a known performance in gelatin.
JD Jones: One quick test I’ve used for years is plastic anti-freeze bottles filled with water. A hit in about a 3-4" circle in the center of the bottle is necessary - off center hits results in blowout on that side too quickly. A steel plate with appropriate hole cut in it saves a lot of time. Back it up about a foot behind it with cardboard. The hole left by the bullet and its fragments are easily seen and size measured - also if they are going sideways. Any bullet that expands in the 4" (approx) thick water column will do well on a deer double lung shot. Just back up to change the distance/impact velocity. Quick, crude, and works for pistol bullets. HV rifles need a much stronger backing. Bottles are about $0.15 each from the manufacturers of plastic bottles.<
There have been many attempts to create formulas to predict the terminal effects of bullets from their velocity, weight and calibre but none have really been satisfactory. In recent years testing bullets on ballistic gelatin has become more widespread and the subsequent data has become more readily available. The problem of correlating this data with real world performance remains.
Recently I came across this webpage and the following formula:
|Minimum Wound Area: MWA (cm2) = [ 1 cm2 / 15 kg Body Mass ] + 60 cm2|
From Jan Friis-Hansen, “Mesolithic Cutting Arrows: Functional Analysis of Arrows Used in the Hunting of Large Game.”, Antiquity, No. 64, 1990, pp. 494 504, based upon research to estimate the minimum lethality required to cause a game animal to collapse from haemorrhage within 10 seconds or 100 yds from a wound caused by an arrow.
Since after damage to the CNS the primary mechanisms for rapid incapacitation from bullets are related to blood loss this may also be applicable to firearms wounding.
We can do some interesting things with this if we accept a couple of assumptions:
If we know the required MWA we can compare it with the internal surface area of the cavity created when a round is fired into ballistic gelatin. Based on this we can derive the following formula:
Where d is the diameter of the wound cavity in inches and l is the depth in inches, to a maximum of 15".
Let us test this formula using a round of known poor stopping power, the .30 M1 Carbine round in FMJ. This was a high velocity small calibre round so penetration will exceed 15" and in absence of better data we will assume the wound diameter to be the same as the nominal calibre. This gives us:
The M1 Carbine FMJ gives us about 40% more wound area than the theoretical minimum required. This is, of course something of a “best case” for the round. Given an average torso depth of 9.4" many hits will travel less than 15". Using 9.4" in the formula gives a value of 0.87. Let us look at the 9mm and .45 FMJ pistol rounds. Both of these rounds penetrate about 27-29" in ballistic gelatin so we will use a wound depth value of 15" again and treat the wound diameter as the nominal calibre for comparison purposes.
In FMJ the .45 is generally accepted to be a more effective round than the 9mm and the predictions of the formula agree with this observation. A .45 FMJ that penetrates at least 15" creates a wound area twice the minimum needed. Put another way, any torso hot of 8" stands a good chance of quickly taking the fight out of an aggressor.
A .22 bullet that penetrates 15" gives us a MWA Ratio of 1. Most pistol fired .22s penetrate 8-11" so give a value of 0.54 to 0.75.
Next we will look at some expanding pistol rounds. The following penetration and expansion figures are taken from the Wikipedia pages on the calibres. We are assuming the wound channel diameter is equal to the diameter of the expanded round and that expansion is 100% reliable, which in the real world will seldom be the case!
|Muzzle Velocity||Expansion||Penetration||MWA Ratio|
|.45 ACP||Federal HydraShok JHP 230 gr||260 m/s|
|.45 ACP||Cor-Bon JHP 185 gr||350 m/s|
|9x19mm||Cor-Bon JHP 115 gr||410 m/s|
|9x19mm||Winchester Silvertip 115 g||373 m/s|
|9x19mm||Federal HydraShok JHP +P+124 gr||370 m/s|
|9x19mm||Remington Golden Saber JHP 147 gr||300 m/s|
|.40 S&W||Cor-Bon JHP 135 gr||1300 fps||0.56||9.8||1.69|
|.40 S&W||Federal HydraShok JHP 155 gr||1140 fps||0.68||13.3||2.78|
|.40 S&W||Remington Golden Saber JHP 165 gr||1150 fps||0.68||12.0||2.5|
|.40 S&W||Winchester Ranger SXT 180 gr||990 fps||0.72||13.0||2.87|
|.357 Mag||Remington Golden Saber JHP 125 gr||1220fps||0.6||13||2.4|
|.357 Mag||Federal Classic JHP 125 gr||1450||0.65||12||2.4|
I’m a little surprised at how well some of the 9mm and .40 rounds performed. 147gr 9mm ammo was designed to “perform like a 45” and it seems that it comes close, at least in the Rem Golden Sabre load tested. Heavier bullets seem to perform better than lighter, which does not surprise me. The 125gr 357 magnum has a reputation as being an effective round, but as I explain elsewhere this seems to be based on the findings of the flawed study by Marshall and Sandow. The MWA Ratios suggest that it is an effective round, but not as effective as heavier bullets such as the .45.
The above figures assume that hollow-points will expand reliably every time, which is unlikely. My personal choice would still be for a larger calibre round such as the 230gr .45 since this will create a larger wound area even if it fails to expand. This is likely to be larger than the wound area created by some medium calibre loads that do expand!
MWA Ratio is a potentially useful tool I am still experimenting with. Rearranging the formula gives us depth = 10.23/p x diameter. This gives the depth of cavity needed to give the minimum wound area for a given bullet diameter. It can also give us the minimum calibre/diameter needed to produce a wound of sufficient depth to produce the required Minimum Wound Area. Using an average torso depth of 9.4 inches in the formula indicates a calibre of at least .35 is needed, which explains the poor performance of smaller and medium calibre pistols using unexpanded ammunition.
There are two cases where you might want to use a high penetration round. The first is for hunting or defensive use against large animals. The second is if shooting through body armour or car bodies. I choose to term the former “Sustained Penetration” and the latter “Pulse Penetration”.
Bullets for use against animals often need far greater penetration than is required against humans. Animals are often bigger, with thicker skins and tougher muscle and bone. Their vital structures are often deeper. Hits against the CNS are often difficult because of the animal posture and temporary cavitation is often reduced because a greater bulk of flesh around the bullet’s track provides greater inertia against displacement. Permanent wounding and exsanguination become more likely killing/incapacitation mechanisms.
What contributes to high penetration in pistol bullets? Is it better to use a light high velocity round or a slower heavy one?
The website at
Penetration (in) = (Bullet weight in pounds x Impact velocity/ diameter of Meplat in inches)/5
Where bullet weight in pounds is given by dividing weight in grains by 7000 and the Meplat is the flat area of the bullet’s nose.
Note that the values that this formulae generates will only tally with real life performance if the bullet was a non-expanding flat point. We can use this formulae to investigate how factors such a velocity and weight will effect penetration if we are aware that the results are just for comparison. We’ll assume that the bullets are all of the same shape.
The results of various bullet weights and their velocities suggest that an increase in bullet weight is more beneficial that one of velocity. This makes sense. To push a bullet through several feet of meat needs a sustained pressure, and a heavy round has more inertia. A heavier bullet will also have more of its original energy by the time it reaches the target.
Hard non-expanding pistol bullets are better to use against large animals since expanding rounds may not have sufficient penetration to reach vital organs.
The other question that occurs about shooting high penetration rounds against animals is will the high velocity medium calibre rounds penetrate more than large bore ones?
To answer this I resorted to the formula again. Comparative results indicate that .45ACP, 9mm Luger and .40 S&W would all produce similar depth penetrations, although the wound channel of the .45 would be wider. (6)
Most 38spl rounds would be similar in performance, although one “hot and heav”y loading (158gr at 1115fps) had a significantly better penetration, on par with 125gr .357 magnums and high velocity loadings of 38 Super.
158gr .357 and 10mm Auto rounds had about 50% better penetration than the .45, .40 and 9mm.
.44 magnum rounds exceeded the penetration of any other rounds and created a broad wound channel.
The muzzle energy of a round seems to be a useful rule of thumb here.
The .44 magnum is the best beast killer, although the weight and controllability during rapid fire may make the pistol less suitable as a defensive weapon.
An 8-shot .357 revolver or 10 shot 10mm Automatic loaded with heavier weight bullets may be a more convenient weapon for constant carry. Medium calibre magnum rounds penetrate better than large or medium bore non-magnums. According to Wikipedia members of the Sirius Patrols in Greenland are issed 10mm Glock 22s for self-defence against Polar Bears. (7)
For standard power bullets, suitably loaded large bore weapons are preferable for defensive use since they create a wider wound. A .45ACP loaded with standard military FMJ rounds will penetrate 29" of gelatin.(8) The .45ACP is ballistically very similar to the .45 Long Colt, which was designed as a cavalry round capable of killing the enemy’s horses.
It is also worth mentioning that higher velocity bullets are often better suited to hunting since their speed reduces the effects of wind and holdover errors at longer ranges or against small targets.
Heavy, pointed non-expanding ammo will also prove effective in this role. However, since the medium being penetrated is usually quite thin, other bullet options are possible.
The first is to use a very light high velocity bullet with a high muzzle energy. This will only be useful within ranges where the bullet’s energy exceeds that of a conventional round of the same calibre. An example of such a round was the THV round. This was designed as a fast energy dump bullet but its shape concentrated all of its initial energy on a very small point, so it showed good penetration against plate metal etc. As an anti-personnel round the THV would only penetrate 6" of flesh.
A similar concept is seen with Russian bullets that have a small calibre steel core surrounded by a aluminium jacket. These are effectively small versions of the APCR rounds once used in anti-tank guns. It is possible that the jacket is broken off on impact, allowing the core to penetrate deeper.
Another technology that may be worth looking at is Abraham Flatau’s Ring Airfoil bullets, marketed as "Ultramag" or "Cyclone"
Another option, although not common in pistol rounds. is the armour-piercing discarding sabot bullet. This also uses a lighter bullet to increase velocity, but the use of a discarding sabot produces a better sectional density and therefore the bullet retains its energy longer.
So called because I first encountered these ideas in the writings of Peter Kekkonen.
The first dodge is to fill the cavity of a hollow point bullet with vaseline.This will increase the bullet’s weight slightly, but I suspect that the main action is to provide a more reliable expansion of the hollow-point. hollow-points work by a hydraulic mechanism, and pre-filing the cavity with a liquid provides a better transfer of energy. Filling the cavity this way may also prevent it becoming plugged with clothing material.
Second dodge is to load standard bullets into a case backwards. I’m not sure these will feed through some weapons. Often the reversed round was loaded directly into the chamber and the magazine filled with more conventional ammo. Reversed rounds provide a round with a broad meplat and what is effectively boat-tailing, useful for the sub-sonic portions of a round’s flight, which for short barrelled guns or weapons like the .45ACP is from the muzzle onward.
The fact that rounds like the 5.56x45mm tumble on impact with a target is fairly well known. What is not appreciated by many is that all pointed (spitzer) bullets have a tendency to tumble, although this tendency is dependent on rifling rate, range, velocity, bullet shape and ratio of length to calibre.
A pointed bullet has its centre of gravity towards the base, and it is a property of projectiles that they are more stable travelling heavy end first. A good example is a shuttlecock, which is launched with the heavier nose to the rear but will turn to arrive nose first. The pointed bullet is made to travel point first because it is spin stabilized. When the bullet hits something denser than air, this will destabilize it and this may occur to such an extent that the bullet flips over to a more stable base first configuration.
How readily this occurs is an important consideration in terminal ballistics. When a bullet is turning sideways it is creating a bigger wound channel and dumping a lot of energy to form a large stretch cavity. Some bullets will not begin to flip until they have travelled through the equivalent of several feet of tissue. In practice they do not start tumbling until they have exited the body so tumbling does not contribute to wounding. Other rounds tumble more readily, although at what depth is important. Some rounds will punch a neat little hole through an arm or leg but cause great disruption if the torso is hit. Some bullets will tumble just once, while others make multiple somersaults.
Range has an effect too. When a rifle round leaves the muzzle it is yawing or wobbling for some distance before it fully stabilizes. This distance can be in the order of dozens of yards. If the bullet hits within this range it is far more prone to tumbling. On the negative side it means that a bullet will not be as effective at penetration of hard cover at such ranges. At long ranges where the bullet’s trajectory becomes very curved the spin stabilization on the bullet keeps it pointing in the direction it was aimed, rather than that which it is travelling. This means the bullet will hit the target slightly side on, and will tumble more readily. Because they have a more central centre of gravity, round nosed rifle bullets are not so prone to tumbling, so may produce their most effective wounds at close or long range.
Hollow-point and soft-point rifle bullets have far more energy available than pistol rounds. so mushrooming tends to be more reliable. The large quantities of energy that can be transferred into a target by rifle bullets often causes the stretch cavity to cause permanent damage.
The 5.56mm bullet does have a characteristic that is not that well known its main mechanism for causing tissue damage is not tumbling but by fragmentation.
When the bullet tumbles it begins to fragment, and the channels that these fragments cut weakens the surrounding tissue and makes it more susceptible to damage from stretching. The result is a large volume wound cavity that may be 7cm across at its widest. The important consideration here is that both M193 and M855 bullets are less likely to fragment at below 2700fps and do not fragment if they strike at a velocity of less than 2500fps. For a 20" barrelled weapon rounds will fragment out to 140-200m range and for a 16" barrelled weapon out to 95-150m range. At ranges under 200m the lighter M193 round has been claimed to have a 200fps velocity over the M855 at the same range, so has a greater likelihood of fragmenting. This actual value can be called into question, given the specifications for the M193 call for a a muzzle velocity of 3,250 +/- 40 fps from a 20 inch test barrel measured 15 feet from the muzzle and for the M855 3,100 +/- 40 fps from a 20 inch test barrel measured 15 feet from the muzzle. Certainly at under 200m the M193 has a higher velocity than a M855. A rule of thumb seems to be that a M193 will fragment at 50m greater range than a M855.
Shorter barrelled assault rifles have a muzzle velocity below the critical level so rounds fired from these will not fragment and will produce reduced wounding and incapacitation. There is some evidence that these bullets will not tumble either, and behave like small calibre FMJ pistol rounds. Minimum barrel length for use with FMJ rounds seems to be 14.5". Tests show a 55gr M193 fired from a M4 with a 14.5" barrel has a muzzle velocity 2,850fps and fragments. The same round fired from a G36K with a 12.5" barrel has a muzzle velocity of 2,650fps and very little fragmentation. Note that the current issue M855 62gr round has a lower muzzle velocity than a M193. Range at which fragmentation will occur with either round from 14.5" barrels is probably less than 100m.
The table here shows that a 14.5" barrelled weapon has a fragmentation range of less than a 100m with M193 and 50m with M855: a difference of 50m compared to the same rounds fired from a 16" barrelled weapon. A 11.5" barrelled weapon has a fragmentation range of only 12-45m, making it considerably less effective than a pistol-calibre SMG.
The Ammo-Oracle page linked to above appears to be corrupted in certain places so I’ll reproduce the table here.
|Distance to 2700 fps||20" Barrel||16" Barrel||14.5" Barrel|| 11.5" Barrel|
Remember that velocity will also vary with temperature, altitude and humidity.
Another factor to consider is that when a rifle bullet leaves the muzzle it is usually yawing, and only stabilizes after 10m or so. This is why against certain materials bullets exhibit greater penetration at medium range than short, despite lower velocity. At very close range a 5.56mm bullet may display tumbling and fragmentation, even though it is travelling slower than 2500fps. Range at which this happens is very short however: a Colt Commando or similar shorty might be an sufficient room clearer, but wouldn’t be a street fighter. A 16" or 20" barrelled weapon is still going to wound a target more effectively. Shorter barrelled versions of assault rifles are marketed as CQB weapons, so it is worth realizing that such guns will have reduced stopping power in the very role that they need it most. This is why I propose the adoption of the 9x39mm round or .300 Whisperâ for CQB even though it means supplying an extra cartridge.
Note that these findings about the effectiveness of short-barrelled 5.56mm weapons mainly apply to when FMJ ammunition is used. Short-barrelled weapons are claimed to work somewhat better with rounds that do not rely on fragmentation such as the Federal 55gr Tactical JSP round.
Another consideration about the 5.56mm round for hunters is that most rounds, including FMJ ammo will not penetrate more than 14", which may make them unsuitable for humane kills on some game.
The 55-grain M193 cartridge used in the M16 and M16A1 is not sensitive to rifling twist rate and can be fired in rifles with twist rates of 1 in 7" to 1 in 12" . The M855 (M16A2) cartridge is best used with a rifling twist rate of 1 in 7" or 1 in 9". If the M855 is fired from a rifle with a slower rate of twist the longer 62-grain bullet can yaw up to 70 degrees in free trajectory through the air, substantially degrading accuracy.
One of the above sources makes the statement that 5.56mm rounds are less likely to inflict serious injury after penetrating interior walls than pistol calibre rounds. The author of this paper (published in “Police Marksman”) came to the conclusion that the 5.56mm was a safer round for police use than pistol rounds and this opinion has been repeated in other publications.
The actual results published call this conclusion into question. In the original paper ONLY TWO loads tested showed reduced wounding after being fired through a wall : the M885 and the Win 69gr JHP. Six of the nine rounds tested showed no change in terminal effects after passing through walls, while the M193 displayed deeper flesh/gelatin penetration but about a third less fragmentation. All of the rounds tested completely penetrated the simulated interior wall they were fired against! If we also consider that a stray 5.56mm is likely to travel five to eight times further than a pistol round then the statement that any 5.56mm load is preferable for general police applications is at best highly irresponsible.
Another Article by Dr. Fackler Mainly written from a clinical perspective
Trajectory and Exterior ballistics of the M855, M193 and 7.62x39mm
For the metrically challenged:
Metres can be converted into yards by multiplying by 0.915.
There are 3.28 feet in a metre.
Centimetres can be converted into inches by dividing by 2.54.
There are 15.485 grains in a gram.
By the Author of the Scrapboard :
Attack, Avoid, Survive: Essential Principles of Self Defence
Available in Handy A5 and US Trade Formats.