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        Earlier articles on the FN P90, HK MP-7 and PDWs can be found at
Compact Machine Pistols
Personal Defence Weapons

Debunking the MP7 and P90.
Very nice, but what are they for, exactly?

        Since the FN P90 and HK MP-7 first appeared there seems to have been tremendous enthusiasm in Military and Police circles for these weapons. Most reviews dwelled on the fact that the guns were very high tech and modern, low recoil, very cool looking and fun to shoot. I have to confess that when reading about the P90 (the MP-7 had yet to be announced) it did all sound very impressive. Then I got to talk with a gentleman who had extensively tested the P90 for the USMC. It is worth noting that despite the results of these tests the USMC continues to express an interest in acquiring either the P90 or MP-7.
        Motivation for this choice seems to be far more Psychological than Practical.

        Several Police and Military organizations have now brought the P90 or MP-7. Buying something just because you want it may be acceptable if it is a new stereo or pair of shoes. When the buying is done with my tax money and a bad choice may endanger our Soldiers I want choices to be made more rationally.

        The big question is:-

        What does the P90 or MP-7 offer that existing service weapons or other alternatives don't?

        The P90 and MP-7 have been offered or have been brought for a number of roles. Supposedly they have been designed as Personal Defence Weapons or PDWs, the intention being that they are used to arm non-combat troops that do not carry rifles and give the capability to engage aggressors at ranges of 200m.
        One might suggest that rather than a new weapon that uses a unique calibre this role might better be filled by a lightweight compact carbine-type weapon that uses the same ammunition and magazines as the standard infantry weapon. Although it may be claimed that the 4.6mm and 5.7mm are intended to replace the 9x19mm round firing at targets beyond 100m is traditionally a role for rifle rounds so it is legitimate to make a comparisom. Also, several companies offer weapons in 5.56mm for the PDW role. Comparing HK's 4.6mm round to the 5.56x45mm NATO standard rifle round is informative.
Round Bullet wt (gr) V0 (fps) E0 (ftlb) Sectional DensityMomentum (ftlb/sec)
FN P90 5.7 x 28mm312,3453780.080.32
HK MP7 4.6 x 30mm 24.7 2,379 312 0.110.26
5.56 x 45mm M855623,100 1,323 0.1770.85

        Compared to the 5.56mm the 4.6mm is        With the exception of calibre all of these properties are also seen with the FN P90 round. All of these properties suggest that the PDW rounds will be more prone to drop and wind drift than the 5.56mm and have a more curved trajectory and longer flight time. This suggests a lower hit probability. The terminal effects of these rounds is also likely to be less than the 5.56mm, given that both rounds have considerably less energy and momentum at the muzzle than the 5.56mm has a 600yds.

        Many users seem to have brought the P90 and MP-7 with the intention of using them in a more traditional SMG role. FN and HK both point out that their rounds will penetrate CRISAT test plates while traditional SMG ammo such as the 9mm and .45ACP will not. The P90 and MP-7 are therefore offered as being superior against targets wearing body armour.
        In Police hands in particular SMGs tend to be used against unarmoured targets so initially it may be prudent to compare the performance of 4.6/5.7mm PDWs and pistol calibre SMGs on targets without body armour. I believe the following clips were originally taken from a presentation by Heckler and Koch to NATO.
        This clip shows a slowed-down film of a 4.6mm round impacting a block of Forensic Gelatin.

4.6mm against Forensic Gelatin
        This looks very impressive. The temporary cavity is mainly in the centre of the block and I'd guess this coincides with where the base-heavy round flips over.
        But before we rush out and all buy a MP-7 let's have a look what a 9x19mm FMJ round does.

9x19mm against Forensic Gelatin

        Here we see the impact actually lifts the block up off the surface and rolls it laterally. The temporary cavity radiates from the entire wound track, but is somewhat larger about 20cm into the 35cm long block.

        Here's a .45ACP FMJ:-
.45ACP against Forensic Gelatin

        We see a temporary cavity that radiates from the entire wound track. Fairly even but possibly slightly broader nearer the entry point.
        And now a word of caution. What we are seeing here is the temporary cavity and this is only a minor component of the tendency of a wound to produce incapacitation. Far more important is the permanent cavity.

         Quite simply the bigger and deeper the hole you make in someone the faster they will loose blood, which causes a drop in blood pressure and loss of consciousness. The Permanent cavity is also the main contributor to the lethality of a wound. The temporary cavity makes no contribution to lethality unless it causes permanent damage.

         We can see that the .45 makes a large diameter wound channel but creates a smaller temporary cavity than the 9mm. It's not clear on the above video clip because it finishes too early but other tests confirm that 9mm tends to produce a smaller permanent cavity and larger temporary cavity than the .45ACP. Real world shooting incidents and extensive tests suggest that the .45ACP FMJ has a higher probability of incapacitating a target than 9x19mm FMJ, and the main contributor to this is the larger Permanent cavity.
        In the MP-7 video it is very difficult to see the Permanent cavity even after the gelatin stops moving. In the video below it is not visible at all, either because it is so small or because it has closed up. Even if the 4.6mm round tumbles it does not appear to create a Permanent cavity comparable to that of the 9mm or .45 ACP. Against unarmoured targets 9mm and .45ACP weapons appear to be considerably more effective than the 4.6mm. I have no videos for the P90 round but in light of its similar specifications it probably displays similar terminal effects.
        For more that a hundred years various trials have established that heavy, large calibre, flat nosed, malleable rounds tend to have the best incapacitation performance. The design of the 4.6mm and 5.7mm appear to be the exact opposite.

        The tendency of the 4.6mm to create a very narrow wound channel may cause problems with cleaning the wound to remove all foreign and potentially infectious material. This makes no contribution to the rounds combat effectiveness but may lead to post-action fatalities or increased suffering of casualties. There may be objections to this on humanitarian grounds, particularly for police forces who issue the MP-7.

        How does the 4.6mm and 5.7mm round work against armoured targets? Looking at the characteristics of these rounds it is fairly obvious that they are specifically designed to pass the NATO CRISAT tests. Let's look at some film of the 4.6mm, 9x19mm and .45ACP fired against Forensic gelatin behind a CRISAT plate.

4.6mm against Forensic gelatin behind a CRISAT plate
9x19mm against Forensic gelatin behind a CRISAT plate
.45ACP against Forensic gelatin behind a CRISAT plate

        The 4.6mm penetrates the armour while the pistol rounds do not. It would be pretty poor armour that did not stop FMJ pistol rounds! Of interest is the shockwave of energy transferred by the pistol rounds to the gelatin below. Would such an impact be enough to slow the target long enough to place a round in an unarmoured region? I don't know. Also is note is that the pistol calibre weapons are using standard FMJ ball, which is jacketed lead while the 4.6mm round has a steel core. Would we see better performance with AP or APCR pistol rounds? I doubt such rounds will equal the performance of the 4.6mm, which has penetrated CRISAT at more than 200m, but they may perform sufficiently at normal SMG ranges. Seems to me that someone should have looked at this before paying good money for a new gun and round.

        The above does show that the MP-7 is a better weapon against body armour than FMJ loaded pistol calibre weapons, but is it the best choice? For the same basic weight and size of a MP-7, P90 or most SMGs there is also a wide choice of 5.56x45mm weapons.
        As has been discussed already, the 5.56x45mm is superior to the 4.6mm and 5.7mm in nearly every respect and is general issue in most armies. The 5.56mm round has a tendency to fragment on impact, creating a permanent cavity much larger than one would expect for this calibre. This picture shows 5.56mm wound cavities.

5.56mm Wound cavities

        Compare these with the barely visible permanent wound track seen in the 4.6mm videos. Even if the round does not fragment, which tends to be a tendency at longer ranges or when fired from very short barrels, the 5.56mm will still create a larger wound channel than the 4.6mm or 5.7mm. Muzzle energy is around four times that of the 4.6mm and 5.7mm. Using a higher velocity round with a greater sectional density also means penetration against body armour is also superior.

        If you want a close range anti-body armour weapon why buy a new weapon and round that doesn't perform as well as the weapons and rounds that you have already?
        The argument that the PDWs are shorter than an assault rifle doesn't hold when there are so many short-barreled versions of assault rifles on the market. There are also 5.56mm weapons of similar size to a Machine Pistol and capable of being fired one handed. In addition to using a more effective and widely available round there are other advantages to using 5.56mm, such as the weapon having exactly the same controls as the rifle the Soldier did his basic training with.
        The argument that PDWs are lighter can also be questioned given that Carbon-fibre rifles such as the Bushmaster Carbon-15 weigh only 4lb with a 16” barrel. Many 5.56mm assault rifles only weigh a pound or so more than the MP-7 or P90 yet fire rounds with around four times the muzzle energy, greater penetration and greater terminal effectiveness.

To recap.
        For the role that the PDWs were supposedly designed for, the defence of support troops, the P90 and MP-7 are inferior to a 5.56mm carbine. 5.56mm carbines of similar weight and bulk to the P90 and MP-7 already exist.

        As CQB weapons against unarmoured targets the MP-7 and P90 are inferior in performance to 9x19mm, .45ACP and 5.56mm weapons. Whether a weapon with such a low level of incapacitation can legitimately be called a defensive weapon is debatable.

        As a CQB weapon against armoured targets the MP-7 and P90 are inferior to 5.56mm weapons.

        An argument that has been made is that the lighter weight of the 4.6mm ammo reduces the soldier's load. A MP-7 and six 40rd magazines weighs less than a 5.56mm rifle and six 30rd magazines. Less weight and a third more ammo sounds pretty good.

        The flaw in this argument is the assumption that ammo expenditure for 4.6mm and 5.56mm will be the same.
        Be it a video game or real life psychologically you tend to keep shooting a target until it goes down or demonstrates that it is out of the fight in some other way (Known charmingly as “F3” or “Fire till the F**ker Falls”).
        Given the very large difference in terminal effects between these two rounds I'd say it is highly probable that the 4.6mm shooter is going to fire more rounds at a given target than 5.56mm, especially if the low-recoil automatic capabilities of the MP-7 are used. The target will take longer to fall, so the shooter will keep firing for longer.
         Exterior ballistics also are a concern. All its physical characteristics suggest that for shots other than a close range the 4.6mm will be more prone to drop, wind drift and have a longer time of flight and more curved trajectory than 5.56mm. All of that seems to suggest decreased hit probability and a higher expenditure of ammo per target.

        The MP7 gunner will probably need twice as many rounds as a 5.56mm rifleman and twice as many would be a very conservative estimate. I wouldn't be surprised if MP7 users end up firing three to four times as much ammo per target.

In Conclusion.
        The P90 and MP-7 are weapons that many members of the military and police are interested in. Claims that the 5.7mm and 4.6mm rounds are superior to those of older weapons do not seem to be borne out by either the manufacturer's own data on the rounds or by testing.
        These rounds are inferior to the 5.56mm in all respects and 5.56mm weapons exist to fill any niche that the P90 and MP-7 can. The very small levels of permanent damage that these rounds inflict makes them a very questionable choice for many of the applications. The possible medical complications that may arise from the difficulty of treating these wounds may also be a concern.

        A lot of people want P90s or MP-7s in their armoury. FN and HK are pushing the products hard. Webpages and Reviews sing their praises and some impressive claims are made for the performance of these rounds but the data and testing tell as different story.

        These weapons appear to have no military role that cannot be better met by other weapons. How can it be justified to adopt a weapon system that offers no advantage over current and alternate systems?

        The Emperor has no Clothes! And everyone wants to buy a suit just like it!

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