American Sniper Association 2007 report on nearly 20 years of police-involved shootings shows an average distance of the police marksman shot at 54 yards. The majority of Police Marksman engagements are probably at less than 100yds, with 200yds being unusual. This is a good application for the flat-shooting 5.56mm.
When they were first issued, standard M16A2s were found to shoot tighter groups than the Army's sniper rifles. Longer barrels, heavier barrels and free-floating barrels can improve on this further and such parts are widely available and can convert a standard AR-15 into a precision weapon for a few hundred dollars. The two guns shown on the right have 24 barrels but it is debatable if this is worth paying extra for in a Police application. A 24 barrel in 5.56mm seems to add about 120-150fps more velocity compared to a 20 barrel. This is unlikely to significantly add much to the terminal effects or penetration of the round and u. With a sing the ballistic calculator this suggests only a 12yd increase in Mean Point Blank Range when using 62gr FMJ.
Buffer technologies produce an Accu-wedge for a few dollars. This improves the fit between the two AR-15 receiver halves and writer Chuck Taylor has claimed this increases accuracy by 35%.
.223 Marksman's rifles are most likely to be used at ranges of less than 400m, although they are capable of accurate fire to much greater ranges. The rifle should have rifling with a 1 in 9 twist to allow use of the full range of 5.56mm rounds. For this application, the heavier bullet weights such as the 77gr and 80gr seem the obvious choice.
Chuck Taylor favours a .223 load using the 45gr Barnes XLC, a hollow point round with very good exterior and terminal ballistics. This round has a Ballistic coefficient of 0.203, an actual calibre of .224 and a muzzle velocity of 3480fps. Such a fast and light load may be more suitable for use in built up areas. With a 200yds zero it has a mid-range trajectory of only an inch.
Police forces are not subject to the Hague convention, so can legitimately use JHP and JSP rounds. Because of the small calibre of the .223 the use of such ammo should be mandatory. The only justified use of potentially lethal force is to protect human life from an immediate threat. In such a situation such force must be applied as effectively as possible.
AR-15/M16A2 weapons are not the only suitable weapon in this chambering. I've heard it claimed that the unchromed barrels of the AR18 shoot even better than standard chromed barrelled AR15s. Unchromed target barrels for the AR-15 are available. There are also various bolt-action varmint rifles available in .223.
Other chamberings of centre-fire Varmit rifle are also worth consideration. Weapons designed for this sort of hunting tend to be flat shooting and very accurate. As well as .223, .222, .22-250, .220 Swift and possibly .22 Hornet weapons may prove useful. Many of the loadings in these chamberings use hollow-points intended for small game which may not have sufficient penetration against a human cranium. Since all centre-fire cartridges of nominal .22-.224 calibre are in reality all .224 bullets such as the 45gr Barnes XLC can be used.
Along with the .223 this is likely to be the main workhorse of the marksman's section. The .270 Win fills a similar niche to the .308, but shoots flatter, faster, reaches further, hits harder and kicks less. The .270 would mainly be used for medium to long range shooting.
An interesting trend recently is the .270 is being offered in several self-loading weapons, such as the AR-10B. Although an anathema for "traditional" snipers, in a hostage situation a rapid follow up shot may be required.
In rural settings or terrain such as golf courses it may be desireable for a weapon with a shorter flight time or more down range energy than the .270 Win.
A common choice is the .300 Winchester Magnum, although the 7mm Rem Magnum does offer several advantages.
In actuallity the .270 equals the .300 and 7mm in speed out to beyond 1000yds, and still has more than enough energy to be effective
Most police departments don't have a requirement for the capabilities of .50 BMG rifles, but there are applications for weapons with greater anti-material effects than the chamberings already suggested. For example, there may be a need to disable the engine of a truck or boat.
The .338 Lapua Magnum, .338-378 Weatherby and .408 Cheyenne Tactical all may have police applications.
It is probable that only a police department that is very well funded, or operates under a wide range of conditions will have all four types of rifle suggested above. Even then there is only likely to be a couple of examples of the extended range or heavy PPRs.
There is some overlap of capabilities in the suggested weapons, and this is no bad thing. It means that if the tactical situation is slightly different to that reported the marksman will still have a weapon that can do the job. Probably the best advice to smaller departments is "pick any three of the above".
The .300 Whisper®
Although gun magazines like to show .300 Win Mags, .300-378 Weatherbys and .300 WSM as police rifles, the vast majority of shots are made at under 200yds.
At this range even the .308 Win is over-powered. A miss or over-penetration can travel long distances and be a danger to the innocent. The .223 Rem, on the other hand, will over penetrate some materials and underpenetrate others, and still has the potential to travel more than 800m.
A round with great potential for police applications is the .300 Whisper® from SSK. This mounts a .30 rifle bullet of between 125-240gr in a .221 Fireball case, which is a shortened .223 case. The .300 Whisper® will fit in any 5.56x45mm weapon although sometimes the magazine needs minor modification.
With a 125gr bullet the .300 Whisper® gives AK47 type ballistics, but is considerably more accurate. 150gr has more down range energy, and is probably a better choice for general use. The heavier bullets are subsonic, which offers numerous tactical options as well as allowing discrete animal control. All bullet weights shoot to 1 MOA or less.
As well as being a good round for a marksman's weapon, the .300 Whisper® is also available in a AR 15 based SMG. With a 150gr bullet the 10" barrel gives 1200fps. This translates as more than twice the muzzle energy of a 9x19mm SMG, and due to the better ballistics of the rifle bullet the .300 Whisper® retains more energy down range. The .300 Whisper® SMG is also quieter than a 9mm.