The Army does not actually consider M855A1 to be armor piercing ammunition, but it is capable of some really impressive penetration. Standard M855 is stopped easily by ¼” AR500 level III body armor, but M855A1 can penetrate even level III+ steel armor.
Bear in mind that the plate in that test was able to stop 5.56mm M193 and 50 gr TSX, .450 Marlin 405 gr JSP, and even a round of 7.62x51mm M61 AP. This plate is seriously tough.
Let that sink in a bit: a standard 5.56mm “ball” round was able to penetrate a target that 7.62x51mm armor piercing ammunition could not get through. Of course the old farts will complain about how their beloved “thuty cal” will carry more energy down range and that is absolutely correct. 7.62x51mm will likely be better at getting through thicker, heavier obstacles like trees and possibly better at multiple layers of light barrier like car bodies. It will definitely be better at chewing up concrete barriers with multiple rounds. Still, at close range, in this narrow use case, a 5.56mm “ball” cartridge outperformed 7.62x51mm AP and that is really saying something. How far away does M855A1 EPR keep that performance edge though?
It is not particularly surprising that it loses some steam and is unable to penetrate at 50 meters, given the light weight of the projectile, but the fact that it can still get through the plate at 25 meters is notable in its own right. There is some limit to the capability of this new round, but it is nevertheless impressive. It is not simply some icepick penetrator, either.
Even when fired from a short barrel, M855A1 shows immediate upset and dramatic tissue disruption. In comparison, M855 shows a much longer “neck,” about four to five inches, actually. The “neck” is the portion of tissue that is relatively undamaged by a projectile’s passing before the bullet begins to yaw, fragment, or expand.
Even when fired from a 16” barrel, the M855 showed a longer neck and less tissue disruption than the M855A1 did from an 11.5” barrel. In other words, the M855A1 is more terminally effective at a lower velocity than M855. There is every reason to believe that it should continue to demonstrate excellent down range performance too, based on the light, three part construction of the bullet. It is refreshing to finally see an effective loading fielded for our soldiers’ 5.56mm rifles. For decades, civilians have enjoyed high performance loads that take full advantage of the 5.56mm cartridge’s potential but our service men and women have made do with an obsolete answer to a question no one cared to ask. The M855 was not particularly good at anything. It was not terrible, of course, but it was far from ideal. At long last, it looks as though the Army has a load that is actually better at penetrating cover than M855 but also produces better tissue damage.