I'm not talking about ballistic accuracy or "umph", I'm talking about the minimum velocity that M855/M193 needs to reliably fragment in tissue. This velocity is about 2700 fps. Below this velocity, these rounds are about as terminally effective as a .22 magnum. From a 20" M16 barrel, these rounds don't drop below 2700 fps until about 150-200 yards from the muzzle. From a 14.5" M4 barrel, it's more like 100 yards, although round-to-round variation can make it as short as 50 or 60 yards (or as long as 150). For a 10.5" barrel, the round is not going to reliably fragment even at 50 yards. So while 50 yards may be "nothing" for a 10.5" AR in terms of accuracy, it's definitely a problem in terms of wounding capability. Of course you can always use tactical hollowpoints (TAP, Mk.262, etc.), but then your weapon's terminal effectiveness is dependent on comparatively rare and pricey ammunition, which kind of undermines the whole point of choosing a rifle in a commonly available caliber like .223. Given these facts, I don't really see the point of an 10.5" AR. "SHTF" survival isn't about bodyguarding VIPs or doing room-to-room house clearing. What advantage does an ultra-compact AR offer? A normal-sized .30-caliber rifle is far more reasonable IMO.
If you want something that's as reliable as an AK and accurate enough to hit a man-sized target at normal combat ranges.... how about an AK. Also, even gas-piston ARs aren't really as reliable as AKs.
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