Q. My department is considering using 10" or 11.5" barrels for our ARs. They are so cool, and everyone knows that all the real go-fast, high-speed, low-drag operators use SBRs. Plus, Robert DeNiro uses one in "Heat." What's the best ammo to use to poke big holes in the bad guys with these?
We dislike this question. We dislike it because of its premise. The premise is that 10" or 11.5" barrels are good choices for law enforcement or defensive use. We strongly disagree with this premise. Some of us actually dislike even 14.5" barrels, in fact.
The primary wounding mechanism for .223 and 5.56 ammunition is fragmentation. The primary factor in fragmentation is velocity. The primary velocity booster is barrel length. 11.5" barrels barely bring milspec (NATO) 55 grain FMJ to 2700 fps (the critical fragmentation threshold for many FMJ .223 rounds). Accordingly, any distance at all drops the rounds below fragmentation velocity. 10" barrels are unlikely to ever get rounds above fragmentation velocity at all.
If you are saddled with a department mandated SBR we recommend the following:
1. A marathon letter writing campaign citing the Ammo Oracle often persuading the powers that be to see reason and potentially save lives by giving you REAL weapons, not toys.
2. Use heavier rounds known to fragment at lower velocities and do more tissue damage such as:
68 grain Hornady Match OTM
69 grain SMK OTM
75 grain Hornady TAP
77 grain Nosler OTM
Obviously, you should probably have a 1:7" twist.
Under no circumstances should you take comfort in the assurances your armorer gives you that the latest soft point or hollow point law enforcement specialty round will solve the problem. Most likely it will not. Soft point and hollow point rounds lack penetration even at high velocity. Because they are not prone to yawing or fragmentation lower velocities will not increase penetration as with many fragmenting rounds.