Contrary to what I've been told my whole life, size does matter, especially with the AR platform. The .223/5.56 in military applications does its damage by tumbling (yawing) and fragmentation once it hits its intended target. The fragmentation is very dependent on the bullet velocity. For 55 grain XM-193 the critical velocity is 2700 feet per second. Reliable fragmentation will not always occur at speeds slower than that. Barrel length has a direct correlation on velocity. With a 16" barrel, 55 grain bullet will have a muzzle velocity of about 3100 fps, but that same bullet from an 11.5" barrel has a muzzle velocity of around 2850. Drop an inch off and now a muzzle velocity of about 2725 fps from a 10.5 barrel. Remember the 2700 fps for reliable fragmentation? You are almost at that threshold at the muzzle. In the event you don't get any fragmentation you will basically have a through and through .22 injury. With a 7.5" barrel you will need an expanding design round, preferably a bonded HP, but not necessarily a varmint round because they don't penetrate deep enough. A very recommended round for the 7.5" is the Black HIlls TSX 50 gr. It can be expected to expand reliably to about 25 yards from your barrel.