I have argued against using the 223 for deer hunting also, but there are quite a few people that do with great success. Many of whom are on these forums. My reasoning for arguing against it is because humane kills require excellent shot placement at short to moderate ranges. However, when shooting at an enemy, a humane kill is of no concern.The .44 mag. and the .40 cal are excellent hand gun calibers but would not be real good for assault type weapons. The .223 is now allowed for deer hunting but I know of no one that uses them because it is just too lite.
The 223/5.56 is far from an optimum round for war fighting, but it's not nearly as bad now as it used to be for self defense because we do not have the bullet restrictions the military does. I would personally prefer to see the 6.5 Grendel become the normal military cartridge in the AR platform, but I always use current military rounds in my primary defensive weapons and the Grendel is still more of a boutique specialty cartridge than anything else. That leaves the 5.56, the 7.62x39 and the 7.62x51.
7.62x51 platforms and ammo are typically heavier than I want as a primary, though I have owned AR10s and am now thinking of getting a M1a.
7.62x39 is a short range round and I want the capability to engage an enemy at greater range than it offers.
That leaves the 5.56.

