the FBI's testing along with some firearms mfg's have determined that when fired from a short bbl, 223 is actually a good cqb round. It can penetrate body armor, and it develops enough velocity to create a ballistic wound channel, which is the tissue damage done simply from the ballistic pressure wave which is the shockwave generated by the bullet entering the body, significantly larger than that of pistol caliber rounds and thus has a higher probability of a knockdown shot. This is only from a short bbl rifle such as an 11" bbl. Beyond that, there is still plenty of time for the powder charge to develop the pressure and continue accelerating the bullet to points where through wall penetration and over pentration of the body, which will transfer less of the bullets energy to the target, becomes an issue.