How many shots with a Luger does it take to kill a cow?

[AGE]

The 5.56 mm (.223 inch) is a whole different story if you are talking rifle cartridges. The 9 mm FMJ pistol bullet velocity of maybe 1100 ft/sec isn't a whole lot more than the 900 ft/sec of a .45 caliber FMJ pistol bullet so the wound damage caused is less due to the smaller bullet area. The .223 US rifle bullet has a velocity of 3000+ ft/sec and the wound damage is generally a lot more than from even a .45 pistol bullet. Also heavy clothing and 1/4 inch thick steel plates (as well as "bullet proof" vests) provide no protection against the .223 rifle bullet.

That said, the .223 FMJ rifle bullet isn't likely to be quickly effective (at least in theory since I have never been shot with one) for poorly placed hits since the bullet will easily exit the target. However, even wounded opponents don't fight real effectively.

The 5.56 isn't better than the 7.62 but the soldier can carry a whole bunch more ammo (at least 3X) and survive longer in a gunfight (hopefully). Actually, the military still prefers 7.62 for long range fighting.