When the .30-06 was the official long-arm 
cartridge of the U.S. Military, the decision to 
standardize on something smaller begat the .308 
Winchester (7.62mm NATO).  Similarly, the 
realization that even smaller cartridges can be 
effective in the field has brought us the .223 
Remington (5.56mm NATO).

    Of course, there is a world of difference 
between the plinker's .22 rimfire and any .22 
centerfire round in terms of power, but compared 
to it's military predecessors, the .223 is indeed 
a pussycat.  At it's introduction in the 
mid-60's, the military underscored the light 
recoil of the M-16 rifle by showing soldiers 
shooting them from their chins!

    The Sporting Arms and Ammunition 
Manufacturers' Institute (SAAMI) warns against 
using military 5.56mm NATO ammunition in sporting 
rifles chambered for the .223 Remington unless 
the manufacturer of the arm has been consulted.  
Evidently, there has been significant variations 
in chamber dimensions used by some makers, and it 
is possible that excesssively high chamber 
pressures could result.  It should also be noted 
that the military 55-gr. M193 Ball cartridge does 
not shoot accurately in any rifle having rifling 
longer than a 1-in-12" twist.