.223 Rem. vs. 5.56mm
These cartridges look identical. They are exactly the same
size, and they both shoot .224" diameter bullets. However, the 5.56 mm military round generates an extra 5,000 PSI
of chamber pressure. Rifles with the 5.56 mm chamber have a much longer bullet jump to the rifling. This is called the leade,
and a longer leade will reduce chamber pressure. That's what allows the 5.56 chamber to safely reach a higher velocity than a .223 Rem. chamber
when using the same bullet. It boils down to this . . . . It's perfectly safe to shoot commercial .223 Rem. ammo in a 5.56 mm rifle,
but 5.56 mm ammo should not be fired in the average .223 Rem. rifle.
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