.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.

      However, the very best accuracy is usually found when shooting .223 Rem. handloads.   Some AR-15 rifles are made with a proprietary .223/5.56 chamber, like the popular Bill Wylde chamber.   Most of these custom chambers have a "slightly" longer leade than the .223 Rem. Rifle.   These rifles usually shoot well with either cartridge.   With so many barrel manufacturers (and custom chamber reamers) out there, I recommend measuring your rifle to see exactly which chamber you actually have.   Handloaders also need to be aware that military cases require reduced loads, because they have thicker brass and slightly less powder capacity.