Chamber Differences For The .223

Ruger's .223 American Predator and other sporti9ng riles of its ilk are typically chambered in .223 Rem. while the AR 15 variants are chambered Mil-spec for the higher pressure 5.56x45 cartridge. — Ruger’s .223 American Predator and other sporti9ng riles of its ilk are typically chambered in .223 Rem. while the AR 15 variants are chambered Mil-spec for the higher pressure 5.56×45 cartridge.

Q: I’ve been given some military ammunition for the 5.6x45mm NATO which I’ve been told is safe for use in my Ruger .223 Remington. But an article in an American magazine warns against doing this. Who is right? Can I safely shoot this ammo in my gun?
Ken Galvin

.223 vs .5.56x45mm Chambering differences.

A: I would be wary about using shooting military 5.56x45mm ammo in a standard sporter. There are significant differences in the chamber throat area; military arms usually have very long throats which allows a lot more room for expanding gases to occupy and reduce breech pressures. Sporting rifles have a shorter throat because they generally use lighter (shorter) bullets which have less distance to travel before they engage the rifling. The NATO chamber stemmed from a need to handle different ammo from different manufacturers of mil-spec cartridges which are normally loaded to pressures, up to 15,000 psi higher than sporting .223 ammo. However, .223 rifles used for big-bore shooting usually have a faster rifling twist and long-throated (NATO -spec chambers) to handle longer, heavier .224 bullets like the Sierra 80gn HPBT MatchKing commonly used in “big-bore” competition. The long NATO throating allows this bullet to driven at up to 2900 fps! This is why NATO-spec ammo is not safe for use in a standard .223 rifle. Some commercial sporters are available which have a fast twist and long throat to handle longer bullets, but they usually have 5.56/.223 stamped on the barrel.