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Technically, 5.56 has a little higher pressure, and a slightly larger chamber, and a longer leade (freebore). So 223 can fire in a 5.56 chamber, but not the other way. Practically, it often does not matter. However, there are reports of Federal XM193 5.56 ammo showing some pressure signs in 223 chambers, namely, the primers pop out on firing. There is an intermediate chamber, called 223 Wylde, that can shoot both.
Lee
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