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There are very slight differences between the military 5.56 and the .223 Remmy, both in the ammo and in the way guns are chambered.
But they are more or less the same round. You can usually interchange them safely, as far as fit and chamber pressures are concerned. Bullet weight/length versus twist rate may not match, but that's another issue.
The 5.45x39 is a WHOLLY different round. External ballistics may be similar, but that's about it.
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