Not trying to revive the whole 5.56/.223 chamber discussion, but I would like to point out one thing, especially if you reload. Actual 5.56 military brass, ie: Lake City, FMC, etc. is thicker than most commercial .223 brass and has smaller case capacity, which creates higher pressure for the same powder charge. This is of no consequence with factory loaded ammo, but the roll-your-own guys like me can create some problems if they get carried away and dump a max .223 load of powder in a 5.56 case.
Before anyone jumps on me for being anal, I've seen this up close and personal when a buddy used his go-to .223 load in a bunch of mixed brass. The first round was a L.C. case and it severely flattened the primer, the case stuck in the chamber, the rim was ripped off, and the extractor on his Colt AR broke. We broke the remining rounds down and the powder charge was within .1 gr. across the board and COAL was fine. The only difference was the cases, which measured considerably less water capacity than the Win. and Rem. commercial cases in the batch. My rule of thumb is, the smaller the case, the less room for error.
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