[nynor]

the difference is mainly in the throat of the 5.56 chamber vs. the .223 chamber. the 5.56 throat is nearly twice as long, thus .223 ammunition can be fired in a 5.56 chambered rifle, no problem.

the main problem arises with firing 5.56 ammunition in a rifle chambered for .223: 5.56 bullets are sometimes seated with a greater COL (overall lenghth) AND the .223 throat is shorter. thus, a 5.56 round could seat directly onto the rifling in a .223 chambered weapon. this is a problem because it will create a sudden, possibly catastrophic, rise in pressure when the round is fired. this problem is compounded by the fact that 5.56 cases have less volume, meaning that they, generally, have a higher pressure to begin with.

so, for the AR15 chambered for 5.56, .223 ammunition is just fine. but a rifle that specifically states .223, without also being marked as 5.56 compatible, shooting 5.56 is looking for trouble.