Martin,

Sorry to hear about your experience! Perhaps you should contact NEF (H&R / Marlin) - that should not have happened. I understand that the primer was pierced - or did it actually blow out? Two different failure modes there. Did the case head separate / rupture?

You are correct about generally not using 5.56 in a .223, although an accident like yours would be rare. See http://www.winchester.com/lawenforcement/news/newsview.aspx?storyid=11
and http://www.thegunzone.com/556faq-nb.html

However (ahem) - it's not all as simple as that - not really too much of a Dr J / Mr H. Yes, milspec may be loaded to higher than SAAMI spec. The "problem" didn't get really serous IMHO until 1) it became a NATO round and 2) the SS109 and longer bullets came onto the scene. Back when the only ammo was US M193, it just wasn't much of a problem.

As Ironraven noted, many weapons handle both just fine - probably in main part because they are designed for 5.56. But be careful with firing 223 in semi-autos - there is a milspec for the energy required to light off the primer, and it is, as a generalization, higher than non milspec. Reason is to reduce / eliminate slam fires cause by a floating firing pin doing an inertia ding of the primer when cycling the bolt into battery. I've never had one, but I load my semi auto ammo with milspec primers for that reason.

Pending hearing back from you, I'm guessing you either had a pierced primer (gun flaw) or the gun fired when it was not completely locked up (also a gun flaw). Frankly, I seriously doubt that one could suffer a case separation in this caliber / action combo unless the action was previously sprung enough for you to notice (and how would THAT have happend? Not enough available energy in that size case).

I'm VERY curious to hear the specifics once you have them. Please share!

Again, glad to hear you're OK.

Tom