I have been told that there are (minimal) differences between .223 Rem. and 5.56 Nato ammo, resulting in 5.56 Nato rounds possibly not chambering properly in a rifle chambered for .223 Rem. This is apparently the result of different production standards being used for .223 Rem. (Saami specifications) and 5.56 Nato (European specifications). Also, as a result of these differences in specifications 5.56 Nato may result in higher peak pressures.
Could this have been the cause of this failure?
Ocin