I would look on that box real close. Make sure it is .223 Remington, not 5.56 NATO. That could (and has for me) mean all the difference in the world. It has to do with what standard the ammo is manufactured to. 5.56 ammo will fire in .223 Rem chambers, but will cause some problems when trying to eject the round. Also look at the cases. If there is a mix of military and commercial brass, that could be a problem too. There shouldn't be much difference in pressure between all the commercial brass, but there is a considerable difference between commercial brass, and military brass. It is enough to cause pressures to jump quite high and cause problems.