SJ - Pistol powder is the usual culprit though it's probably ruled out since these were the only two incidents. You neglected to mention if there were signs of a barrel obstruction. A short charge with enough powder to stick the round between the gas port and the end of the barrel might still function the rifle but leave an obstructed bore resulting in catastrophic failure with the next round. Stage 3 of your 650 should have the powder check die in it to detect short charges. All the acceptable .223/5.56 powders I'm familiar with fill the case nearly full, any double charge makes a "mell of a hess". I routinely use compressed charges for my match ammo (kiddies don't try this at home) and haven't had any problems. Quarantine ALL of the ammo from that batch. If you used quality components, a digital scale will spot short charges. Otherwise - burn the lot. (Or make the guy that reloaded them pull the bullets.
) My best guess without more info - somebody else may be along shortly with a better idea. I'm glad nobody was seriously injured.
Peregrino