Took my AR to the range today (Stag Arms M4gery, 1/9 twist barrel marked 5.56) and I had a couple of significant ammo failures. The rifle had been completely stripped and detail cleaned last week and had not been shot since.

I was firing Winchester Q3131A that I bought from a fellow board member that appeared to be in good condition. After a few rounds I had a jam where the bolt would not close completely so I dropped the mag, cleared the rifle, and pulled back on the charging handle, locking the bolt in the rear most position. I then noticed that there were little metal star shaped objects just before the chamber. I dumped them into my hands and they appeared to be the innards of a primer. I then searched the brass on the ground, and sure enough, two empty cases were primerless. I immediately stripped the rifle looking for damage, and couldn't find any. I then reassembled and continued firing using UMC .223 without incident. I couldn't find any stress marks on the spent UMC cases.

What would cause this to happen? Was it just a bad piece of ammo, or should I avoid the rest of the ammo I purchased (500 rounds).

Thanks!