Hey everyone, this is my first post and I am hoping that someone can help me out.
The rifle is a Mossberg 100 ATR that I got over Christmas this last year. It has had maybe 50 rounds put through it. The scope that it came with was garbage and I took the bases off and heard a little rattle. I took out the bolt and this fell out. I've looked at the owners manual from Mossberg and couldn't identify this piece from their schematic.
It is possible that you were shooting some reloaded cartridges that had been reloaded to many times. I have never had a loose primer pocket on my reloaded brass but I have split the case necks from use. That is why you inspect them before loading and throw away anything that doesn't look right. I don't try to load my ammo to the max and that is something that is very hard on the brass.