This is twice in about 500 rounds that I have seen this happen:
Instead of popping out whole, the primer fractured and only part of it came out of the primer pocket. I thought the first instance was just a very odd fluke. But this is twice in a very small set of used brass and I don’t recall this ever happening before in the over 100K times I have removed primers.
Update: I had this happen with three more cases out of about 150. All with R-P headstamps. I bought this brass used. It would be interesting to know the history of these cases.
New brand primer, same both times? The picture makes it look like the primer cap material is brittle.
These were used cases I bought cheap (about $0.05 each, including shipping). See my update to this post.
Does the primer seat tightly? Did you notice more effort being required to seat the primer? Might try one of those cutting tools that makes the primer pockets uniform.
I did not reload these shell casings. This was when I tried to remove the old primer from used brass I purchased.
Wet tumbler process, not properly dried.
That’s what I sort of suspected too. You can see some corrosion in the pocket.
Either a prior reloader didn’t properly clean out the primer pockets, or you are dealing with heavily sealed primers.