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Yikes this 5.56 empty was on the ground at the range. Not sure what's going on here but I can only think the shooter's gun fired out of battery. Major picker factor I bet
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Another TLDR post from 44hunter45...
Google "incipient case head separation". The two main causes are excessive headspace and pushing the shoulder back when re-sizing. The two are related in that when you have excessive headspace, your case stretches to fill in that longer chamber space and then it gets pushed back when you resize. The stretching happens right there at the base as in the OP photo.
It can be obvious sometimes if your re-sizing die leaves a bright ring around the case near the base. More so with small base dies.
You can make a tool to feel for this by finding a piece of stiff wire that will fit through the case mouth and bending a 90° leg at the bottom about 1/8". I use an old coat hanger. You can now feel if there is a ring around the inside of the case at the base. Those cases should be rejected. I check any rifle brass I pick up and reject the failures to the melt bucket. With a little practice you can feel it easily.
Brass which weighs really light after sizing and trimming is a good candidate to check.
OR chuck....
Highly tapered cases like 22-250 or 6.5x55 are more prone to this.
Weapons which regularly have this issue should be checked out thoroughly. One of the causes is an excessively soft heat treatment of the action or bolt which allows it to stretch under pressure. If your fired brass does not want to re-chamber or your bolt is stiff to open, you may have an issue other than over-pressure loads.
Old military surplus rifles need to be watched for this. My 6.5x55 Husqvarna had to be re heat treated and re-barrelled. If you can scratch the bottom of an old military action with a file, it needs a deeper inspection by a gunsmith.
Crimped primer is new info. I think then your assumption is right that this came from a rifle with an issue.thanks for the info I've been loading for 16 years and have encountered most classic head separation resulting from multiple reloads...this one threw me since it was Lake City brass I picked up from somebody else's ammo and the one side had a bulge while the other showed no signs (bright ring or slight bulge). Since it had a crimped primer I assumed the brass hasn't been reloaded.