That case separation is a fair way away from the head.
But if you can feel a ridge inside thats what it is.
Section that case with a hacksaw & clean it up to have a good look at it, grab another that looks like it's starting to do the same & section that one too, it will give you a better understanding of whats going on, chuck a picture up here as well if you can be bothered.
If all the cases are showing signs bin them now before they cause more serious issues.
If you have the cases standing in a reloading block they will get a shiny mark around them just like that from being moved around & rubbing on the block......
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