When I was a kid, my grandad bought me a used single shot Remington .22 bolt action. It had been shot a LOT and I put many more thousands of rounds thru it. Eventually one day I noticed a huge bulge in the fired case. It was occuring right in front of the case rim of where the extractor grabs the rim. The extremely thin part of the barrel, at the breech, which is cut out for the extractor had apparently worn down to almost nothing, and a tiny amount of case was now unsupported. The gun was near worthless anyway and it was simply trashed. I am wondering if this wear occurs because of the rather strong spring scrapping the extractor against the barrel each cycle. Is this something that is typical of high round count rimfires? I suppose it is possible my assessment of what happened is incorrect and it was simply a headspace issue. But the swelling was occuring in just one spot at the extractor.