Thinking about this some more, maybe the barrel band(s) kept the barrel seemingly tight? I can see the barrel threads getting battered with repeated shooting and getting even looser. The previous owner could have had a bubba do some work to it and he did not even know.
I think you are going to need to do some close examination of the threads looking for damage and checking the head space to figure out what to do next.
You might look at this thread and the links within it for some helpful ideas:
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