Here are a few pics to illustrate what I'm talking about.
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I've never barrel wear this extensive on another brand. A lot of used Sigs look like this, granted I think if you keep them lubed properly, they probably won't wear quite as much. I have my suspicions the previous owner never really cleaned or oiled it, at least not how you are supposed to. I'm not worried about it. Sigs are pretty resilient and this one seems to work fine but it is an interesting thing to see nonetheless.