The problem is that the lower feet of the barrel lugs are worn down where the slide stop contacts it and arrests the forward movement of the slide. It's mostly just a cosmetic issue, but if it bugs you then you'll need to install a new or unworn barrel.
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D. Kamm
USGI M1911/M1911A1 Pistols Website
http://usgi1911.tripod.com
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D. Kamm
USGI M1911/M1911A1 Pistols Website
http://usgi1911.tripod.com