The biggest culprit is plastic residue left in the barrel. Rust develops underneath that plastic and can pit the barrel. Besides that, recoil increases. Extraction of the shell is less efficient, especially with the semi-automatics.

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A clean gun, one that functions properly, is worth more than a rusty one.

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Upland hunters might have something as simple as a weed seed that has worked into the gun, into the trigger group or the bolt, causing a malfunction. Maybe a bur has developed on a part.

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