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.
We see more and more guns not getting routine maintenance. At the end of the season, (a hunter) should clean the barrel of all that residue and lightly oil it; external surfaces, too.
A clean gun, one that functions properly, is worth more than a rusty one.
