http://www.shotgunworld.com/bbs/viewtopic.php?t=357470It is not just Browning. If you are referencing discoloration I had the same thing happen around the tang and safety of a Beretta silver finished DT-10. Used that shotgun a good bit at one time and came to accept it as normal wear and tear. Flitz would remove most of it, but it would return quickly.
Perhaps varying acid/base balance of differing individuals hand moisture may have something to do with it.
