gen metal is good. but yo need more stuff on your texture. how bout some stickers, text, or tape. text could be a warning, "made in china", that sort of thing.
why does everyone want to add such things to guns?? somali and brazil thugs do that...
it´s a weapon, not a guitar...
the mac loks ok, but the rust is reeeeeeeeeally overdone.. do you want to add it to some sort of somali pirate context?? only weapons that havent been maintained and are exposed to salt water will look like that after a while..
noone who´s got some brains left would fire such a weapon ;)
imho tone the rust down, focus on making the metal parts get a little different hue.
often bolts and pins shine more due takedowns, sooth on the muzzle and near the ejection port, grease stains etc etc.