they make some stuff thats supposed to chemically remove rust, i cant remember the name of it but they sell it at brownells and midway, comes in a blue and white tube. That works well on light rust and didnt affect the blueing on the gun i used it on. The next step up would be 0000 steel wool and some type of gun oil. There is also a special metal sponge you can buy for rust, its a little more aggressive than steel wool, its like ribbons of metal and its sharp. Ive used all 3 with different levels of success...at a certain point though you will start damaging the blueing