Bronze wool, a penny, electrolysis and a wire wheel.
Depends on the gun, the rust, etc.
"Wire wheel" is going to get some attention. Not my first choice, but many years ago I had some stubborn rust on a rifle barrel and I was in a hurry.
Chucked up the wire wheel in my drill press and went to town.
This particular Chinese wire wheel may be of of the softest "steel" ever made. It has been tossing wires for years and it is worn down to just about nothing.
I have used this wheel to safely remove rust from dozens of guns, more barrels than I can remember, small parts, etc.
I will be sad when I finally have to give up on it, as finding another such poor quality version will be unlikely. Or maybe Harbor Freight will have what I need.
