Swab the gun out so it is mostly oil free and dry. Take off the wood furniture, then give the gun a shower rinse of very hot water to wash away any remaining salts, then put the gun in an oven at about 150-175 deg F to dry fully and fast so the water does not cause new rust. Otherwise, salts from the corrosive ammo will keep attracting moisture no matter what oil you apply. Then re-oil the gun fully. Nothing wrong with a paint brush, new motor oil, and rest a few days in a bucket. Then use brushes and swabs to see what comes freely from the gun. The gas piston may need a bit rubbing with 0000 gauge steel wool, but the crap is probably just sitting on the chrome bbl lining.

The barrel rust is proabally steel jacket deposits from the on top of the chrome like RS39 sez.

Hot water thru the barrel, action and gas system.

Happy face = all came out in the wash?

Glad it worked out!