I'd avoid the steel wool, and scotch bright, at least to start with. More antiques have been turned into neat old junk with these products in the hands of a novice than can be counted without a calculator. Shoot the parts down with a good industrial penetrating material, like AreoKroil ( the best). If you can't turn up AreoKroil, liquid wrench, or WD40 ( the worst), might get you there. Disassemble the gun, take the metal parts, and spray them with the solvent, and wrap them in plastic wrap, and leave them in a nice warm place for a few days. Much of the surface rust will wipe off. Repeat the process, and if you get stubborn spots that won't come off, work on them with a piece of wood with a beveled edged cut on the end, and some more solvent. I use a popsicle stick with the afore mentioned bevel cut on the end.
Hungry Horse