If you are truly serious about giving your shotgun an aged appearance, here is what works for me when I want a "salty" look on a reproduction of a vintage firearm:
Save your blackpowder cleaning patches. You know, the ones that are soaked and crusted with firing residue and such. Sweaty hands or salt water wet rags work well too. Intentionally handle and touch the metal parts all over and leave unattended for a day to two. Wipe down using the dirty cleaning patches. You should get the beginnings of fresh red rust and some loss of the bluing. Clean off, and repeat. Avoid using cleaning solvents or oils. Repeat. Do this every day or two for a couple weeks, and you should have a nice patina.
That is how antique gun finishes develope over many years. Some rust and oxidation from handling, lack of cleaning, and exposure to sweaty skin, and the elements.
The dirty BP cleaning patches contain sulfuric acid residue and other combustion products. They are also nice for giving an aged patina to bright modern brass.
Aging the wood is a little more difficult, especially if the wood finish is a modern polyeurethane varnish. If an authentic oil or varnish finish, you can give it the veteran look by inflicting minor dents and scratches using a dog leash chain, manually wirebrushing, and rubbing it down with some paint remover, and then applying woodstain. Wipe on some fireplace ashes, wipe off, and rub in some gun stock finish. The more you work the wood and inflict wear, the closer you get to artificially aging it.
