As someone who dabbles in Smith restoration on non-collector grade guns, I would not use anything caustic. I also don't think the finish would look correct on a Smith if the barrels did somehow survive the salts. For something faster than traditional rust bluing, I've had great luck with Mark Lee's Express Blue. It uses the same process as slow rust bluing (rust, boil, card, repeat), but the "red rust" step requires much less time. The final finish is slightly more black (and less blue) than some slow rust solutions produce,in my opinion, but its very close, very durable, and looks "right" on guns from this era.