Someone I know of buys old guns and renovates them to sell on.

He was struggling a bit to restore one stock off a rifle, and reached the point where he thought he would not be able to do anything with it, so, out of desperation, he put it in the dishwasher.
Apparently, it came out looking like new. All the ingrained dirt and old oil stains had gone.

I'm not sure I would try it but it is worth knowing about as a last resort.