If the wood at the head of the stock was lower than the metal because it was hammered down as with a dent, or otherwise compressed, then yes, steam or water will swell it and it will stay raised up. If the metal is proud of the wood because it was sanded off too low originally or in a prior refinishing job, it will not raise very much, if at all, and it will shrink back very quickly as the moisture content stabilizes with the ambient humidity. You could move from the dry southwest to damp coastal Oregon, but that might not be practical.
Furniture joints which are wetted to swell quickly loosen back up. Loose hammer heads which are wetted to tighten the handle quickly loosen back up when they dry. Loose wagon wheels may stay tight for a while because they will be driven through mud and water frequently. But if they dry out, they will also loosen up. White Oak (quercus alba) is frequently used for wagon wheels because it has the property of swelling a lot when moist. This is why white oak is also used for wine and whiskey barrels. It is self sealing when filled with liquid. If you can figure out some way to keep your stock constantly wet without rusting the metal, you're good to go. Otherwise, this would be a waste of time. Sorry.
