I bought a beat up low serial number 1895 Chilean Mauser rifle that had numerous dents, and used a cotton pillow case made moistened from the faucet, then a heated copper cooking pan from the stove top. Applied the pan onto the old folded sheet, listening for the steam, this really worked well. The dents came up like an elevator.
I had to be careful about the steam getting onto the metal parts of the gun, simply oiled every part of receiver and fittings when done. Let it thoroughly dry, and with the exception of a torn area, the rifle really looked much better than before. :grin: