If I was in your position, I'd certainly be seeking legal advice.
Am I right to think that this person not only did work you hadn't asked for, but then charged you for it ?
As it stands, you are worried about using YOUR gun, YOUR heirloom, because of the quality (or lack of it) of the work carried out without your say so
The end result, as you say , could be that the gun becomes a wall hanger. You've lost money and you've lost the gun ?
Don't let him get away with it. He could be destroying other people's pride and joy AND risking causing serious injury to the person pulling the trigger.