I am not an expert but..... When I do mine I first cut out the spot on the barrel, sharp hard chisel or mill or whatever but not too deep maybe a 1/64" - 1/32". I get the cut shaped nicely then undercut the edges with hand engravers and then I cut a piece of what ever metal I am going to inlay the hole with, IMHO the metal filler sheet should be twice as thick as the depth. I bevel 3 sides making as tight a fit as possible so it has to be popped in. I then anneal the inlay metal and fit the piece into the hole and using a large diameter punch with flat polished end hammer the soft metal to make the edges drift or expand to fit the undercut hole, dress polish and engrave . Experiment, have fun make mistakes on a practice piece, take a chance on piece of mild steel rod.