I bought a costum made star stamp form a firm in England. Cost 30£ + shipping.
The star is 2mm (0.08") tall. Max depth of imprint is 0.9mm . I tested it on a breech plug I had messed up. I figure the steel in the plug is similar to the steel in the gun barrels. Three moderat whacks with a 2 lb hammer made a 0.9mm deep clean impression. I tried filling the impression with silver without undercut. It fell out when I turned it upside down.
I then did a shallow undercut with a knife graver along the arms. The silver stuck in the impression and was almost impossible to pry out. Amazing how little undercutting is necessary. Did a quick and sloppy rust bluing to see if scratching would mess it up. No problem. Going to do some more test rounds to try to find out how deep the impression has to be. The shallower I can make it , the less chance I'll screw up the barrel. Also a shallower impression will give sharper points on the star.
Best regards
Rolfkt