I don't know how long you have been hunting, but when you eat game that has been shot, there is always a chance of finding a shard of a bullet or a piece of shot and I clean my small game and try to get all the shot out of the meat. I can't even tell you how many pieces of shot I have bitten into over 50 some odd years of hunting and small pieces of bullets are no different no matter how careful you are. What should you do? It aint no big deal, chalk it up to eating wild game and I wouldn't be so sure it wasn't a piece of your bullet, you do know brass is made from copper and zink? In addition, they do not clean the grinder between deer so there is a possibility it may have been from the last deer. I wouldn't go running to the butcher and accuse him of giving you someone elses meat without more evidence of what you have. You could asked them if they mix peoples meat when they grind.