As it will likely be done in either a drill press or by hand with a drill, I don't recommend a carbide bit, standard high speed steel bits will work just fine. Don't get them from Harbor Freight though, you don't want to try to remove pieces of drill bit from your barrel. Buy a single numbered standard HSS bit from your local hardware, use a suitable cutting oil and a hand drill and go slow, letting the bit do the work. Midrange on the RPMs, nothing crazy, mild pressure for the drill. It will follow the existing hole and the burr in the bore will disappear with the next shot that goes downrange. Easy peasy.