Ideally, a .290" or .295" mould would be best for use with a .018" patch or perhaps a .020" patch. Lots of guys are finding out or were originally using a full, bore diameter ball, ie: .300" in your case. Seems to me, this mould is available from Lee. Even a .308" mould would work. You merely have to have a nice radius on the muzzle's crown. In the thread "What makes a Good Hunting Rifle" - check out the crown on the huge 2 bore's muzzle. Notice the crown starts wider than the depth of the grooves, then angles out to the tops of the lands ie: bore size. Polishing this cut angle ensures a tight fitting ball and patch swages into the bore for a tight, gas sealing fit and shoots accurately. You can finish with your thumb, and make the initial cut in your barrel with an electric drill and a case-mouth chamfering tool - RCBS is the one I've used for 35 years. It has a stud on the outside taper reaming end that can be held in an electric drill's chuck. With the barrel in a bench vise, line up the drill straight, have a friend help with this, and give it a quick spin in the muzzle. Pull it out, check it and make it deeper is need be. Polishing can be down with 220 or 320 grit Emery cloth between your thumb and the muzzle. Point your thumb well into the muzzle and twist back and forth, etc, for 15 seconds, then turn the barrel 90 degrees, 90 degrees, etc. until the muzzle's crown is nice and shiny and smooth.
