I have used the smaller .520 balls, though not in this rifle, and they worked OK. The reason for that is ease of loading in the field, as in repeat shots at game. I just bought a .527 LEE mold at an estate sale and remember that size was popular with the Green River Rifle Works people. Haven't cast any balls yet. I did not cone the muzzle of the rifle in question but used a short starter for loading. Not needing to do that is better in hunting, thus the search for a loading combination that works well with the smaller ball. One possible solution is to load a .530 ball of known accuracy for your hunting but carry several .520 rounds in a quick loader, for speed of reloading should that be necessary.