I believe that .62 is the largest you can safely go, and GM used to make such a critter, but dropped it from their standard listing. So, I had Ed Rayl make me a .62 caliber straight 1 inch barrel 36 inches long and built a nice left handed flint Hawken to shoot elks with, ifn I ever get to. You just have to be careful with your dovetails, as they have to be fairly shallow and the same with the rib screws unless you are soldering it. Mine is screwed on. I have been sighting in with 100 gr of FFg and plan to use somewhere around 120-140 grains when all said and done. Ed builds a super barrel and I shot a 3-shot group off the bench at 25 yards that measured .72 inches center to center--in other words, the three .610 round balls were all cutting the same hole.