Bear in mind that the Civil War Springfields, and other Minie Ball guns, had twists of 1/72". I shoot a 570-grain Minie (Lyman 57730) in my .58 Green River Rifle Works barrel, 1/60" twist, and it is very accurate! It is also very accurate with a round ball, both using 120 grains of FFg. I also get good results from a .50 Bill Large barrel with 1/72" twist when shooting the 370-grain TC Maxi Ball. See below.