The trick to using a maxi or Conical type bullet in a roundball twist is light weight. for the 50 caliber a 300 is best but 350 will work and a max powder charge anything less will not yield good results as you have already seen. In a 54 caliber Great Plains, max powder charge at 400 grain conical (Oh bullets must be flat based, cups bases don't work well at all.) and max powder charge if I rememebr right 110 or 120 grains yielded 1.5" at a 100 yards.