I shoot my.72 caliber Kodiak Double rifle with between 90 and 140 grains of GOEX FF using conicals I cast with very good results. I had a mould custom made for the conical that weighs 1.525 oz's or about 669 grains.( See pic below). The .72 conical is beside a .58 caliber REAL bullet cast for my 1861 Springfield. I wonder if your .58 conicals are the right bullet for your rifles twist, or were they made for more traditional rifles, like Hawken rifles or Civil War Springfields or Enfields. I would bet the problem is your conical, not your powder charge.