Wow, 4 years and first reply. I have used duplex loads in 45-100 and 32-40. I use 5% of the full BP charge weight for the smokeless primer charge with IMR-4227 or AA-Rel#7. I want to try AA-5744 next. I also have a few pounds of 5749 left. However, I have a load for the 32-40 that I save that stuff for. Also, I reduce the original full BP charge weight by the same weight as the smokeless charge. No grease cookie, just a soft coconut oil/bees wax grease.
Oh, Fg seems to be the best performer and highest velocity of the BP options with a large pistol match primer of choice.
Like you, I can shoot as long as I want or can without wipes between shots with duplex loads. The biggest cleanup job are the cases, not the rifle.
My favorite 45-70 lever silhouette load is a Paper Patch bullet as well. However, on the opposite end of the weight options. I use a 45 Colt 230 gr .452" plain base bullet wrapped with three layers of trace paper and 14 gr of Unique. She gets out the barrel at around 1,600 fps and rolls all targets over without issue. The recoil of a 410 shotgun and quicker lock time has really helped my scores.