Carole killed this mule deer with a .40 caliber. She and her husband Carl probably killed deer most years of their about 40 years of married life, some with muzzleloaders. She has killed them with her TC .50 caliber, but it is too hard for her to load and too long and heavy for her to shoot well. So we designed this custom flintlock that I built for her, and she shoots it very well. (They go to the National Youth Hunter Safety meets at Mansfield, PA and Raton, NM). But here in Utah, legal ball weight for ML deer hunting is, I think, 150 grains and the .40 weighs only about 92. The game warden suggested double balling, there being no conicals on the market. I experimented until satisfied it would work well. Velocity drop from a single .390 ball and 35 grains of Goex 3F to double ball, first patched and seated and second patched and seated on that, was about 435 fps if I remember. So I upped the charge to 70 grains of Goex 3F to get about the same velocity. At 50 yards the balls would strike about 1/2 inch apart, on point of aim. Accuracy was fine and the pressure was not too high. Lyman's Black Powder Handbook and Loading Manual, 2nd Edition, does not list .40 caliber loads, but their max for Goex 3F and a .36 roundball is 70 grains, and for .45 caliber it is 120 grains.
She saw this buck at about 70 yards, walking into heavy brush, and shot a raking shot to penetrate forward through the midsection. One ball hit the hip bone and stopped and the other, perhaps only an inch away, penetrated the body and severed an artery. The deer laid down, but I think she used a finishing shot on him.
A single ball would have done the same thing, if it missed the hip bone, but that is not legal here. Using double ball is a tricky thing. I do not recommend it, and will try to have a conical for her next season. Both balls must be down on the powder and stay there. If for any reason that top ball oozes forward, you have an obstructed bore, which will be damaged upon firing. So you have to make CERTAIN the second ball is down on top of that first ball, and probably recheck it while hunting. I have not had any separate, but shot them only from the bench. If you check the seating, then you need to dump your powder pan or pull the percussion cap. My statement is that you better have a really good reason to shoot double balls, then be very careful that the second one stays seated. I do not recommend the practice for that reason.