Wheel weights are again a much higher BHN then the normal lead your normally use to make your black powder bullets. Wheel weights run from 10-15 BHN and pure lead is 5 BHN.

It sounds like you have the right mold for the right rifle. Do you have a small mag light flashlight that will slide down the barrel of the rifle and slide back out? I bought a small one at Wal Mart and use it to check the bore of the rifle. It will slide down a .54 caliber and larger but not fit into a .50 caliber. If you dropped that down the barrel you could see if there is even rifling in the barrel.

If it is like the others on that page you posted, it would have a 1:48 twist which would shoot conicals and round ball. Those minnies you make should fit and shoot just fine.

The last couple things I would try is, get some JB Bore Paste and scrub the bore to smooth out any ruff spots. Last I would try shooting those minnies with as little as 50 grains of powder and see if perhaps they do not like to be pushed fast. This rifle is a real puzzle.