I don't know the Rockwell hardness number for 60/40 lead, but its HARD. That is plumber's lead. Fine for sealing joints on iron pipe. Not so good for balls and bullets.
We use Soft lead to cast balls for the rifles, then wrap them in a fabric patching, so that the diameter of the ball is smaller than the diameter of the land to land(bore) measurement, in most cases. The soft lead HAS TO EXPAND to force the fabric into the deep GROOVES of the rifling, to seal the gases behind the PRB. Otherwise, the hot gases cut holes or burn the fabric, and melt the ball or just cut off chunks of the ball before it even leaves the muzzle! :shocked2: :idunno: Accuracy is non-existent.
We sometime use Wheel Weight lead to cast balls, because while the lead is harder, the lead is still soft enough to expand to some extent. Those balls cast with wheel weights will be Lighter in weight, than balls cast from pure lead, AND LARGER in DIAMETER than balls cast from pure lead.
I can only speculate, but I suspect that balls cast from 60/40 mix of lead/tin, will weigh even less, be larger in diameter, and be much harder, meaning they are not likely to upset at all to help push the patching into the grooves.
These might work in a barrel with a fast Rate of Twist rifling, with shallow grooves. You can stabilize round balls out of any ROT rifled barrel, as long as the grooves are deep enough to hold a tight patch tight enough to transfer the spin of the rifling to the lead ball. A VERY HARD LEAD ball simply compounds that problem, as without upsetting on firing, its difficult to get the patch to "grab" the ball well enough to insure consistent spin. You probably will need to resort to using a mallet to seat a thick patch/hard ball combination in such a barrel.
If I were in your situation, I would trade that lead for some pure lead, and avoid the situation altogether. There are plenty of other uses for such an alloy lead. Even lead sinkers, and line weights used by fishermen would be a better use for that lead than making balls for a MLer, IMHO. :hmm: :surrender: :thumbsup: