Muzzleloader powder charges seem to be all over the board. Published data seems quite low which follows along with smokeless powder. How does one go about developing safe loads for a muzzleloader?
For a gun of recent manufacture, I stick with the manufacturer's recommendations. That's what they've pressure tested to. Going beyond that, regardless of whatever logic you apply, seems foolish. Unlike cartridge guns it is pretty hard to pressure test a muzzleloader.
For older guns usually there are some rules of thumb. I tend to push a bit beyond where I want to actually shoot, set the gun up lashed/bungee-d to an old tire, and fire it a few times via a string around the trigger. How many such shots are needed to determine safety? Not sure. Whatever you do is a gamble. As an example, I have a rifle made in 1842 as best I can tell. That suggests flintlock converted to percussion some time in the far past by a gunsmith of unknown skill / competence. I've seen "rule of thumb" suggestions of putting a round ball in your hand and add powder to cover the ball, then pour the powder down the barrel. I've seen rule of thumb suggestions of ratios of ball weight to charge weight. I don't recall for sure but it was something like 7/4 lead to powder. Those two put me around 40 grains plus or minus for my .40 cal (or whatever it is). On the other hand I've seen specific recommendations around 70-75 grains max for a .40. With a gun that old I prefer caution so it mostly just hangs on the wall where shyte doesn't blow up.
Regarding that gun, I have not figured out ideal diameter. It seems to be looser at the muzzle. It's roughly a .40 or slightly below. .395 round balls don't want to go down the barrel. .390 round balls go further but don't quite want to seat on the powder. .385 round balls are friggin' loose even with a thick patch. Someday I may have more time to struggle with it but for now .. it's nice to look at.
