For a powder charge, start at 50 grains, and work up to 90 grains. I would recommend FFg BP, but you need to try both FFg, and FFFg powders to find out which granule size your particular rifle likes. For that size caliber, you want to be using nothing less than .015" patches, and .018"- .020" will hold more lube, and do a better job for you. The thicker patching is less likely to shred, or be burned by the larger powder charges.
Measure the Bore of your rifle. Don't Guess at this. Then, AND ONLY THEN, can you make an informed decision on what diameter RB you should use in the gun. Unless the bore is substantially larger than .580" in diameter, I would think the largest size you should be thinking about it that .570" RB. That, with a .015-.018" thick patch material should make a very good seal.
To decide the patch thickness question, measure the depth of the grooves in the barrel. The patch needs to be at least as thick, as the groove is deep, plus 50%. So, if you have a groove that is .012" deep, you need a patch that is at least .018" thick. On these larger caliber barrels, there is a lot of room for the fabric to stretch over the lands, and get down into the grooves. The Grooves, in turn, can hold a lot of tightly compressed fabric. A lubed, or wet patch can be expected to compress at least 50%, and sometimes more in the larger calibers.
However, IF you have a shallow groove barrel, then go with a tighter fitting ball( .575") and the thickest patch you can get to go down the barrel without bruising your hand on the short starter. ( probably no thicker than .015"). For loads using more than 70 grains of FFFg, use an OP wad to act as a firewall, between the powder charge, and the PRB. That will protect the thinner patches from burning in the barrel, allowing gas to cut the lead ball, too. Shallow groove barrels can fire a PRB, but the ball/patch combination needs to be tight to the bore, so that you do not expect the lead to expand to force the patching into the grooves when the gun is fired, to spin the ball as it moves out of the barrel. With those shallow grooves, there is just not enough room for much expansion of that lead ball, except FORWARD in the barrel. As to those shallow grooved barrels with a fast ROT, a PRB will shoot accurately no matter what ROT you have in a gun. The key is the tight fitting ball/patch combination to the bore, and then measuring your powder accurately from shot to shot. Fast ROTs are less forgiving to variations in powder charges, using black powder, than are the slower ROTs. Faster, or slower, MVs will result in different POI at 50 yds, and beyond. :hmm:
