.50 cal? You might try 35 gr of 777 FFG behind a .490 or .495 patched ball using pillow ticking which can be bought at any fabric shop. It's easy to ID as it is off white and has blue lines sewn in. Cut the ticking in strips 3/4 inch wide then just cut off a bunch at 3/4 long. You need to wash the pillow ticking a couple of times to soften it up. I dip the patches in water then squeeze the water out with my fingers when shooting matches. The semi wet patch won't be in the barrel long enough to wet the powder. You can just substitute the water with crisco shortening for hunting. You need to try the two different ball sizes to see which one your rifle bore likes. My old Thompson .50 loved .495 with the pillow ticking. You can work your way up with the powder charge but you don't have to get crazy and will probably settle on 55/60 gr. Shooting matches with 35 gr is very accurate with no kick whatsoever. Just make sure you seat the ball all the way down as I heard one of the reason H&R might have gone out of biz was perhaps not because they made a bad rifle but because folks will do what they do. Douglas once had issues with their BP barrels also. I shoot a .45 cal Kentucky using a Douglas barrel and it's the most accurate BP ML I own.
Ref powders: I use Trip7 and love it as it burns fairly clean and is very easy to clean. It does burn a hair hotter than Goex but it's is also a little bit tougher to ignite. I use Goex in my .45 Flintlock ML to prevent hang fires.
I also use a drop tube made from an arrow shaft with a static free funnel JP'd on top. I slide it down the barrel to make sure the powder goes straight down to the breech and does't hang up to be mushed by the patched ball going down. It keeps everything consistent.

Here's my kit for the range. I use the rubber mallet to tap around the breech after I stick in the powder. It ensures I get the powder down into the breech channel. Feel free to ask any other questions.
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