I have not been impressed with the energy of a blackpowder round ball rifles. I have a 54 caliber and it shoots a 235 grain round ball. When you chronograph the thing, some 44 Magnum pistol loads, of the same bullet weight, go faster.
I have been told that 58 caliber Muskets, with their 460 to 510 grain Minie balls, have an amazing knockdown power for a blackpowder rifle. I have a musket and can tell you that it truly whacks my 100 yard gong hard.
For those round ball rifles, I have known a number of folks who have had good success with Maxi Ball loads. The Maxi ball is a huge cylinder of lead, and if you are going to use a 50 cal muzzle loader, that would probably be a better choice than a round ball.