I use 38 grains of FFFg in my .45 Colt. Under a 270 gr soft lead bullet, that is a heck of a load. That is the old Elmer Keith load. Way too much for such a fragile child.
I got to thinkin and dug out some info here that is pretty amazing. Now before we get into it, the thing we dont know is what rifle that you're planning on using, and we do not know how it is breeched. So, do you have a ridge on the inside that is gonna prevent the ball from seating down on the powder if we go too low? That you are going to have to check out to see just how low you can go.
Now here is the neat stuff. I have an old book on .45-70 original loads. They use a gallery load for the .45 using a round ball for OUTDOOR practice that may be what you are looking for. Dont forget to check that chamber volume, cause if'n ya get some air space in there, its on you, not on me. You might want to put some filler on top of the powder. Using a lubricated .457 round ball with no patch, and 7.5 grains of 3Fg, they were getting 680 fps. With 10 grains of 3FG, 815 fps. That was producing 2-3" groups at 50 yards. (They were pushing that RB down into the cartridge and jumpin it into the rifling, so they didnt have to worry about the chamber. They did have problems with air spaces if they did not twist the ball when they pushed it in to get the air out from under it.)
INDOOR loads of four to six grains were commonly used with 5 grains of FFg yielding 570 fps, and 5 grains of FFFg yielding 630 fps. This load with unpatched .45 RB yielded 1 to 1-1/2" groups at 75 feet with no smoke and very little sound.
They used 230 gr pistol bullets over 20 grains of 3Fg for 870 fps. 25 gr-950, and 30gr-1020. These loads being used at 50 and 100 yards for practice.
The only thing that would bother me here is whether you can load clear to the bottom of your chamber. That is a must check. But, this sure clears up some loads and velocities. I realize that you are talkin about a .50, so I suppose that it would be a little slower than the .45, but this information should get you into the ball park.
If'n you guys were to use some filler, what would you use, corn meal? I'd sure patch that ball also, to keep from leading the barrel. They said that 25 shots will lead the snot outta your barrel and then you gotta clean it good. But, you can go as low as it will push the ball out the barrel. It looks like your sweet spot is going to be between 10 and 25 grains of 3Fg.
Hope this helps.
Bill