Some have said these rifles regulate with 405gr. factory ammo- ie: 1,320fps with 405gr. Jacketed. That being the case, Ed can duplicate those loads for speed and weight of bullet, however, there will be more recoil with the black powder, thus perhaps the regulation will be changed anyway, just due to that.
The only way to find out, will be to try it and see what it wants for regulation.

Big Mike - if I had one of those rifles that regulated properly at 1,760fps, I'd be VERY happy. It's the perfect muzzle speed for a 405gr. RP flat nose bullet, for all big game hunting in N.A. Easy to shoot, too.