Yes. Many do. Including me.
I fill up a 44-40 case with pyrodex P or RS and put a bullet on it. Standard primers. Sometimes I use pyrodex .44/.45 cal , 30 grain pellets. The pellets don't have to be compressed, they already are. I use pyrodex because it doesn't react as badly to the standard lubed bullets I use. Any gun must be cleaned of all smokeless residue and conditioned [bore butter etc.] for BP before expecting it to make it through a match. Shot shells: pyrodex RS, #7 1/2 shot, AA hull, plastic wad. The amount of pyrodex is determined by the needed volume that compresses and allows a crimp with your shot charge, wad, etc.
DON"T mix shooting BP and smokeless in any gun without cleaning. The fouling will be bad.
IMHO
Best BP rifle Winchester '73.
Best caliber for BP: 44-40.
Joe West