I have certain BP guns that prefer certain powders. My 20 year old T/C Thunderhawk will only shoot loose powder accurately. For some strange reason with even pyrodex pellets it is all over the place. Put 90 grs of Pyrodex RS down its throat, and it drives nails. My CVA don't like loose powder, only 777 pellets.
I think one gets a better burn with loose powder. More surface area with all the nooks and crannies from the granules to catch fire when the primer ignites. That what the hole in the pellets is supposed to offer. A burn spot to assist from the inside out.