FYI, there are increased risks of firing black powder firearms on indoor ranges.
BP has a fair amount of unburned propellant ejected from the muzzle. This can accumulate & light-off. I know someone who was injured in an indoor range incident whereas the material which combusted (not BP) was thought to be non-flammable.....well, nearly any dust can comust. My associate ducked the fireball and now can joke about the time his eyebrows, eyelashes and other facial hair was singed off...he got a nice temporary tan, too. Just like he came back from a tropic vacation, but without the airfare.
Take care,
Slim