Made some grenade cartridges with a combination of Unique and black powder and used a cardboard disk to cover the neck opening and sealed with a dab of paint. New tennis balls that are still fuzzy fit tight but used worn smooth ones need to have duck tape wrapped around them to seal them better. All balls need duck tape placed on the bottom of the ball facing the muzzle to prevent a hole being blown in the ball.
It was fun and the ball only went a little over a hundred yds because it's too light and could be caught by a guy down range. If interested I can try to find the powder charge I used. Ray