For m/l blanks as DJS suggests a homemade paper tube works, you could drop the powder to a grain or 2 quite easy and all that would come out the barrel would be a bit of smoldering tissue. Reliable ignition would be your biggest problem. If you used pyrodex it would go off with more of a pfut than a bang if its not under compression or much of a restriction ie ball in front of it.
“If a cricketer, for instance, suddenly decided to go into a school and batter a lot of people to death with a cricket bat, which he could do very easily, I mean, are you going to ban cricket bats?” :- Prince Philip said after Dunblane