I did that with my BP shotgun many years ago. A guy in my house wanted to "shoot" it new years. I was in a place where I was okay with new years, but July was just too much fire danger. This was near Chico, CA.
I put in some powder, as I recall, I put a tiny piece of a cotton ball next, just to keep the powder in place. On top of that, I put some firecrackers, then a simple piece of cardboard to hold it all in. It worked as expected, the flash lit the shortened fuses and they went "pop" up in the air, he got a big kick out of it.