I have one that the firecracker is the charge, fuse comes out the breech and the cracker mess is contained in the tube. Nice for garage shooting. Once, the first cracker went off and lodged in the muzzle. Not wanting to expend the effort to push it out, I loaded a second behind it. Was amazed at the power of a cracker when contained, blew the first one out with a loud report and considerable force.
I have another that you load with BP and put the cracker in fuse first. The BP lights the fuse and you get a little aerial detonation from the cracker. Timing is dodgy. Sometimes close to the muzzle, sometimes on the ground.
Always fun, MIKE