Good evening everybody, my first post here.
Blaf, your cracker looks very nice, i make similar ones but somewhat bigger and with Black powder
Size shown is a bit smaller than it actually is, they are big indeed.
The main object here is to plug the ends REALLY well, i use some sort of cement which dries in 10minutes and makes rock hard plugs that will stay in the tube no matter how crappy your BP is. The explosion is a deep boom which is a characteristic of black powder.