Too many years ago to think about comfortably, I remember setting up battles with 1/32 scale plastic soldiers and various Dinky(TM) type artillery pieces. I can't remember how I came by the method, but I would pull a dozen or so caps off the old roll types (not sure if you can still get them) and push them into a straw, the waxed paper type. Push the straw up the barrel and light the free end. The straw burned very slowly and produced a thick, whitish yellow smoke which would spurt out of the end of the barrel (and some from the breech too ) every time one of the caps burned. There was no real bang, but the smoke looked good, especially as it continued to curl out of the muzzle after the 'shot'. I know you can get the plastic type caps still (try joke shops and fancy dress places), though I expect the straws are long gone now. Perhaps you could just wax impregnate any old paper and roll it up. Light the end and use a sail winch to pull back the 'hammer' which could also provide the 'spurt' of smoke out of the muzzle. Just a few thoughts...
Mike