I wouldn't use granulated black powder for a fuse. It burns too fast. Fuse train powder is formulated to burn at a very slow rate (fractions of an inch per second vs. yards per second). In fact if you confine black powder as in a straw or coffee stirrer it will burn faster even than in the open air. Slow match used in commercial aerial and set pieces comes with a paper sleeve over most of its length, with the igniting end exposed. That part burns relatively slowly, but as soon as the fire reaches the paper sleeve, it really takes off. I almost ate a 5" aerial shell before learning the proper way to fire.
Do as the man says above: buy some of that red or green colored sheathed fuse used in M80 salutes and backyard aerial display fireworks. It is small diameter, water resistant, and with good quality control has a uniform burn rate. Test it for burn rate first, then cut to a safe length, insert in the cannon vent, light and "retire".
Have fun!!