This cannon is moderately powerful but fun. DO NOT, UNDER ANY CIRCUMSTANCES HOLD IT. DO NOT POINT AT A PERSON OR ANIMAL. TREAT IT LIKE A REAL GUN. Please, be careful. If you do choose to make one, I take no accountability for anything you may do with it. It is not a toy. If anyone is hurt, that's your fault. This is purely  educational. DO NOT TRY THIS AT HOME. I will add pictures soon.

Step 1: Materials

Paper index card
toilet paper
Bamboo skewer
BBQ Lighter

Wow what a way to blow yourself up. We use to do stuff like this to until we overpacked a item and a piece of copper wizzed by our heads. Stuff like this doesn't need to be on instructables. Kids look at this and think its a good idea since in movies and games they get a reset.
I understand your concern, but it has less power than an airsoft gun and the explosive potential of a small firecracker, as well as a VERY STRONG BODY. I have fired one more than ten times in a row without mishap. The only real risk is the gunpowder itself, and if you own some, than you know how dangerous it is and will automatically respect it.