How many grains of black powder should we use? And what kind?
Once you've determined your's is actually safe to shoot, etc., etc.
What I've done for cannons about this size--- that we built (on the 'sly') in High School Metal Shop:
Un-roll 3-4 'Black Cat' fire crackers. Yeah, it was 3-4,,,, no, not 10 or 12.
Pour that silver looking, flash powder down the barrel.
Put one of the 'black cat' fuses where it needs to go,
then cram the unwrapped paper from one of them down the barrel with a common pencil.
There ya are..
But.... NEVER!!!... I mean it now...
Never!!, ever!!! stuff a sling-shot ammo, type of ball bearing down on top of that powder charge.
And if you do,,,,, NEVER!!! light the fuse... I'm warning ya--- Don't do it..
If you do, when it comes out of there--
That ball bearing is more than capable of going through the door of your Mom's 1964 Buick 'Electra 225'.
It can also knock BIG!! chunks off of bricks.... like the ones your parent's house is made with.
One of those ball bearings can also go through both sides of a (old style) steel garbage can.
So, Never!!!,,, never/ever.....
No--not even once, should you put one of those evil things in your cannon.