All you need is a rubber band. The barrel has three openings to adjust the tension of the rubber band which can be used to dial in the right amount of force.
You can hold the cannon without the base and shoot it like that as well. If anything, that might be the easier way to do it.
All of my dice fit inside this cannon: https://www.thingiverse.com/thing:2838067
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Not sure if 100% infill is necessary for all parts but that is what I used.
Wood Grain finish!
1) Sand it (coarse grit)
2) Sand it again (fine grit)
3) Clean off the dust/particles (burn the print with a flame for a cool burned look)
4) Stain stain stain
5) Dry time is very important, let them dry before the final step
6) Polyurethane Finish, glossy in this picture