The spring mortar is dirt simple, dirt cheap, and there is NOTHING illegal or unsafe about it since nothing comes out except an arcing cloud of BB.
No gas, no explosives or anything that can explode, no heat, no fire. 100% re-useable.
If you want more power, experiment with better springs, or a longer tube, or a canister that would contain more ammo.
A film can contains about 300 BBs. (My memory fails, but it's at least 150)