Airsoft mortar, safe version;
PVC tube, about 1 inch diameter and a meter long.
35 mm film container, empty, screwed to the top of a long spring that is just less than 1 inch diameter. (Imagine a piston concept)
Attach other end of spring to bottom cap of PVC tube.
Fill film can with BB.
Use a dowel to compress the canister to the bottom of the mortar tube, and a simple cotter pin to keep it all there.
Pull pin, spring pushes the container of BB up real fast, and stops because all is fixed except the ammo. BBs carry on at low velocity, creating a shower.
No gas, no explosives, no danger, and nothing gets lost except the BBs. Also dirt cheap to make AND re-useable.
No, this is not a new idea. I posted it in the forums before.