Originally Posted by Talraga
your mortar Idea is all fine and dandy, Greylocks, but where's the fun in that? It's essentially a big shot shell. It's more realistic and interesting to have a single shot round that has a "splash damage" effect on impact.
Greylocks idea is a GREAT basis for experimenting with. The spring idea is awesome, no heavy propellant canister falling on your head. You could fab up a firing mechanism that would let you fire it with the spring at different tensions. You could also fab up the firing mechanism to be a very close approximation of a real one.
As for the shot shell effect, you could envelope the BB's in some sort of sabot enclosure to have a more direct fire effect. And being spring loaded, you can experiment with this to your heart's content without needing $$$ for CO2 or other propellants.
Go. Have fun. Experiment...