Why not get a metal tube, and a 40 mm airsoft shower grenade shell.
take a bottom plate and weld a small "pin" in the centre.
Weld the plate so the bottom pit is facing inside the tube pretty dead centre.
Take the grenade shell and use a grinder to grind off the "lip"
Once completed, just fill up the grenade shell, drop it in, it slides down... the pin inside triggers the shell to go off and BOOM.... a blast of bb's raining down.
