You can make a fast shooting BB gun from a cap gun.
Now why didn't we think of that as kids!
Start off with a dollar store cap gun, the eight-shot revolver variety, some caps and projectiles (in this case air soft pellets were used).
Watch how it's done in this instructional video.
The velocity of this $2 total cost gem is around 200 fps and will shoot off eight quick shots with this BB gun.
This is one of those cases we ask ourselves "Why didn't I think of that?!"