Ok here's a little gun i made out of knex. It's small and is good for making a few to play with friends. Also, it shoots pretty far and is easy to reload :)

Step 1: The Hopper and Shooting Platform

the gray connectors and blue rings are where the green "bullets" are kept. and the little black connector between the red rod and red connector is so the bullets don't fall through when you reload

Step 2: Locking Thing

this is to lock the firing piston after you pull back the pump

Step 3: Pump and Firing Piston

yup..

Step 4: Locking Thing + "launching Pad"

connect the end of the red rod on the locker to the yellow connector on the launchpad. then the orange connector to the side of the write connector

Step 5: + Pump and Piston

slide the pump on so it points tward the front, then the piston in over it
make sure the free-moving red connector on the locker is to the right of the rod on the piston

Step 6: Handle and Trigger

put a handle on. and a trigger by sliding a grey connecton onto the stub of the leftover red rod. then use another red rod to connect the grey and free-moving red connector. get 2 medium sized rubber bands, one thick and one thin.

Step 7: Rubber Bands

use the thin rubber band to pull the pump back into the starting position, and the think one to fire the piston.

and then you're all done! just fill the top with about 5 small green rods. put your thumb on the back of the first red rod (next to the trigger) and pull back on the pump and let go of it. then push the trigger down with your thumb and it shoots :D