This one is sort of an exercise in how small you can make a pneumatic spud gun, or how studly you can make a Nerf gun.
I like the incorporation of the bicycle pump into the assembly, as the thing becomes quite portable that way. I suppose the projectiles from my pedal gun would work in it The valve is a lightweight version of the one we used in a D-cell gun with a 1.25" barrel 8'long and a 4" concentric plenum. The length was great enough that a wood base was required to keep the barrel from sagging.
Air is added through a schrader valve located near the rear, on the bottom of the plenum fitting
Opening the valve at the rear allows the internal piston to retract, dumping the plenum through the barrel. There is a spring behind the piston to return it to the forward position. The piston has a rubber sheet glued to it to seal against the back of the barrel.
If you attempt to build one of these, the piston should also have a rubber pad at the rear to prevent it from damaging the back of the gun.
Of course this also means that my next gun post will have to involve gunpowder. OK.