Some time ago, Eli asked me to build a toy shotgun. We wanted something that would give a real “bang”, but be simple and safe.
Provided that a large mass of metal was collided into another at sufficient speed, the user would experience both a “bang” and a “kick” (although inverse).
We milled, drilled, and fit a bolt into a square aluminum tube. A spring connected the front of the bolt to the front of the tube. When the bolt was retracted and released, it would collide into the square tube with the desired effect.
And, enjoy the pictures.