This is a project I stumbled across a few years ago.  It’s a “Tater Tot Gun” – a pneumatic gun which uses a small bike pump to pressurize a chamber and hold a semi-rigid diaphragm in place.  When the chamber is very slightly decompressed, the diaphragm falls out of place releasing all of the pressure and accelerating your projectile.  Apparently it’s capable of more than 350 ft/s versus 1000 ft/s for an actual firearm. If this project at all interests you, definitely check out the two or three different models he describes.

Bottom line:  it’s crazy dangerous.  So, naturally, I bought two mini-bike pumps.  I not going to be completing this any time soon. 1

1. I bought those bike pumps about three years ago… [↩]