Jeremy Cook advances the potato cannon arms race from muzzle- to breech-loading eras with this .602 caliber pneumatic spud gun prototype that fires from a high-pressure gas reservoir instead of via the familiar fuel-ignition system. He writes:
The actual sliding bolt mechanism was made from a piece of Delrin, which is a type of plastic with a very high resistance to abrasion and fairly low friction. It was also what was available to me at the time. There is a hole down the middle of it which allows air to pass through when fired. One end is tapped for 1/4 inch NPT to allow a 1/4 inch air fitting to be screwed in. The other end is cut just smaller than the inside diameter of the 1/2 inch pipe that it will be putting air into. To keep air from escaping when fired, a slot is cut for an o-ring to provide for a tight seal. Additionally, a 1/4 – 20 hole is tapped through one side to screw the [operating handle] into.