I don’t recommend trying this and can’t guarantee that it’s safe. PVC pipe is generally not rated for holding air pressure.
Years ago, I made a cannon that shot grapes, tater-tots, or cut potatoes with a 1/2 inch length of PVC pipe. The innovative part of this was that it used a bolt-action mechanism to open and close the breech. This meant that instead of having to stuff the projectile down the muzzle of the “gun” into the “chamber”, it could simply be placed in the breech and closed. Bolt-action rifles were, of course, invented long ago, so I can’t take the credit for this. One that fired potatoes would not come until much later though.
Here is what I came up with:
The chamber is opened with, literally, a bolt. In this case, a 1/4 – 20 bolt. When the chamber is opened, one can put a pre-cut potato or other projectile into it, close the bolt, and then blow air down the chamber with the valve of your choice (more on this later). The outer (1 inch) piece of pipe is cut out to guide the bolt down into the barrel. The cutout then turns to the right, locking everything in place.
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 bolt into.
Shown above is the sliding part of the bolt. The “chamber”, shown to the right, is made out of a 1″ piece of PVC pipe. Inside of this, another piece of Delrin is cut on a lathe so the outside is a tight press fit on the inside of the 1″ pipe. The middle of this Delrin spacer is drilled out so that the 1/2 inch PVC barrel can be press fit into the assembly. Additionally, the spacer and 1/2 inch PVC pipe facing inside is cut at 45 degrees to allow the projectile to slide into position.
The cutout illustration to the right should illustrate how everything fits together – even if the scale is off a bit.
Once everything is in place and sealed, air is sent down the barrel, sending the projectile down range. Hopefully this post explains pretty well how the bolt and barrel works, in the next post I explain the pneumatics that allow this gun to get more than one shot per air fill.