Here's a little side project I did the other night:
It is a coaxial air pistol, only 9 inches long, but produces 386 fps with poor fitting ammunition. I need to get some 5/16" ball bearings for a better fit, so I'm using 1/4" bearings in a double patch for now.
Construction of a coaxial is quite simple. The barrel is concentric with the air chamber. This makes a very compact design which lends itself well to pistol construction. The barrel is made from a 6.5" length of 1/4" nominal size type L copper pipe. The chamber is made from 1/2" nominal size type L copper and is 6" long. The barrel is attached to the chamber with a 1/2" -> 1/4" reducer fitting. On the trigger (or more technically, the pilot) side, a 1/2" threaded female fitting is soldered. This is what the body of the pistol looks like:
Here's a look down the snout:
The barrel must remain centered in the air chamber for a proper air seal. With longer coaxials, barrel supports often have to be added. Since this barrel is short and light, it remains centered on its own:
The valve is made from a rubber piston which rides in the air chamber. Here is the piston inside the chamber:
This piston was simple to make. First, I took a 1/2" long 10-24 machine screw and chucked it in a drill. Next, using the drill to turn the screw evenly, I turned the head down on a file until enough material had been removed to fit into the barrel. The screw was then removed from the drill and two faucet washers were selected. I used a flat 00 washer for the sealing face, and a tapered 00 washer for the bumper side. A dab of superglue is used around the head to make a airtight seal. A nut is the torqued on the other side until it fits nicely in the air chamber. A drop of superglue on the threads to keep the nut in place, and the piston is now complete:
Since this valve is pneumatically actuated, a blowgun makes a great trigger. A schrader valve is used to connect the airgun to a shock pump. This allows it to be charged all the way up to 400 psi. This is what the trigger assembly looks like on the airgun:
Starting at the pistol, the first fitting is a 1/2" -> 1/4" bushing. A 1/4" close nipple connects it to the 1/4" T-fitting. Since my schrader valve had 1/8" threads, I used a 1/4" -> 1/8" bushing to connect it to the T. The blowgun attaches to the T with another 1/4" close nipple.
One last thing is needed to make this pistol functional. There is a bit of dead space in the copper 1/2" threaded female fitting. This piston will get stuck in these threads and refuse to reseat. To fix that, I took a piece of high pressure hose and stuck it into the 1/2" bushing. This reduces the pilot volume too, making the valve more efficient as well. It also absorbs the impact of the piston when it opens as well:
So now I have a working coaxial air pistol, powerful enough to pop right through 1/4" oak plywood. I think with the proper ammo, it would be suitable for small game hunting. A bit of wood, and this will turn into a nice homemade airgun. Hope you like it, stay tuned for the completed airgun.