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Advanced Homemade Airgun

Building the Powerplant and Trigger
Here is a picture of the sprinkler valve I modified for use with the airgun:

I removed the valve cap, drilled and tapped it to 1/4 npt. I then threaded a street elbow to it an attached a blowgun for the trigger. On my next one, I'll not put the street elbow in, so I can pull the trigger instead of pushing it up! I epoxied the area around the street elbow to prevent air leaks and make the connection much stronger. Also, if you remove the solenoid, be sure to fill the holes with epoxy as well. You can use the solenoid instead of the blowgun assembly, but in my tests the valve does not open as quickly. This design allows the diaphragm inside to open quicker, dumping the air in a powerful blast. Do not exceed the pressure rating of the sprinkler valve unless you want a plastic grenade! I taped the chrome plated blowgun with camo tape, as I did the rest of the gun to make it less flashy.
***Be extremely careful*** with this design, as there is no safety. When the gun is pressurized, it can fire and will fire if that blowgun trigger is pushed!

Breech Loading Mechanism
Muzzleloading can be a pain, so I made this simple little device to load from the breech:

It consists of a t-fitting, and 90 degree elbow, and a 1/2 in copper to 1/2 npt fitting. To load, unscrew the end cap, put in an arrow or patched lead ball, push it past the transfer port with a short rod, and screw the cap back on. It works wonders and reduces the loading time by quite a bit. Always be sure that the cap is secured before firing, that wouldn't feel to good if it popped of into your face.

Fill Adaptor and Barrel Support
Here is how the fill adaptor and barrel support look:

Ok, so the block of wood needs painting, but it does work. I used a pipe clamp to hold the barrel securely to the air tube. The fill adaptor is a high pressure schrader valve threaded into a plug. Crude, but works well with no leaking. Drill and tap the plug to 1/8 npt, lightly coat the threads of the schrader valve with epoxy and screw it in securely. Do the same thing with the plug when it is screwed into the coupler.

Building the Sight Rail
Here is a closeup of the sight rail:

You'll want to have 2 t-fittings and about 10" of copper pipe. In addition, you need one of the weaver rails sold at walmart for lever action rifles. Chuck a bastard file into your drill press and grind it out so the copper pipe slide through it. Make sure it is fairly tight. Slip that over the barrel and solder into place. Solder about a 6" length of copper pipe into the other t-fitting. Now for the hard part. Join the two fittings together and line the 6" pipe with the barrel. Take your time on this, as this will affect the accuracy of your sight. Solder it in place and get out some quick steel and the weaver rail. Rough up the copper pipe and the underside of the weaver rail with 80 grit sandpaper. Put down a bed of quick steel and set the weaver rail on it. Press it down solidly as you line the weaver rail with the copper pipe. Work quickly and strive for perfection, a hard combination to achieve. After the quick steel has set, you can mount a red dot sight or scope and you'll be all set for some action.