I decided last summer that my winter project this year would be a "Scratch Built" PCP rifle. Perhaps I
should call it a "Custom" since I will be buying some of the parts, such as the barrel, trigger group, tank,
and stock. Regardless of the semantics, this project is a step up from purchasing parts (with a few bits I made
thrown in) and assembling them. Where possible, given my limited skills and machine tools, I will be making
most of the metal parts from billet. I have christened the gun the "Hayabusa" which is the Japanese name
for the Peregrine Falcon (the fastest animal on earth).
I plan on building two guns, one this winter and the other next (assuming the first one works. This year's
gun will be interchangeable .22 and .25 cal using 0.630"D x 23.8" long choked Lothar Walther barrels. I hope
to test a polygonal barrel in .25 cal as well. The follow-up, next winter, will be interchangeable .30 cal
and 9mm, using 28" LW barrels. The main tube is 1" OD x 0.065" wall 4130 tubing. It will be fed by a 22ci. 3000
PSI tank and Ninja regulator set (initially) for 2000 psi. The entire main tube is downstream of the regulator,
and the plenum volume (i.e. between the regulator and valve) will be about 7 ci (115 cc), nearly the size of
the entire air reservoir in a Disco. The valve is larger than a Disco valve at 0.87" OD which allows larger
porting, although the poppet is from a Disco. I really like their over-molded design and it's plenty big enough
to allow a 5/16" throat, although this gun uses 1/4" ID at the valve seat and 3/16" through ports and plenty
big enough for a .25 cal.
The valve is only the back end with the poppet, ports, O-ring and mounting screws. There will be a spring
to hold the poppet closed when the reservoir is empty. However, there will be basically no restriction between
the plenum and the valve, making the valve volume virtually 115cc. Even with an air-gulping 9mm I expect the
plenum (and therefore valve) internal pressure to stay above 1600 psi (i.e. average ~1800). In the .25 cal
the pressure should average around 1950 psi during the shot cycle. It is this high AVERAGE pressure that I
hope will produce some very satisfactory results.
I designed and built the "reversed" tank block so that the tank is turned around, with the regulator at the
front and the tank itself becomes the fore-stock. The tank block (and the valve) will be mounted with three
10-32 (high tensile) low profile SHCSs with the head spreading the bearing load out into the tube. The calculated
safety margins for the various parts under load (at 2000 psi) are as follows:
Tube: 4.7:1. Screw Shear Strength: 3.9:1 Tube Bearing Area: 3.2:1 Tear Out: 6.6:1.
In the event of a regulator failure, the lowest common denominator (the tube yielding around the screw heads)
will still have a safety margin of over 2:1 at 3000 psi. The tank block is threaded for the output of the
regulator and acts as the front tank mount. It contains a gauge for the low pressure side so that I know what
the pressure is in the plenum. On the top is a machined shoulder with two tapped holes to mount a barrel
band. On the bottom is a 3-slot Picatinny Rail for mounting a Bi-Pod.
I hate the way Crosman mounts their breech with a tiny screw under the bolt. I also wanted to eliminate any
hole in the tube for the front trigger mount. I therefore came up with a rather novel assembly which will
mount the back of the tank, the front of the trigger, and wrap around the main tube. It will be pulled upwards
with two 6-32 SHCSs coming down through the breech on either side of the barrel.There will be a notch in the
bottom of the breech (below the barrel) to clear (and locate) the top of this tank band/trigger mount assembly.
Once the bolts are tightened, the entire assembly will become rigid.