A short while ago I built a .25 cal Carbine Pumper based on a shortened Disco Tube, the linkage from a Benji 392, and an MRod barrel.... While working on it, I started collecting ideas and parts for a rifle version using a full length Disco tube and a .25 cal choked Lothar Walther barrel.... Instead of having an auxilliary reservoir made from a 16 gr. CO2 bulb under the main tube, this gun would have an internal reservoir and a separate check valve.... After about a week in the shop, here are the main internal parts....
The pump linkage is an extended Billet linkage from Mac1 Airguns.... It is stronger, and 3" longer than stock, although it maintains the same 7.6" stroke, giving a swept volume in the Disco tube of 54 cc.... I wanted to mount the pivot pin as far forward as possible, so instead of shortening the Disco tube I made a threaded end plug to help spread the end load of the pivot pin into the tube.... The smaller stub on the front will mount a barrel band.... The piston is the same as the one I made for the Carbine.... Immediately behind that is a check valve, sealed both sides with O-rings, mounted into two new holes in the Disco tube and secured with high tensile 8-32 low profile SHCSs.... There is a 2.5" long, full diameter, (18cc) air chamber between the check valve and the extended front end on the valve.... The valve extension is 3/8" ID for most of its length, providing additional volume, and enough wall thickness to mount the gauge and a male Foster fitting on the left side just behind the gauge location.... That will enable the gun to be filled from a tank or stirrup pump, both easier than the onboard pump, whose main purpose is for topping up in the field.... Also shown is an MRod trigger with Challenger trigger guard.... Everything will be mounted in a inletted Boyd's Blaster stock from Norm at Discos-R-Us in Nutmeg laminate....
In the second photo are details of some of the parts.... The front plug was pretty tricky to machine.... I had to turn and thread it, mount it in the already slotted Disco tube, and then machine the slot and drill the pivot hole in the proper orientation.... It worked out beautifully, and the tube is full wall thickness at the pin location.... Below that is the check valve.... It is a simple aluminum housing, drilled through, and carrying a shortened check valve pin from a male Foster fitting, retained by a piano wire pin.... I tried one in my Carbine and it functioned perfectly, and the thin pin diameter left room for the screws on both sides.... Beside the check valve is the valve poppet and spring.... You can see how I have streamlined the head of the poppet, and narrowed the stem behind it.... The front of the valve spring has been expanded to 9/32" ID, to match the spring seat ID in the extended valve front end, for additional flow.... The brass part is a custom gauge mount I made from brass pipe fittings.... I used a 1/8" male to 3/8" female adapter and a 3/8" male to 1/8" female, then bored them out for additional volume, and threaded them together and sealed it with solder.... The Blaster stock is plenty deep enough for that assembly, and it added over 3 cc more volume to the valve.... The total volume is now 28.5 cc, slightly bigger than the Carbine was.... The Disco valve has been bored out inside to 0.62" ID, the throat drilled to 0.25", and the exhaust port milled out to 0.203" on a 20 degree angle.... It still needs a little TLC with a Dremel to round and smooth the inside corner.... The porting is 16% larger in area than what I used in the Carbine.... That combined with the longer barrel and slightly larger valve volume should gain a bit of velocity.... I'm hoping for about 10% higher FPE in this version at the same pressures.... With the additional leverage of the extended linkage, the effort should be the same at about 1800 psi as it was at 1500 in the Carbine.... so I have little doubt the peak power of this rifle will be higher....
This project resulted from my joining the Green Forum because of interest in their "Millenium Pumper" project, the concept of which was to bring MSPs into the 21st Century.... There was no concensus of opinion over what it should be like, so I undertook this project as my own interpretation of what could be done easily, starting with mostly Crosman parts and adding a few custom bits into the mix.... There are lots of other interpretations of the idea possible.... this just represents mine....
Bob
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