.25 cal Disco Pumper Carbine

[SeanMP]

I like it!How about a 2 stage piston to keep the pump force under control

[rsterne]

In a future version, maybe.... I have had zero luck finding info on them....

Bob

[rsterne]

I got the valve front end done today.... Here are the parts....



The extended front end incorporates a shortened check valve from a male Foster fitting.... It is held closed by a 1" long 0.020" wire spring sitting against a washer located on a shoulder inside the valve front.... I opened up the last coil of the spring to 5/16" so that it can't get past the washer.... Behind the washer is a QB hammer spring, and the location of the shoulder for the washer was chosen to provide the same force holding the valve poppet closed as on a stock Disco valve.... This was achieved by drilling progressively deeper, assembling the valve, and pushing the stem against the stem of a stock valve until they opened at the same time....

The rear (spring) chamber of the valve front is drilled to 3/8" and the front portion to 5/16".... and then a 1/8" NPT hole intersects the front part for a gauge mount.... For initial tests, I will be using a simple male to female 1/8" NPT extension to mount the 1500 psi gauge, which is located in the stock position for a Disco.... At the moment the rear portion of the valve is dead stock....The total volume as built, including the gauge mount, is ~5.5 cc.... That gives me a bore/valve volume ratio of 3:1 with the MRod barrel.... I plan on boring out the rear portion of the valve to unshroud the poppet and increase the volume by nearly 1 cc.... and to drill the throat to 1/4", thin the stem, and increase the port diameter and angle it for improved flow.... but I think I'll try it with the stock valve initially.... Then comes the fun part....



I have made a gauge mount that connects a 16 gr. thread CO2 cartridge to the valve.... It has an internal volume of just over 20cc, and is rated at 250 bar MSWP, >500 bar burst.... It installs into the mount with 3/8"-24 threads which will be either glued or soldered to seal them.... The cartridge just clears the main tube, and will live in a hollow in the stock just in front of the gauge.... It will be invisible when the pump arm is closed.... With the cartridge installed, the total valve volume will be ~27cc, and the gun would, of course, be tuned to retain air and operate like a PCP with an onboard pump to top up the pressure between shots....

I'm pretty excited to see all the parts coming together for this project.... It won't be too long before I can install the parts in the tube and try out the pump.... I still have other parts to make before the gun can be fired, however....

Bob

[rsterne]

I assembled the pump today, using a stock Disco rear valve half.... and a conventional gauge mount.... Valve volume is ~ 5.5 cc and the swept volume is ~ 54 cc.... The following pressures were achieved:

5 pumps - 600 psi
10 pumps - 1100 psi
15 pumps - 1500 psi

After sitting for an hour, the pressure is still 1450 psi, which I would imagine is the effect of cooling.... I can't comment on the pumping effort as I had no stock on the gun, and it's not exactly easy to pump sitting upside down across my knees and pushing on the 1/4" wide metal pump linkage.... I would say the effort is significant but do-able.... and ergonomically shouldn't be a problem once the stock is fitted....

I'm quite pleased with these results.... I have to make the barrel band and the transfer port and a bolt, so initial testing isn't too far away.... I'm not expecting too much with the stock valve and porting, but need those results as a baseline.... I also have to modify the stock before any serious testing can be done....

Bob

[rsterne]

Well I couldn't wait any longer.... I made a new bolt and transfer port and threw the gun together for a few shots.... no stock, harder than heck on the hand to pump and all.... First a string with the Disco hammer spring set just off coil bind.... varying the pump numbers and recording the pressure and velocity.... I used 25.4 gr. JSB Kings....



The gun started retaining air at over 10 pumps (1100 psi).... It delivered 490 fps (14 FPE) on 6 pumps (700 psi) and 590 fps (20 FPE) on 10 pumps (1100 psi) and peaked out at 667 fps (25 FPE) at 15 pumps (1500 psi).... To run the gun as a pump and dump, without changing to a heavier hammer spring, the most I can get is ~590 fps with 10 pumps to refill to 1100 psi....

I then started playing with the RVA (power adjuster), to find out how that affected the velocity and the pressure remaining after each shot.... It turns out that I can back the adjuster out about 3 turns and only lose 10 fps but the pressure drop for the shot is about 150 psi less, which means a big gain in efficiency....



At 7 turns out, I can get two shots at 590 fps (20 FPE), and still have ~350 psi remaining.... It takes 12 pumps to refill after the two shots.... or about 5-6 pumps after the first shot.... Compare that with the pump and dump, same power but 10 (easier) pumps to refill.... At 3 turns out, I get one shot at 656 fps and 7-8 pumps to refill to 1500.... with having a gauge, it is a simple matter to know when to stop pumping....

Now remember that these are preliminary results, using a stock Disco rear valve half, stock barrel port, and matching transfer port (all 0.150").... The next step is to bore out the valve and port everything out to 3/16".... While I'm at it, I have to get the stock done, this thing is killing my hand pushing on the 1/4" wide pump lever....

Bob

[AirGunEric]

I like the idea of retained air so as to reduce the number of pumps required for a second (or third shot).

Any idea when you will have it fully assembled for us voyeurs to see the final configuration with the gauge and caplet installed?

[rsterne]

I made the barrel band and have cut the stock and mounted the rear part.... I have to slot the foregrip for the linkage and install it.... Then comes boring and porting the valve.... and more testing.... At that point, you will see photos (with gauge).... I'll give you a hint, the overall length is only 35.5".... The extra reservoir comes after all that.... so it will be a while yet.... You won't see it anyway, it will be hidden inside the stock....

Bob

[rsterne]

Got the stock modified and fitted today....



Good ergonomics for pumping, total weight 5 lbs. 6 oz., LOA 35.5"....

Bob

[Dukemeister]

Holy crap! Another masterpiece.
Bob, you are outstandingly amazing!

[rsterne]

The last couple of days I got a chance to bore out the valve, enlarge all the ports, and add a male Foster to the gun.... Here are the photos.... First up the shortened Disco tube with the slot cut, holes for the front block, and a new hole on the left side just behind the gauge hole for the male Foster.... Also shown is the 392 linkage mounted into the front section of the Disco stock....



The next photo shows the larger ports in the barrel, breech, and the new transfer port, made from 5/16" teflon.... All the ports are 3/16" ID, up from about 0.150" stock.... Also shown is the new bolt.... Note there is no O-ring groove, it lives in the end of the MRod barrel.... Also in the photo is the piston, which now has a packing groove full of string which will be soaked in oil to keep a thin film of oil in the pump tube.... similar to the felt used in the old Crosman 140s.... I deepened the O-ring groove a few thou and changed to a 90 Durometer O-ring as the previous 70D one was failing from extruding into the slot....



The next photo shows the guts of the valve.... The Disco rear half has been modded with a 1/4" throat, it is bored out to 5/8" ID, and the exhaust port is 3/16" diameter and angled 20* and smoothed with a Dremel.... The stem has been reduced to 0.109" and shortened 0.060" to increase hammer travel.... The extended front end which incorporates a check valve and a gauge mount has now been drilled and tapped on the left side to 1/8" NPT for a male Foster fitting.... The check valve is shortened from a Foster fitting, with a light return spring.... The valve spring is from a local industrial supply, and between the two is a thinwall aluminum sleeve that acts as the spring seats and has a window in the side to clear the valve in the Foster fitting.... Also shown is the 1500 psi gauge with it's extension, the male Foster QD fitting, and the custom front block / barrel band assembly....



The final photo shows the assembled valve.... The Foster isn't necessary until I add the reservoir onto the gauge mount.... but I don't really feel like pumping this thing 70-75 times to fill it, so now I can fill it from a SCUBA tank or a stirrup pump.... I tried the Foster today, and it works great, but you sure have to be careful filling a 6.1 cc valve from a SCUBA tank.... I only did it once....



I put everything together just before dinner and let it sit to check for leaks and there weren't any.... I then remembered I forgot to shorten the back of the bolt to increase the loading room, so had to pull off the breech and do that before I could do any testing.... Results to follow....

Bob