.25 cal Disco Pumper Carbine

[rsterne]

I started work on the internal parts for my .25 cal Disco Pumper today.... It will use the MRod barrel and Disco breech I mated up the other day, plus an extended Disco valve with a check valve built in just ahead of the gauge port.... The valve extension will include a gauge mount for a 1500 psi gauge.... Here is the piston and the front mounting block to fit a Benjamin 392 linkage into the Disco....



There will be a barrel band mounted on the stub of the front block which projects 3/8" ahead of the shortened Disco tube.... The barrel band will be 1/8" behind the muzzle of the MRod barrel....



The next step is to cut the Disco tube to length, and machine an 8" slot in the bottom for the linkage.... and drill the hole for the front pivot pin....

Bob

[FRANK]

If this works out great you will have to make plans and let us copy them for free,
can't wait to see your results.

[rsterne]

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

Bob

[rsterne]

With all due respect, Frank, perhaps you should keep notes.... When I want plans I often just add dimensions to a photo.... Detailed drawings are important when you need to reproduce something.... Since virtually all of my projects are "one-off" designed for my own entertainment, and not to make money.... why would I take the time to make detailed drawings and then "give them away".... Sounds like a government "make-work" project.... except they get paid for it....

Once again, here is how it works, for me.... I do my own thing.... I don't make parts or guns for anyone else.... I try and document what I do and share it with others, and I do that free of charge.... Beyond that, you are welcome to borrow any ideas you see, and make any parts that are copies of mine.... but I think it's a bit much to ask me to do drawings as well....

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]

Today I installed the reservoir.... It is made from a threaded 16 gr. CO2 cartridge mounted on the side of a gauge mount I made for this purpose.... With the mount, it adds nearly 21 cc to the volume of the valve, so the total is now 27 cc....



I modified the stock so that when it is installed it hides the reservoir completely.... I'm very pleased with how it turned out.... You can see the Foster fitting for filling it from a SCUBA tank or stirrup pump, on the side of tjhe tube, just above the gauge.... I'm really glad I added that.... For the initial testing I never even used the pump....



I also installed a Fitco 2-7 x 32 AO scope.... For pumping you can either hold the scope, or the neck of the stock, and I find tucking the butt under my arm works well....



I couldn't stand the suspense, so I cranked the RVA to the max, setting the Disco hammer spring just shy of coil bind, filled the gun to 1500 psi from my tank, loaded a 25.4 gr. JSB King and fired the first shot through the Chrony.... How does 836 fps, for 39.4 FPE sound?.... Yeah, I was pretty pleased too !!! .... Here is a graph of the velocity vs. RVA setting for the first and second shots.... Obviously there is no way the gun can dump 27 cc.... the pressure available for the second shot was 1000 psi with the RVA maxed out, and gradually increased to about 1370 psi with it 10 turns out.... At 2 turns out, I get 820 fps (38 FPE) on 380 psi pressure drop....



You can see that the curves cross over at about 3.6 turns out on the preload, and at that setting you get two shots of 780 fps, which is 34.3 FPE.... and the pressure after the two shots is about 960 psi.... That works out to about 1.10 FPE/CI, which I am very pleased with.... By backing off the RVA more and reducing the fill pressure, you can reduce that power level.... That will mean fewer, easier pumps to refill.... Filling to 1250 psi, with the RVA about 6 turns out, gives two shots of 30 FPE and requires refilling from about 750 psi.... I shot a series of shot strings at each RVA setting, and graphed all shots that were within 10% of the maximum for each string.... Here are those results....



You can see that as the hammer spring preload is reduced, you get more shots, a flatter curve, and the peak velocity moves to a lower pressure.... In addition, you will notice that the shots at the high pressure end get dropped from the string due to partial valve lock.... These strings aren't ideal for some of the RVA settings because I always started at 1500 psi.... In fact, that is not the way you would use the gun at lower power levels, you would fill to a lower pressure.... As an example, look at the blue curve for 6 turns out on the RVA.... It uses about 250 psi per shot, and the peak of the curve is at ~1150 psi.... If you filled to about 1400, you could get three shots of about 720, 745, and 720 fps, ending at about 650 psi.... That is very close to 3 shots of 30 FPE with an ES of about 3%.... getting 90 FPE on about 85 CI of air, or 1.06 FPE/CI.... That might be the nearly perfect setting for hunting, and you would just top up after 1, 2, or 3 shots depending on what you needed.... The better your first shot, the less pumping you have to do....

I plan on doing a bit more playing around before deciding on a final tune.... but I think I can call this project an unqualified success at this point.... I will also be keeping track of the number of pumps to refill, today I was lazy, using the SCUBA tank.... Oh, I nearly forgot, I tried some other pellets with the RVA maxed out.... Here are the results....

H&N FTT 19.3 gr.... 911 fps.... 35.6 FPE
JSB King 25.4 gr.... 836 fps.... 39.4 FPE
H&N Baracuda 30.9 gr.... 779 fps.... 41.6 FPE
EunJin Dome 35.2 gr.... 737 fps.... 42.5 FPE
EunJin Pointed 41.7 gr.... 678 fps.... 42.6 FPE

So there you have it, a .25 cal pumper that shoots over 42 FPE on 1500 psi.... built with mostly Disco parts, an MRod barrel, and a 392 pump linkage.... with a few custom parts thrown in.... and Eric, the answer, as you know.... is no....

Bob

[FRANK]

well I love your idea it's thinking out side the box and if it works great I'm going to copy your palns if you let me and guide me to build mine.

[Fastair]

Nice to see you are on to another project.
I liked what you did with your uber pumper and you shared all your ideas with everyone.
I will be following this project too and suggest that Frank print off all the info you post.

[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

[rsterne]

I decided that I liked the way this gun responded to a lighter hammer, so I had another one kicking around that weighed only 40 gr. (30% lighter than a Disco hammer).... I installed it, and the gun responded in exactly the way it should.... I lost about 8 fps at the top end (with greater efficiency) with the Kings, and I needed one more turn of preload on the RVA to get back to my "3-shots at 31 FPE" tune.... and they are a bit more consistent, with a whisker less air use.... I got one additional benefit.... With the RVA at maximum, filled to 1500 psi, and shooting 42 gr. EunJin Points, I get two shots at 655 fps (40 FPE) that are only 2 fps apart.... Should I wish to tackle something larger and that requires extra penetration, that combination would be just the ticket....

Bob