.25 cal Disco Pumper Carbine

[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]

It was time today to fix a couple of things that cropped up during building and testing.... I had noticed that there was a problem with the hammer dragging during testing.... If I squeezed off a shot instead of "yanking" it and holding the trigger back, I lost about 40 fps and the velocity was very inconsistent.... For that reason, all the previous testing was done with the "yank and hold" technique.... I pulled out the hammer and found exactly what I had expected.... a dent.... circled in the photo below....



Remember I shortened the valve stem 0.060".... Well that extra hammer travel was allowing the hammer to impact the back side of the sear, if it was still sticking up like it would be if you just squeezed off a shot.... I've seen this before, that is why I knew what was going on.... I had another Disco hammer handy, that had been lightened 20% by removing the front half of the rear portion of the full diameter part, making the "waist" longer.... Therefore, the section where the impact could occur was no longer there.... Since I wasn't going after maximum power, and I had lots of RVA adjustment to play with, I simply dropped in the lighter hammer and that fixed the problem.... You could squeeze off a shot and get exactly the same velocity as if you "yanked" on the trigger....

The other small issue I addressed was the "clacking" of the pump arm lever against the tube on closing.... I took a small piece of 1/8" thick UHMW Polyethylene about 3/4" square and forced it into the end of the pump arm flush with the end and down all the way inside it.... It stuck up about 1/16" above the arm and forestock so that the plastic is what hits the tube.... I drilled through it for the rear pin to retain it, reassembled the forestock, and voila, much quieter on closing.... BTW, somebody asked about holding onto the scope while pumping.... I find it quite comfortable if I put my hand right over the front scope ring.... Holding the gun there I'm pushing straight down right at the ring, and in all the testing I did today I didn't notice any looseness or POI shift.... I don't think it's a problem....

Now it was time to see if the new hammer made any difference.... It wasn't a big difference in weight, but the valve should open slightly faster, have a bit less dwell, and close quicker.... I expected to see a slight loss in velocity , but in fact I gained a little bit at maximum preload, with the 25.4 gr. JSB Kings hitting 845 fps (40.3 FPE).... I guess the faster opening helped a bit, or possibly the stock hammer was just catching the sear before I could yank it out of the way before.... In any case, I'm now getting over 40 FPE with the Kings for one shot.... In addition, each shot is using slightly less air, so the second shots are a bit faster.... I get two shots of equal velocity with the RVA only 3 turns out, and consequently they are more powerful, at 796 fps (35.7 FPE) instead of 780 fps at 3.6 turns out previously....

Now it was time to get to the serious setting up of the gun.... I'll spare you all the tribulations, but I eventually settled on a 1400 psi fill and 6 turns out on the RVA.... Here are the results....



The 4th shot is included just for interest, although it could be used for a "coup de gras" if necessary.... The exciting part is that I get 3 shots within a 3% ES, averaging 31 FPE.... I was only testing indoors, but those three shots kept going into one ragged hole.... With a 3% ES, I wouldn't expect any significant POI shift out to about 70-75 yards between those three shots.... I also pumped the gun up after 1, 2, 3, and even 4 shots, and recorded the number of pumps, as follows:



It takes 12 pumps to refill after the first shot, another 15 for the second shot, and 18 each for the 3rd and 4th shots.... The pressure drop per shot is 190 psi for the first shot, 230 for the second, 320 for the 3rd, and 410 psi for the 4th shot.... That gives efficiencies of 1.39 FPE/CI for the first shot, 1.23 for the 2nd shot, 0.83 for the 3rd shot, and a dismal 0.49 FPE/CI for the 4th shot, which only leaves 250 psi in the gun.... Like I said, the 4th shot is for emergencies only, at close range, to finish off a wounded animal.... The average efficiency over the first 2 shots is 1.31 FPE/CI, and over the first 3 shots is 1.10 FPE/CI.... I'm pretty darned pleased with that.... It sounds like there is a bit of hammer bounce on the 3rd shot, and it is definitely present on the 4th shot.... but with the good efficiency on the first 2 shots it really can't be an issue....

I think I'll stop here for a while, and do some work on a PCP that has been calling my name.... In conclusion, with 25.4 gr JSB Kings, I have 3 tunes as follows:

Max. @ 1500 psi.... 1 shot at 845 fps.... 40.3 FPE
3 turns out @ 1500 psi.... 2 shots at 796 fps.... 35.7 FPE
6 turns out at 1400 psi.... 3 shots at 739 fps.... 30.8 FPE

One other thing.... When filling from a SCUBA tank, I would strongly suggest using a regulator set to just below where you want to fill (I'm using 1350 psi).... I damaged one gauge by overfilling the small valve by just 200 psi.... It wasn't a safety issue, just a PITA and cost me a gauge.... The regulator prevents that from happening again....

Call it a day, I'm done like dinner....

Bob

[rsterne]

I machined the Disco tube today.... cut it to length and squared the end and then machined the slot for the pump linkage....



The tube is 1/2" behind the front of the MRod barrel, and the 3/8" barrel band will fit over the stub of the aluminum front plug.... leaving the front of the barrel band 1/8" behind the muzzle.... The pivot pin is 5/8" behind the front of the tube, and the slot is 8.2" long.... The stroke is 7.6" and the piston is 6.7" from the link pin to the compression face.... That leaves the compression face 0.3" ahead of the front of the gauge hole, which leaves room for the O-ring to seal the valve....

The next step will be to make the extended front end for the valve incorporating the gauge mount and check valve.... Incidently, the front of the Disco stock comes to just behind where the spring sticks through the lever (the bump in the photo).... It will be cut just in front of the gauge, with the forestock becoming the pump handle.... Should work perfectly....

Bob

[jgoodz420]

Looking good Bob

This thing should be a beast can't wait to see the numbers you get out of it...

[rsterne]

The swept volume is 54 cc.... My Uber-Pumpers with the 2100/2200 tube are 24 cc.... so over twice the volume per pump.... over 3 times what a 13XX pump moves.... The down side is that at 1500 psi the load on the piston is 650 lbs. instead of 450 lbs.... I am not aware of anyone using a metal Flat-Top Piston with a 392 linkage before.... The 392 valve can't be machined flat without exposing the check valve, so they just shim the pump cup to reduce headspace.... There is a lot of new territory being explored here.... or at least if it's been done before it is not documented (that I can find).... If the linkage buckles, I'll get a billet linkage from Mac1 Airguns.... I already have one of their exteneded ones, with a 3" longer lever, for the followup rifle version with internal reservoir for retained air (ie PCP with onboard pump).... and I just got the tube and valve from Eric today.... Pretty funny, haven't even finished this one, and I already have plans for the next one....

Bob

[jgoodz420]

The swept volume is 54 cc.... My Uber-Pumpers with the 2100/2200 tube are 24 cc.... so over twice the volume per pump.... over 3 times what a 13XX pump moves.... The down side is that at 1500 psi the load on the piston is 650 lbs. instead of 450 lbs.... I am not aware of anyone using a metal Flat-Top Piston with a 392 linkage before.... The 392 valve can't be machined flat without exposing the check valve, so they just shim the pump cup to reduce headspace.... There is a lot of new territory being explored here.... or at least if it's been done before it is not documented (that I can find).... If the linkage buckles, I'll get a billet linkage from Mac1 Airguns.... I already have one of their exteneded ones, with a 3" longer lever, for the followup rifle version with internal reservoir for retained air (ie PCP with onboard pump).... and I just got the tube and valve from Eric today.... Pretty funny, haven't even finished this one, and I already have plans for the next one....

Bob

WOW!!! that is a serious amount of air


Maybe making a new head for the valve altogether and moving the check back slightly would allow for a bit more per stroke?

I do the do same thing while making/drawing things up, get half way through something, and start thinking "Oh, I'll need that too" or "I should make three versions of this and see what difference they make"

The on-board reservoir sounds pretty awesome as well, ill be watching it unfold closely

[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]

I'm making a whole new front for the Disco valve, which of course has no check.... I have to have an O-ring between the piston and the gauge port, and there is only 0.3" of room there.... However, that means the valve will be as long as the Disco valve and gauge mounting block combined.... and over twice the volume.... My goal is at least 5 cc.... A 13XX valve is 1.6 cc, and a 392 valve is about 4 cc....

Bob

[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