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rsterne · Moderator

I've been quietly working on this project since Christmas, and finally got to shoot it today.... Based on a 22XX tube, it has a Discovery valve (exhaust port corner rounded) bolted to a QB79 tank block.... Only the tank block and the valve are pressurized (O-ring between).... but both are bolted into the tube for extra security.... The tank is a 9 oz. CO2 tank, which is rated at 1800 psi.... At the moment it has a 14" barrel in .22 cal.... more to show just how compact it is, and to find out what kind of performance I can get out of a PCP Carbine.... Twisted Evil

I started out at 500 psi to make sure I could take the tank off under pressure (no problem, the QB79 block vents perfectly).... Then shot a few shots at 1000, 1200, more at 1400, even more at 1600, and then a full string at 1800.... Using my Disco pump, it takes 20 pumps per 100 psi, so it's 360 pumps from empty to 1800.... but you only have to do that once.... Rolling Eyes

.... basically you get a shot for every 2 pumps, which I think is pretty cool.... Cool

The first short strings, I only shot down to 98% of the peak (starting) velocity, but the 1600 and 1800 psi strings I shot down to 96% of peak.... That gave the following results, using JSB Espress 14.3 gr pellets:

Starting Pressure; Average Velocity; Number of shots over 98%; Number of shots over 96%

1000 psi; 671 fps; 6 shots
1200 psi; 712 fps; 7 shots
1400 psi; 747 fps; 10 shots
1600 psi; 771 fps; 15 shots; 26 shots
1800 psi; 771 fps; 29 shots; 47 shots

As you can see, pumping to 1800 psi gives nearly 50 shots within a 30 fps range by shooting down to 1350 psi.... It would require about 100 pumps to refill.... If you start at about 1750 psi and shoot down to 1450 you can get over 30 shots within a 20 fps range and it will take about 70 pumps to refill.... Shocked

I'm only just starting to play with this platform.... at the moment it has a stock transfer port (0.140").... so although the barrel port and valve are relieved somewhat, the gun is restricted to resemble a stock Discovery at the moment.... I did try it with a 24" barrel, charged to 1600 psi.... It acted quite differently with the longer barrel, with the shot string starting out at about 805 fps, peaking at 828, and dropping back to 800 at about shot 45 (1150 psi).... This is a flatter string than a stock Discovery, and nearly twice as long.... Shocked

.... My stock Disco (charged to 2000 psi) with the same pellets started at 800 fps, peaked at 838, and fell back to 800 at shot 24.... Since the 24" barrel has about a 50 fps advantage over the 14" barrel, I think I'll do the rest of my development work with the longer barrel.... That will allow direct comparisons with the Discovery, just for giggles.... Laughing

Next step will be a larger transfer port.... and some playing around with the velocity adjuster.... Waiting in the wings is a JDS Airman 13 cu.in. tank with a 1600 psi regulator.... I have my PAL, and the finished product will, of course, be registered.... Twisted Evil

Bob

broommaster2000 · Moderator

You ARE going to put something nice and stock-ish on there aren't you? Mr. Green

I bet the balance is way off now?

Still, a nice set up. If I ever go PCP, I know that there will be plenty of stuff to convert all my CO2 rifles to PCP.

Already thinking of getting a PCP reservoir for the QB 78, or Airmagnum. Mr. Green

rsterne · Moderator

Actually by keeping the tube as short as I did it is very comfortable using the tank as the foregrip.... Even though the tank is aluminum, it is slightly front heavy, but a wooden rear stock would make it balance very well.... When you see the rifle version you will realize it's not as front heavy as it looks.... Wink

Bob

yourdaddyjoe · Supporting Member

Hey Bob,

I built a similar set up for a buddy. It is still a work in progress.

I am doing a kick a$$ fore stock and inserts for the butt stock to balance it out. He has the larger JDR tank and a RJ riser breech.

If you go with the JDR Tank... You will need to change out the foster fitting to a "Myth Low Profile" foster fitting to clear the barrel in order to remove the tank if it still has pressure in it. Or you will be limited to filling the tank while it is on the gun.

With the High rise breech and the low profile Foster... You can swap out tanks between HPA & Co2 at any time... Great for in the field when you run out of HPA air... The QB block has a pressure vent in the top. I am sure you have figured that out all ready...

Even with the Valve and Tank Block butted up against each other... Don't go over 2K psi... The o-ring sandwiched between makes no difference in safety... Unless you turn a physical joint with o-ring and they slip together.

***Edit*** If you have not done so all ready... A 8-32x1/8 set screw through the gas tube on each side parallel with the OE valve screw will secure the valve. (Never mind Duh... Disco valve) ***End Edit***

Also drill & tap the top of the gas tube as far forward as possible and place another set screw in the tank block for 3 points of contact. This will also eleminate the tank from pivoting on the tank block mounting screws.

Use Loctite, Green or Blue to lock down all the securing screws. The kick that HPA has creates a tendency to loosen them all and invite leaks...

Just some suggestions that I hope help...

Oh... one more thing... if you want to go higher in psi... Buy one of the Stainless Steel gas tubes from... TreeRatHunter... Still remember you have to factor in what the valve and block will handle...

Stainless 304 tubing with a 0.065" Wall thickness:
Theoretical Bursting Pressure: 11145 lbs/sq inch
ASME Code suggests a safety factor of 4 - use 2786.25 PSI
Burst pressure based on temps between -20° F and 100° F.
These are typical/average values and are not a guarantee of maximum or minimum values.

Burst Safety Info from:
http://www.onlinemetals.com/merchant.cfm?pid=12931&step=4&showunits=inches&id=312&top_cat=1

Joe

broommaster2000 · Moderator

Alstone · Moderator & Site Supporter

I’m quite impressed with the 22xx conversion Bob, 50 shots at 2 pumps per shot I think is pretty good going. The connection block between the tank and gun is it a bought part or did you make it and fit the disco valve in, I would be interested on how you did that?

Al

yourdaddyjoe · Supporting Member

My buddies loves to have the hammer spring adjuster tight... Less bounce...

AirGunEric · Site Admin

Nice work Bob- now go get yourself a SCUBA tank and stop pumpin' like a 14-year-old with his mom's Victoria's Secret catalog. Razz

rsterne · Moderator

yourdaddyjoe.... Excellent critique and information.... many thanks.... Cool

The Disco valve and the QB block are actually bolted together with a hollow bolt and sealed with an O-ring.... the mounting bolts for the valve and block are redundant.... the tube is NOT pressurized (no O-rings on the valve or block).... If the valve and block ever do break apart, the pressure will vent around the valve and block.... scare the heck out of me.... but as I said, the tube sees no pressure.... Rolling Eyes

Alstone.... Next time I have the valve out I'll photograph how I bolted the valve and block together.... the tank block itself is from a QB79.... Wink

AirGunEric.... Getting some cardio by pumping was part of the price I paid to the better half to get my Disco.... Twisted Evil

Bob

rsterne · Moderator

I drilled out the transfer port today to 0.159".... and shot some strings with various hammer spring settings.... The number of turns "out" is measured from coil bind using a Discovery hammer spring.... All testing was done with JSB Express 14.3 gr pellets and the 24" barrel:

As you can see, at 5 turns out, the 96% shot string starts at about 1700 psi, peaks at about 1350 psi, and goes down to about 1100 psi.... the 4 turn string starts at 1800 psi, peaks at about 1500 psi, and goes down to about 1300 psi.... The 3 turn string is already on the sweet spot at 1800 psi, peaks at about 1650 psi, and goes down to about 1400 psi.... The 2 turn string is starting from the peak velocity at 1800 psi and ends at about 1500 psi.... Here is a summary of the results:

Setting; Average velocity; Energy; Shots within 98%; Shots within 96%; Pumps to refill:

5 turns; 822 fps; 21.5 FPE; 42 shots; 64 shots; 120 pumps (to 1700)
4 turns; 858 fps; 23.4 FPE; 36 shots; 52 shots; 100 pumps (to 1800)
3 turns; 884 fps; 24.8 FPE; 30 shots; 36 shots; 80 pumps (to 1800)
2 turns; 911 fps; 26.4 FPE; 13 shots; 22 shots; 60 pumps (to 1800)

It seems to me that the best setting with this setup would be about 3 1/3 turns out.... I didn't shoot a string that way (yet), but it should start at the beginning of the 98% sweet spot (at about 863 fps), peak at about 880 fps, and produce about 32 shots between those numbers.... with an average velocity of about 872 fps (just over 24 FPE).... If you find the 96% curve acceptable, then the 4 turns out curve is perfect as is.... Cool

I am very pleased with this platform so far.... It is capable of over 900 fps with standard pellets.... or it can shoot more than 50 shots at between 832 and 867 fps.... It looks like with the 1600 psi regulated bottle, I can expect to be able to adjust the velocity between about 800 and 900 fps.... Twisted Evil

Bob

yourdaddyjoe · Supporting Member

Slavia · Moderator

Nice job! The gun is cool, but I especially appreciate the detail that goes into your reporting. Thanks for the effort!

rsterne · Moderator

Just fine tuning today.... drilled the barrel port out to 0.166" (#19 drill) and the transfer port out to 0.161" (#20 drill).... I also extended the bolt probe by 0.150" and shortened the back of the bolt by 0.060" (to allow more room to load pellets with the longer probe).... Those mods didn't affect the velocity any significant amount, but they changed the shape of the curve a bit.... I tested this version with five different pellets: 11.9 gr. RWS Hobby, 14.3 gr. JSB Express, 15.9 gr. JSB Exact, 18.0 gr. JSB Heavy, and 21.6 gr. Kodiak (all weighed).... The pressure curve is close enough to the same for all pellets, so I only show one on the graph.... The tank was pumped to 1800 psi, and I ended the shot string when the velocity dropped to 96% of the maximum for that pellet.... I had the velocity adjuster set 3 turns out from coil bind with the Discovery hammer spring for all testing....

As you can see, the Hobbies had lots of velocity, but a much shorter shot string.... as the pellet weight increased, you got more usuable shots, more FPE per shot, and the total energy in the usuable string increased as well.... Here is the summary:

Pellet; Average velocity; Energy; Shots within 98%; Shots within 96%; Total Energy within 98%

Hobby; 956 fps; 24.2 FPE; 18 shots; 30 shots; 435 FPE
Express; 884 fps; 24.8 FPE; 32 shots; 41 shots; 795 FPE
Exact; 849 fps; 25.5 FPE; 33 shots; 40 shots; 840 FPE
Heavy; 806 fps; 26.0 FPE; 35 shots; 44 shots; 909 FPE
Kodiak; 736 fps; 26.0 FPE; 42 shots; 50 shots; 1092 FPE

My conclusions are that the Hobbies are too light and the Kodiaks are too heavy for this gun.... The Kodiaks have a lovely long shot string, but don't have any more energy per shot than the JSB Heavies and a lot less velocity.... I kind of expected this, but it's nice to have the numbers to look at.... If I were doing a .177 version of this gun, pellets of over 10 grains would probably produced great results for FT work.... Twisted Evil

I think I've pretty well figured out this version.... so the next step is to try the 3000 psi JDS 13 cu.in. tank with the 1600 psi regulator.... It looks like 1600 psi is right in the middle of the sweet spot for all pellets except the Hobbies.... so I guessed right when I bought the regulator, anyway.... Rolling Eyes

Bob

sniper · Silver Status Member

bob.

beautiful thread. just beautiful! Very Happy

i aspire the likings of your brain! Thank-You

rsterne · Moderator

I've been impatiently waiting to get to this stage.... I fitted the JDS Airman 13 cu.in. bottle today.... 3000 psi regulated down to 1600 psi.... It's lighter than the 9 oz. CO2 tank, and the complete gun weighs in at an ultralight 4 1/4 lbs.... Shocked

The gun balances beautifully.... about 2 inches in front of the trigger guard (ie further back than my Disco).... and it's very comfortable using the tank as a foregrip.... Wink

It took 305 pumps to fill the tank to 3000 psi from empty, and the Disco pump worked prefectly.... I let the gun cool for a half hour, and lost about 100 psi, so the string started from 2900 psi.... I shot 20 shots with JSB Express 14.3 gr, and the total variation was only 8 fps.... tons of pressure remaining, so I shot 10 JSB Exact 15.9 gr and then another 5 Express.... Still lots of pressure left, so I did 10 shots with JSB Exact Heavy 18.0 gr, and then another 5 Express.... This brought me up to 50 shots, and I was only down to 1900 psi... I shot three shots groups playing with the velocity adjuster, from coil bind to 8 turns out, and then a final 3 shots back at my initial setting of 3 turns out.... The pressure was down to 1500 psi, but the Express were still shooting within 20 fps of the highest.... This was after a total of 78 shots.... Shocked

The graph above shows the first 60 shots with the 1600 psi regulated setup, adjuster at 3 turns out from coil bind.... I left the curves for the unregulated setup with the same three pellets there for comparison:

JSB Express 14.3 gr averaging 876 fps - 24.4 FPE
JSB Exact 15.9 gr averaging 843 fps - 25.1 FPE
JSB Heavy 18.0 gr aberaging 794 fps - 25.2 FPE

All shots for each pellet were within 1% of each other in velocity.... so the regulator is DEFINITELY to way to go for consistency, compared to the unregulated setup.... The pressure drop in the tank with the adjuster at 3 turns out was almost exactly 20 psi per shot.... so from 3000 psi down to 1600 psi that means a completely flat 70 shot string.... Shocked

The second graph shows what effect the velocity adjuster has.... First of all, note that I can get about another 10 fps by cranking the adjuster in one more turn (to 2 turns out).... but after that there is virtually no increase.... My guess is that it would take a larger valve volume to take advantage of the increased lift/dwell of even heavier hammer strikes.... Basically, the gun is dumping all the available air in the valve at 2 turns out.... As you decrease the hammer spring tension from there, however, the velocity goes down, and the gun get progressively quieter, until it eventually becomes quite a "clipped" pop.... presumably at a much higher shot count.... just the ticket for plinking.... I have no idea how long the shot string might be at the lower velocities, but it would be loonnnggggggggg.... Cool

To say I'm delighted with the results would be an understatement.... I'll be doing a bit of fine tuning, to be sure, maybe take a couple of threads off inside the valve, play with the porting a bit more.... and optimize the velocity adjuster to get the most velocity without shortening the shot string too much.... The gun will get a scope, and possibly a "Packhunter" stock from sniper.... and likely a barrel band at the front of the tank.... but just the way it is , I have a gun that will fling 14.3 gr pellets downrange at 876 fps, and deliver 70 shots from 140 pumps to top the tank back up from 1600 psi to 3000 psi.... Twisted Evil

The unregulated version has its advantages too.... only an 1800 psi fill, shoot it down to 1400 psi, get 30-35 shots within 2% of each other.... and you only have about 70 easy pumps to refill.... In addition, you can change the bottles without removing the barrel, so you can carry a spare, and they are a lot cheaper.... Either version looks like a winner to me.... Cool

Bob