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rsterne · Moderator

I decided to build a new 2260 HPA using a similar tank setup as I did for the last QB 78 I built in this sister thread.... http://airgunhome.com/agforum/viewtopic.php?t=7992

I'm using the reversed tank block idea which I built my Hayabusa around, where the tank is underneath the main tube and ends up being used as the forestock.... This results in the 2260 main tube becoming a very large plenum which stores regulated air, and with suitable valve modifications that air keeps the pressure up at the valve seat during the shot cycle.... I have found in past builds that 1 cc of air available for each FPE you want is a good number to keep up the efficiency.... In this build, like the QB 78, I have kept the full volume of the tube, which will be about 60 cc.... That will be plenty for a .25 cal, which is the eventual plan for this rifle.... Initially, as I have done with the QB, the gun will be tested as a .22 cal.... and with the two guns being so similar in concept, a "head-to-head" showdown is in the cards....

The tank block is virtually identical to the one I made for the QB, with the exception of the attachment to the main tube.... Whereas the QB has an external collar that threads onto the tube, the 2260 uses internal thread for the original CO2 cap.... The threads are plenty strong enough, but you have to provide a seal inboard of the threaded portion where the tube is full wall thickness for strength.... In addition, you have to allow for "thread clocking" which means that every end cap will tighten up in a different rotation compared to every main tube.... Below is the solution I came up with....

I machined a 1/2" long collar to fit the inside of the tube, drilled to fit over a 3/8" NF bolt, with an O-ring seal at the mid-point of the outer surface.... Then I cut a recess in one end of the collar for a 3/8 x 1/2" O-ring.... I drilled out the CO2 cap to 3/8", and machined the outer face flat, again with a recess for another 3/8" x 1/2" O-ring.... They fit over a 2" long bolt with a 1/8" hole through the middle of it....

The bolt threads into the tank block, and once tight, the O-rings seal it up.... In order to look after the orientation problem, you simply thread the CO2 collar into the end of the tube and mark it on the bottom.... You position the tank block in line with that mark, and tighten the bolt.... Then when you thread the assembly into the main tube, the block stops pointing straight down.... The 10-32 Low profile SHCS with O-ring provides a seal for the internal air passage in the tank block that connects the regulator to the hollow bolt.... I machined a 3-slot Picatinny Rail into the bottom of the tank block for a bipod, sling swivel, etc....

I think this is a perfectly workable solution that avoids having to shorten and drill the end of the main tube to attach a tank block.... In addition, it allows the full length of the 2260 tube to be used.... That means that a 9 oz. CO2 bottle will fit, and can be removed, and still leave ample clearance between the tank and the trigger group.... I built the tank block with enough offset to allow the slightly over 2" CO2 tanks to clear the main tube.... although I will be using a 2" diameter HPA tank.... initially a 13 CI regulated to 1600 psi, although an 18 CI tank will fit.... This reversed tank arrangement can be used with either a shortened 2260 or Disco stock and trigger group.... or the 2240/50 trigger group and a skeleton stock.... I'll be using the 2260 trigger group (with lighter trigger spring and 3-screw mod) and a shortened wooden stock....

The next step is to modify a Disco valve and secure it into a 2260 tube.... so stay tuned....

Bob

robert w · Senior Member

NOW this is what i like to see. im not into your gun but i like to see what you can do with tools . as for air guns im a springer guy but not blind to others . and all i can say is thats some nice machine work there

Sholo · Veteran Member

Awesome work as usual, Bob! Ingenious idea for the tank block - tube attachment Thumb Up!

Hey Robert W, hang on for a few more of Bob's posts here, he'll astound you at what he can do. Makes you wonder if he is 100% human at times! Shocked

Todd

rsterne · Moderator

I worked on modding the valve today, and drilled the 2260 tube to secure it.... Rather than use the Disco retaining screws, which are of unknown quality, I used some 1/4" long 8-32 low profile SHCSs which are 180,000 psi tensile strength.... I had to shorten them to 3/16" to fit the valve.... Instead of drilling the bottom hole in the 2260 tube out to 5/16", this allowed me to leave it stock, as it was a perfect fit for the screw head.... I drilled matching holes in the sides of the tube using a size "I" letter drill (0.272").... I'm very pleased with the results, and the extra safety margin provided by the high-tensile screws.... They now have a 3:1 safety margin at 3000 psi and are no longer the "weak link" in converting a 2260 tube to a PCP....

I made one of my extended valve front ends to cover the roll-stamped area of the 2260 tube, eliminating that safety concern as well.... Although I plan to run 1600 psi on the regulator, if I decide to run higher pressures I no longer have to worry about running a 1.8K burst disc as there is over a 2:1 safety margin on all components at 3000 psi in the event of a regulator failure.... Once again, I worked on one end of the valve spring, forcing it over first the shank of a 17/64" drill, tapping on it with a hammer, and then switching to a 9/32" drill bit and repeating.... Once I had the ID of the spring increased to 9/32", I tempered it in an oven at 450 degrees for 1/2 hour to relieve the stresses, and drilled out the spring seat to 9/32 for increased flow into the valve....

In addition, the valve stem is slimmed to 0.112" and the valve throat is drilled out to 0.234".... The exhaust port is angled 20 degrees and smoothed out with a Dremel so that the smallest point is at the exit, where it measures 0.166", the same diameter as the transfer port.... The valve is bored out inside for extra volume, but mostly to ease the flow past the head of the valve poppet.... In addition, the threads are shortened to 0.3" on both halves.... The front end of the extended valve is drilled to 1/2" back to where the milled flat is for the trigger screw, and then behind that it is about 3/8" back to the valve seat....

This completes the pressure portion of the gun, and I will likely assemble it and set it aside to check for leaks as I work on the hammer, spring, and RVA group.... plus shortening the stock to miss the HPA tank....

Bob

rsterne · Moderator

Well, this afternoon went really well, and I had the gun assembled and shooting before dinner.... I "temporarily" used a Disco hammer and hammer spring, but to my surprise I probably won't need to change it.... First of all, here is the gun....

I cranked the hammer spring preload to just shy of coil bind, and right out of the blocks, this gun outperformed the QB 78 I did recently which used the same air tank, regulator pressure, and overall concept.... I tried several hammer spring settings, plus I tried a heavier hammer spring (made from 0.040" wire).... Here are the results....

At maximum, this gun shoots the 18.1 gr. JSB Exact Heavy pellets at about 980 fps, which is about 40 fps faster than the best the QB did.... It exhibits the same huge increase in air consumption when you try and get the last few fps of velocity.... but you don't have to dial it back as much as the QB to get efficiencies in the 1.0 FPE/CI range.... it does that while producing 3 FPE more than the QB.... I got 23 shots at 38.5 FPE, but I was able to get 37.7 FPE at 40 shots, which was better than the QB did when set for the same number of shots at 34.7 FPE.... By the time you dialed the gun down to 33 FPE, it was giving about 65 shots, at identical efficiency as the QB at the same power level, 1.30 FPE/CI....

I cranked the hammer back up to maximum and shot my collection of heavier pellets, with the following results....

18.1 gr. JSB Heavies - 978 fps - 38.5 FPE
21.0 gr. H&N Baracudas - 943 fps - 41.5 FPE
25.3 gr. JSB Monsters - 889 fps - 44.4 FPE
28.4 gr. EunJin Domes - 856 fps - 46.2 FPE
32.1 gr. EunJin Points - 813 fps - 47.1 FPE

The maximum power level reached with the EunJins is 5 FPE more that the best achieved by my QB78.... and I'm delighted at the power of this gun, considering it's only operating at 1600 psi....

The only problem I found during initial testing was the occasional low velocity shot.... which I'm sure is the hammer hanging up slightly.... I'm not happy with the way I have the trigger adjusted, and the second stage is too light.... plus I think I may well grind a slot in the bottom of the hammer so that once it is released by the sear, the sear won't touch it again.... Other than that one minor gremlin, there is very little to do on this gun in .22 cal, other than try it at different velocity settings and choose a final tune.... That will have to wait until the snow melts and I can get out to the range....

All in all, this gun and the QB 78 came out pretty close in performance.... They both will give 60-65 shots on a fill at 33 FPE, shooting 18.1 gr. JSBs at just over 900 fps.... If you go for broke, then the 2260 based gun comes out on top, both in power, and in efficiency at the higher power levels.... but the difference is not that large, so which gun is "better" is largely a matter of personal preference.... It will be very interesting to see how they compare when they are converted to .25 cal.... I'm really looking forward to that....

Bob

rsterne · Moderator

I pulled apart the gun today to determine what the problem was with the low velocity shots.... It turns out that by shortening the valve stem 0.080" the rear shoulder on the hammer was sometimes hitting the back of the sear on firing.... I could have ground the shoulder back a bit further, but I wanted to make a new hammer anyways so that I have the option of running a bigger hammer spring when I go to .25 cal.... I made a new hammer that is the same diameter and length as stock, but I drilled an 11/32" hole for the spring instead of 5/16" so that I can fit a QB hammer spring.... I also drilled and tapped the bottom of the spring seat to 10-32 so that I can fit either a spring guide or a shaft to make it rear cocking.... I moved the cocking pin back 0.060" to allow more travel before it hits the front of the slot, since the valve stem is shorter.... I milled a slot in the top to miss the 4-48 breech screw should I wish to use a 22XX breech, and it is further back as well.... On the bottom, I milled a slot for the sear.... long enough that it cannot hit when firing, and with a nice taper on the front to ease in recocking.... The top hammer in the photos is the original, with mine below it....

My new hammer weighs 59 grams, 2 gr. heavier than a Disco hammer an 1 gr. lighter than a 22XX.... I repolished the sear where the hammer had been striking it, and reassembled the gun using my heavy hammer spring which is made from 0.040" wire and is 1.75" long.... The gun was immediately more consistent, shot to shot, in fact, it has one of the smallest extremes between high and low shots I have ever had, regardless of hammer spring setting.... I tested the gun with all hammer settings between just shy of coil bind and 6 turns out, with the following results.... Immediately below the results for the 2260 are the results from the QB78 for comparison....

I lost a small amount at the very top end, but the flat shot curve more than makes up for it.... I wanted to set the gun up the same as the QB78 HPA I did and found that 3.5 turns out on the preload gave me virtually identical performance, and the best shot string I have ever had.... I got 60 shots from 3000 psi all the way down to 1300 before the velocity dropped below 900 fps, and the high was 912 fps using 18.1 gr. JSB Exact Heavies.... The average was 906 fps which is 33 FPE, at an efficiency of 1.30 FPE/CI....

Here is a photo of the two completed HPA conversions....

They are alike as two peas in a pod, really.... I got a bit more top end power from the 2260 (about 20 fps), but they both give 60 shots at 33 FPE on a fill at the same efficiency, 1.30 FPE/CI.... If you set them up for about 1.0 FPE/CI, the 2260 shoots about 953 fps while the QB 78 "only" does about 928 fps.... a difference of 25 fps in favour of the Crosman.... I tried to set the two regulators as close as possible to the same pressure, but that difference could actually be just a small pressure difference in the regulators.... I'd pretty much call it a draw.... which is very satisfying because, to me, it means that I managed to wring them both out at 1600 psi.... It will be interesting to see what changing to .25 cal does to the results....

Bob

radar · Silver Status Member

That's a lot of high power shots. Very nice consistancy.
Whats the tank volume and pressure for the last graph with the JSB 18.1 grainers?

yourdaddyjoe · Supporting Member

Thanks Bob, Now I'm going to have to make more 1760/2260 valve extensions Mr. Green

I all ready have the materials cut. No more brass, going to T6 and having them anodized so they aren't so dang heavy.

SMP · Silver Status Member

Woah....That's the nicest string I have ever seen.

yourdaddyjoe · Supporting Member

rsterne · Moderator

That string is from a 13 CI tank (yes, 215 cc, the same volume as an MRod) starting at 3000 psi and shooting, believe it or not, all the way down to 1300 psi (the 61st shot took it under).... The regulator setpoint is 1600 psi, but the gun is shooting a "bell curve" (admittedly a very flat one) from 1600 down to 1300.... It's about as flat a shot string as I've ever seen.... ~1% over 60 shots.... Mr. Green

Bob

yourdaddyjoe · Supporting Member

I wonder what it will do at 1800psi. It's the only 18ci tank I got left.

My 2260 Disco wanabe needs some more mods...

rsterne · Moderator

Probably add about another 3-4 FPE with little efficiency loss.... Should put the EunJins over 50 FPE....

Bob

yourdaddyjoe · Supporting Member

I likey!

rsterne · Moderator

I spent the day fitting a .25 cal 24" long TJ's Barrel Liner that I got from Stan at AeroTec.... I machined the transfer port in the same location as a Crosman barrel and made it fit a poly port, so it's interchangeable with the .22 cal barrel.... it's even the same OD.... I had to make a new bolt as well, and the first few shots across the Chrony showed that it shoots 25.3 gr. JSB Kings at nearly the same velocity as the 18.1 gr. Heavies shot in .22 cal.... Here is how the velocity and efficiency worked out at various hammer spring settings....

I didn't bother testing lower settings, as the whole point of a .25 cal (to me) is power.... At maximum preload, I only got 18 shots, the gun was using over 80 psi per shot, and MAN was it loud.... but it was shooting over 50 FPE with the JSB Kings.... At 1 turn out on the preload, I only lost 8 fps, I picked up 3 shots, and the gun was noticeably quieter.... Another turn out, the shot count increased to 28, and I was still getting 47.5 FPE.... At 3 turns out, the velocity was averging 886 fps (44 FPE), it was probably about half as loud as at maximum power (or less) and I got 40 shots from 3000 psi down to 1300 (300 psi below the setpoint).... The fastest shots (by just a few fps) were two-thirds of the way through the shot string.... and the efficiency at that power level was 1.16 FPE/CI.... This is about the same power I have my .25 cal Disco set up for, but I only get 11 shots....

I tried the heavier pellets at the maximum setting, with these results....

25.3 gr. JSB Kings - 945 fps - 50.2 FPE
30.9 gr. H&N Baracuda - 892 fps - 54.6 FPE
35.3 gr. EunJin Domes - 853 fps - 57.0 FPE
42.4 gr. EunJin Points - 794 fps - 59.4 FPE

So if I don't care about shot count and noise, this gun shoots 50-60 FPE in the .25 cal version.... Knowing that, I detuned it to 2.5 turns out on the hammer spring preload, which gave me an average velocity of 907 fps with the JSB Kings.... and another beautiful shot string....

The gun is using 50 psi per shot to produce 46.2 FPE, and I got 33 shots between 904 and 912 fps (a 1% range).... Even though the setpoint of the regulator is about 1600 psi, I can shoot down to 1300 before the velocity drops below 900 fps.... I can't wait until spring when I get a chance to find out what the accuracy is like....

Bob