Extended Valve Front End for a SAFE 2260 HPA Conversion
After refining my "Uber-Carbine" pumper, and then spending a lot of time working on my Disco and my 22XX PCP project (which ended up regulated), I decided to see what I could come up with in a PCP doing the minimum changes to a 2260. Modifying a 2240 or a 2250 carries other problems related to PAL and non-PAL and the possibility of ending up with a restricted firearm in Canada, so I decided to start with a 2260 since they are PAL to start with, and a rifle, no legal problems in modifying them here.
There has been a lot of interest lately in doing these PCP conversions, and safety is always a concern, especially since the 22XX main tubes were never designed by Crosman to be pressurized. When I looked up the specifications for mild steel tubing of the same dimensions, I found a safe working pressure of 2000 psi. The QB series (based on a previous Crosman design) DO have factory accessories which allow you to bulk fill them, and it is well documented that CO2 pressures can reach 1900 psi at 120°F, even when the reservoir is not overfilled. The QB uses the same size tubing as the 22XXs do. Many 22XX HPA conversions run 1400 psi, and it is common for such set-ups to have an 1800 psi burst disc. It seems to be the consensus of those who mod these guns that pressures up to about 1800 psi are not unreasonable, but one thing worries me - the Roll Stamp.
The main tube of the Disco has the information painted on, while the 22XXs use a process where the information imprinted on the side of the gun is done so by pressing the letters into the side of the main tube by a method called "roll stamping". I have seen several examples of 13XX and 22XX tubes where the letters were so deeply stamped that you could see imperfections in the inside of the tube when you look at a bright light through it. This just HAS to weaken the tube! The roll stamp on the 2260 goes about 1.5" forward of the front of the valve, so I decided to only pressurize that portion of the tube forward of the roll stamp. Since I would be using a Discovery valve, and I wished to have a gauge installed similar to that in the Disco.... it didn't take rocket science to figure out that making a new front end for the Disco valve incorporating a gauge mount and moving the O-rings forward of the roll stamp cured all the problems at one go. I discussed the idea with Joe at Muzzle Mack, and found out that we were on a “parallel course”, and that he was simultaneously working on exactly the same idea for his 2260 Wannabee Disco Carbine. Great minds think alike – fools – oh forget that part, we know we’re great !!!
I decided to start with a stock Disco valve, as the performance I am after (and the very limited air reservoir) won't require improvements. Since the 2260 breech screw is further forward than the one on a Disco, I machined a small notch in the top of the valve behind the transfer port to clear the 4-48 screw. Optionally, you could use a Disco steel breech and a Disco hammer. Joe chose a third option, which was starting with the brass 2260 rear valve half, which he modified to mount in the same manner as a Disco valve. I drilled the 2260 tube with three holes for the heads of the Disco valve mounting screws. These are in the same location as the transfer port, and 90 degrees apart around the tube. I measured the gauge location in a Disco and decided to use the same location for the gauge port in the 2260 tube so that a Disco stock would fit perfectly, should I ever want to do that. Joe did the same, but will also offer a version without the gauge port because of thread orientation issues.
Now it was time to make the valve extension. I machined the rear threaded part to be identical to the Disco valve front end so that I could use the original O-ring and spring - same threads, same spring seat - so the valve is identical inside to the original Disco part. I machined in a notch to clear the front trigger screw. The extension is drilled 1/4" right through, and the front end is counter bored for a bit extra volume. All three O-rings are the same as on the Disco valve, you just move the O-rings over to the new front end. Both outer O-rings are ahead of the front of the roll stamp on the 2260 tube, hence the roll stamp area is NOT pressurized. One word of caution here - if you had to replace the rear portion of the valve on a front end with a gauge port, it is extremely unlikely that the notch for the trigger screw and the gauge port would line up because of thread "clocking".
I used a B&A Bulk Fill adapter equipped with a male Foster fitting to fill the tube. You could also use a Disco fill fitting. Since I will be using a hand pump ONLY, I have not bothered to fit a 1.8K burst disc, but if I was going to fill from a SCUBA tank, even a regulated one, I would add one between the trigger screw notch and the gauge port for safety. Joe will be offering this as an option. I measured the length of the reservoir, and it comes out to just under 9" for a volume of about 65 cc, roughly half that of a Disco. This means you can expect about 10-15 shots at Disco velocities using a Discovery hammer spring. You can also use the stock 2260 hammer spring and get lower velocities at lower fill pressures (but still better than on CO2). I think it is entirely possible to make a 12 FPE version for England, and maybe even a non-PAL (under 500 fps) version for Canada. This may require a smaller Transfer Port.
Above is a photo of my finished Carbine. Considering it only has a 14” barrel, I was delighted by the performance. With 14.3 gr. JSB Express pellets using a dead stock Disco valve back end and a Disco hammer spring, when I adjusted the hammer preload to produce a nice 10 shot string, I got a high of 840 fps and a low of 800 with the average being 825 fps which is 21.6 FPE. This is virtually the same performance as my Disco had, although it could get 24 shots set up for that velocity with its 24” barrel. I could also dial it down to get 15 shots averaging 735 fps or 20 shots averaging 655 fps. By changing to 18.0 gr. Crow Magnums, with a bit of readjusting, I was getting 14 shots averaging 20 FPE, with a total velocity spread of only 5%. My thanks to Joe for his collaboration on this exciting project.
Bob Sterne
Coalmont, BC