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://www.canadianairguns.com/showt...w-QB-78-on-HPA
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