AirBorne Arms Co .25cal Benchrest build

My next build on the bench has been more than 3 years in the planning and design stage.. not because it overly complicated or fancy, in fact its a very simple design, but I had been waiting for JSB to design and manufacture an new heavy weight high BC .25cal Diabolo pellet suitable to my needs.
Some of you know I have always pushed the 18.1gr .22 pellet to the extreme range of its capabilities and made kills just past 200m but have always pondered what a heavier .25 cal could achieve.
The .25cal JSB king is limited by a relatively low BC (in fact its almost the same as the 18gr .22 pellet)
so 200m is about its limit for reasonable accuracy and energy retention.
Well JSB have finally got on board and released a very high BC 34gr heavy .25 cal pellet that has me excited again to push the modern PCP boundaries and capabilities.

Soooo.. I have under way a .25cal Benchrest build that will push the new .34gr JSB pellets past 75ftlbs with the goal of MOA past 300m.
On paper and running the figures through ballistic programs it looks very doable and my early and very crude test platform for this new pellet has landed 5 shot shot just outside of 4 inches at 300yds.

http://www.airgunnation.com/topic/jsb-new-heavy-25-cal-kings-100-yards-half-inch-group-and-normal-kings-group/page/2/

The platform will be running a fast twist BSA barrel a Huma regulator and hold 580cc of on board air.
Although this with primarily be a bench rifle its likely to get plenty of pest control work too so it will be light enough to carry in the field and set up on a hill side some where to pop long range bunnies.

This build started with a hunt for suitable air cylinder materials. It needs a big supply if I want to shoot it un tethered or I will be forever filling it so even 300cc in my std air tubing wouldn’t cut it on this build.
I considered a 500cc Carbon fibre bottle for a moment trying to push past the ugliness of the “supersoaker” look but once a reg is added it can get very long and IMO not very safe having a high pressure reg suspended out the front off std low pressure paintball threads that these regs come with. this also would require about 70-80cc of plenum space to some how be incorporated behind the reg to get my desired velocity and economy.
Instead I have opted for a much larger alloy tube that will incorporate a Huma reg and large plenum in front of an off set valve.
Here’s the tube next to my std tube;

Here’ the front plug that will incorporate a 28mm Wika 350bar (5000psi) gauge and an 8mm fill probe port.
The first pic shows one of my std 1″ front plugs next to it:

The larger tubing exposes the end plugs to a much higher stress so larger and more pins are needed to keep the same safety factor.

With the front plug finished and fitted I moved onto the regulator and monster plenum.
Here’s the plenum nearly finished (still needs O ring grooves and atmospheric vent port) next to a .25cal Huma Marauder plenum for scale.

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Final cuts on all the O ring grooves and fill probe/reg breather holes which completed the front plug, regulator and plenum.

Once hydro tested the cyl got an overnight 3400psi leak down test which it has passed. I will run another leak down of the low pressure regulated side also once all reg and valve parts are fitted.

Set up the 4 jaw chuck to finish of the valve body with the off set port.
I have offset and recessed the valve seat to keep the transfer port as short as possible to keep this .25 monster as economical as it can be. The shortest and straightest air path to the pellet will use less air and lower reg pressure to achieve the high velocities and highest shot count.
I am using a raised 45 degree valve seat with a 90 degree contact face to make the most of a regulated 130bar quick plenum dump.

The final sized valve seat port was then cut.

Refitted the 3 jaw chuck and it was on to the valve poppet.
Im using an 1/8″ stainless steel rear sprung stem running inside a brass sleeve to keep it slick.
I threaded it and wound on a brass nut then turned it down for a small shoulder to stop the stem hammering through the Delrin poppet over time.

Decided to push the cupped face of the valve back as far as i could to gain another 15cc to the plenums size.
It now holds a total of 70cc of regged air. 75ftlbs here I come!