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.
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!
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Thanks Donny, but I really am just a back yard hack with a cheap hobby lathe and mill with plenty of ideas that I attempt to execute.
Some work right away some need loads of development or complete redesign. I am self taught and use some really crappy make shift tooling so most of my machining is not as good as it could be with decent tooling. I guess I make it hard for myself as I love a challenge 😉 🙂
Here’s rough drawing of what it will look like when finished:
Phil, do you think the thick skirts of the MK1s are better for long range? I am thinking of trying to tune an EDgun long .25 with Mk1s or MK2s and hit soda cans at 300 yards. I can’t build my own bad ass gun like shown in this thread so I am stuck asking questions on the best approach with what I have access to.
Ed has changed out a weak part in the Matador design with the new upgrades so I believe it can handle 65 foot pound. I don’t think I can get it safely to 75. I mean I can tune it to shoot that fast but I don’t want to teach people how to break guns. I still use your .177 data to tune guns for people with fantastic results, so I am waiting to steal from you again.
The gun is coming along but I always admire your machining skills. Awesome work Wingman!
Thanks Brian, I hope to make a few vids of this one when its up and running..
Valve dwell time is actually controlled more with the valve port diameter and seat design. Well there is a little more to it than that but that is the basics of it. The raised seat actually directs the rush of air back at the open valve causing a slightly longer dwell which is beneficial on a low pressure regulated larger plenum.
However this does have a valve spring, it is on the rear of the valve stem like my pistol designs have, this way there is no side loading causing bent valves and flexing of the poppet resulting in valve breakage or jamming. It also means the valve can be made lighter and an O ring can be seated under the return spring on the valve stem which not only stops any blow by but also acts as a depinger..
Here’s one out of my 9mm pistol:
The barrel is a piece of rocking horse crap that I have had stored for years for this very build.. it is one of the very sort after early production hammer forged BSA lonestar barrels that are still winning all the awards in the 100m comps throughout Europe, its brand new and unfitted..
It is a 1- 15-3/4″ twist.
It has a very nice personality trait of retaining the highest B.C of all barrels I tested.
I will be shooting the Mk 1s as they seemed to out shoot the mk2’s in this barrel by a whisker at 100y.
The barrel sports the factory BSA choke which is quite tight. The old BSA barrels also had a wider slanted 12 land bore with very fine grooves, where as the new BSA barrels look more like the L/W bores.
Old BSA .25cal bore print on JSB king
New BSA .25cal bore on Benjamin dome and JSB king
Phil that gun looks awesome, good luck on your vision quest. I hope you have time to make a video.
Couple questions…
How do you control valve dwell time without a valve spring?
What is the rate of twist?
Are you going to shoot the MK2 heavies?
How much choke are you going to have in the gun?
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Ed has changed out a weak part in the Matador design with the new upgrades so I believe it can handle 65 foot pound. I don’t think I can get it safely to 75. I mean I can tune it to shoot that fast but I don’t want to teach people how to break guns. I still use your .177 data to tune guns for people with fantastic results, so I am waiting to steal from you again.
My .177 data? Refresh my memory on that one?
Although not confirmed with 300 yard testing I believe the thicker skirts will be the better choice for this mission.
I certainly is the case with the two different skirt punches in the 18gr .22cal JSBs in my matador at 100 yards. I have yet to get the thin skirt .22s inside 2″ at 100y where I can consistently shoot the thick skirts inside an inch and even 1/2″ when the conditions are in my favour.
IMO Eds .25 platform is not up to the task. ( I considered it myself) It is possible but it would need loads of custom bits.
The .25 Matador power plant is too small to deal to the 34gr JSBs. The hammer is too light and the plenum is too small.
You could wind the reg to 165-170bar but the valve will come apart and Im not sure the hammer will have the energy.
You could possibly use a .30 Matador for its larger plenum and hammer but mod a .25cal valve for the stronger 3mm .30cal valve. You need the same transfer port, valve seat port and breech as the std .25 matador, the .30 cal valve is not set up for the 130bar reg setting on its larger plenum so its dwell would be out for a .25 barrel.
Lastly the matadors L/W barrel doesnt seem to shoot the new 34gr JSBs all that well.. they need the 1-16 twist of Edgun L/W but his choke dimensions seem to clash with this combo.
Id recommend fitting a BSA barrel or a CZ barrel to get these pellets to perform at the longer ranges.