Theoben Rapid MK2 mods

[Hobbyman2007]

Figured rather than hijack other threads not to mention I just wanted to share my “end of winter project” . Rifle started out as a .25 cal ( that’s staying ) with a 23” long barrel and a standard 500cc aluminium cylinder and unregulated with free flow valve.It’s gone through a bit of a transformation. Here’s the list of mods and changes. None of which are my designs , mostly borrowed from members from various forums. Thanks gents , you know who you are.

-cleaned , steamed out dents then oiled up the stock

-Install missing rear sling stud and added a front stud

-fabricated cheek riser and wrapped to match

-fill port cover ( aluminium sleeve and Delrin)

-580cc best fittings bottle ( wrapped in CF vinyl)

-aluminium 30gr. Hammer with adjustable PIP and peek tip

-SSG with multi springs ( mild 30 fpe to wild 65fpe)

-17” LW barrel hand lapped , with a two port set up one at .2” the other at .150” , oring sealed . Clock barrel 90 degrees to change .

-force reduction valve stem and end cap to match . Got the idea from Mubhaur. Only one additional Oring

-picatinny rail added from m lock section

-custom roller bolt with oversized knob

-all ports matched from valve to match the block at .2”

I’m still waiting for a RAW V block and will have to modify the stock to fit. I currently have it tuned to peak at 40 FPE with a 20 fps spread over 4 magazines. That’s not terribly impressive but that’s from a 150bar fill shooting down to 100 bar. Basically 1 bar per shot. Very pleased with how it turned out. Classic rifle with a bit of a modern twist.

 

[Ranchibi]

Wonderful mods! You'll enjoy the v-block once you get it! Martin also manufactures a mini plenum extension for the front port under the barrel, I have one on my .22 HMx.

 

[Hobbyman2007]

Unfortunately, Martin won’t ship to canukistan. I ordered from rapidairworx. I wouldn’t be able to use the plenum as that’s where my valve stem protrudes to open atmosphere. Although I’m still working out a plan to incorporate a cobra chamber.

 

[Hobbyman2007]

Up to some more tricks. Valve version 4.0. This time I was able to reuse the stem I made but remade the end cap. This time with enough room for a cobra chamber. So this added two more o rings . The cobra chamber or “air spring” has a .052 vent and is roughly 1cc ,minus the area of the valve stem going through it. Give or take . I’m still going through the tuning but so far is much better that the stiff valve return spring I was using. I’m shooting for about 35 ft/lbs or 790fps with 25 grain JSB’s. I already have a heavy hitting 25 cal ( Leshiy 2) . Pics are of the parts disassembled and complete as drop in. I’m also still running the 7075 aluminum hammer and after three tins of pellets shows no signs of wear. Thanks for that tip Motorhead.

 

[Hobbyman2007]

Well, change of plans yet again. Seeing as my Leshiy doesn’t like the 33 grain JSB’s . I figure I may as well use the full potential of this unregulated rifle. With the valve system the way it is now , I’m going to try to get to the 65ft lb mark. It’s going to be a little tricky with the short 16” barrel. If that doesn’t do the trick I can revert to the 23” barrel I received the rifle with.

The cobra chamber was also a bust, while it did work and shortened the dwell. Whenever I shot down to 120 bar the next shot would dump the tank. I don’t think I had a big enough volume in the chamber and the vent was way too close to the valve throat.

 

[Motorhead]

Interesting mid of mish-mash tech going on all at once
Hope it finds a good tune that serves you well

 

[Hobbyman2007]

Interesting mid of mish-mash tech going on all at once
Hope it finds a good tune that serves you well

ya it’s gone through a bit of an adventure. The easy open valve is going to stay as it’s working well. I didn’t and still don’t have a clear vision for what this will end up looking like. If this rifle likes the 33 grain JSB’s I’ll stick to those at whatever velocity it chooses.
I’m two years sober this week and this rifle has helped me get through the last few months. Been a long road especially through the winter months here.

 

[Motorhead]

ya it’s gone through a bit of an adventure. The easy open valve is going to stay as it’s working well. I didn’t and still don’t have a clear vision for what this will end up looking like. If this rifle likes the 33 grain JSB’s I’ll stick to those at whatever velocity it chooses.
I’m two years sober this week and this rifle has helped me get through the last few months. Been a long road especially through the winter months here.

Just understand with these " Mono Block" receivers the available plenum volume really throttles DOWN the power when going to the larger calibers, heavier weight projectiles or seeking high output.

I have a personal Theoben MK2 in .22 cal that has a custom balanced valve in it, light hammer & SSG. It certainly makes easier power than the OEM valve, is snappier and better efficiency too, but due to the limited plenum volume it kinda self stabilized output about the 31-32 fpe. Trying to extract more oomph ... lol just made for a harsh shot cycle and lousy efficiency.

 

[Hobbyman2007]

Just understand with these " Mono Block" receivers the available plenum volume really throttles DOWN the power when going to the larger calibers, heavier weight projectiles or seeking high output.

I have a personal Theoben MK2 in .22 cal that has a custom balanced valve in it, light hammer & SSG. It certainly makes easier power than the OEM valve, is snappier and better efficiency too, but due to the limited plenum volume it kinda self stabilized output about the 31-32 fpe. Trying to extract more oomph ... lol just made for a harsh shot cycle and lousy efficiency.

No regulation on this one, has a fast flow standard inlet. During testing of V 1 , 1130 fps with 25.4 grain . Mind you that was the long barrel.

Do you think the onboard volume may have effect on power? I know there are a few bends in the passages.

 

[Motorhead]

No regulation on this one, has a fast flow standard inlet. During testing of V 1 , 1130 fps with 25.4 grain . Mind you that was the long barrel.

Do you think the onboard volume may have effect on power? I know there are a few bends in the passages.

Forgot you were fast flow ( Non Regged ) .... yea a bit of power as the "Free Flow" to get from bottles volume to the valve feeding the shot is still somewhat throttled by the smallest passage getting there.

As you were good man .... Rainy day here thinking out loud and not thinking that hard before typing ... oups