Theoben Rapid MK2 mods

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Hobbyman2007

Hobbyman2007

Member
Nov 17, 2015
817
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Ontario, Canada
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.

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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.

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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.
 
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Motorhead said:
Interesting mid of mish-mash tech going on all at once :D
Hope it finds a good tune that serves you well ;)(y)
Click to expand...
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.
 
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Hobbyman2007 said:
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.
Click to expand...
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.
 
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Motorhead said:
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.
Click to expand...
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.
 
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Hobbyman2007 said:
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.
Click to expand...
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 :censored:
 
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Ok , it’s done. No more fooling with it. I finally got the fill block from RAW. Yes I had to cut the stock to fit the block. In my defense, the stock was/is not collector grade to start with. Also added some black dye to the checkering to give a bit more contrast and I think it turned out pretty well. I am using the stock hammer , 9mm length silent pip home brew. Kept the easy open valve stem as I have had very good luck with it. As it sits , with an SSG with just the slightest free flight. 230bar fill, start at 810 FPS a slow rise to its peak of 830 FPS at 200 bar back to 810 on the fifth magazine ending at 165 ish bar on the new gauge. This is shooting the 33.95 MK2 king heavies. So just over 50ft/lbs. more than plenty for paper or small game. So in the end that’s 50 shots at 50 ft lbs on a 580cc bottle. I’d say that’s pretty awesome. Even if it’s not the most efficient, not that I’ve run the equation since Lloyd’s calculator was taken offline. I could get over 70 fpe out of it but it’s way too loud with the short barrel and that’s a Korean cliff tune. No thanks .
I may get a 300 bar gauge to replace the one supplied with the V block as it tops out at 250 which is close to the fill pressure. 23mm Sehkmet if they ever come out with one.

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Hobbyman2007 said:
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.

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Click to expand...
Like the gun rack
 
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