.25 cal Carbine Based on MRod Tube - Includes Build Details from Reply #33

[rsterne]

I am thinking of replacing my .22 cal Grouse Gun with a .25 cal, and of course it will need more air.... I can use a 2024-T3 aluminum tube that is 1.25" OD x 0.095" wall (like a Marauder), and install a regulator inside, and get about the same number of shots I have now (16 is desired, ie 2 mags.), at the same gun weight.... My current gun fills to 2000 psi and has a 14.5" barrel and uses a 2260 tube, and looks like this....



Imagine that with an MRod diameter tube, and about the same length .25 cal barrel, and you get the idea.... Velocity with a 25.4 gr. JSB to be between 750-850 fps....Now for the question....

It would make sense for me to use an in-tube regulator, such as the Huma... I want a plenum volume between the regulator and valve of 28-40 cc, and I am wondering what is available?.... What brand of regulator would you recommend?.... I am particularly interested in what Motorhead has to say about this.... What setpoint pressure would you recommend for 800 fps with a 14-18" barrel?....

Bob

[YEMX]

Huma has a high power .25 plenum that's 30cc's 

[Motorhead]

Built this but never used it ... IT WOULD ONLY REQUIRE A BREECH TO RECEIVER SPACER to get barrel above tank a bit. or a custom tall breech block ?
Gen 2 marauder based. 13cu in ninja bottle w/ regulator.

INTERESTED  in it just PM me.

[rsterne]

Thanks, Scott, but I want to build something more compact and lighter (I assume that is a steel tube?).... Now that you are here, what would you use for an in-tube regulator to fit an MRod tube?....

Tom, thanks for that info, that sounds about right.... Is their .30 cal plenum any bigger?.... Do you know what the overall length is for the high-power .25 Huma?....

Bob

[Motorhead]

Most compact is a HuMa universal sized at 1.050" .... how you navigate a plenum can be using the gauge manifold ( hogged out ) or a HuMa with attached plenum with or without the air tube gauge that shows plenum pressure and not HP.

[YEMX]

Thanks, Scott, but I want to build something more compact and lighter (I assume that is a steel tube?).... Now that you are here, what would you use for an in-tube regulator to fit an MRod tube?....

Tom, thanks for that info, that sounds about right.... Is their .30 cal plenum any bigger?.... Do you know what the overall length is for the high-power .25 Huma?....

Bob

I don't- But I think I have a .30 plenum in my parts bin, I'll see if I can find it, and take some measurements for you...
EDIT: I found the plenum, it sticks in my mind that it's a .30 plenum.  No idea on age.  If someone knows better (i.e. based on measurement), please correct.  The measurements are as follows:

OAL: 3.358"
OD:  1.056"
ID:  .9755"

All measurements are as accurate as I can get, given my digital calipers and telescoping gauge set aren't the highest quality. 

[rsterne]

Hey, Tom, is that LOA including the regulator, or just the plenum?.... What is the inside length of the plenum?....

Bob

[YEMX]

Hey, Tom, is that LOA including the regulator, or just the plenum?.... What is the inside length of the plenum?....

Bob

That's just the plenum.  Add a touch over an inch (Motorhead's length sounds about right) for the OAL with reg (that is, IF this is a .30 plenum like I think it is).  The plenum is just a tube that the reg sits on, so inside length is the same as the OAL.  WELL... There's a groove for an o-ring on one end, but that's about it... 

[rsterne]

So the plenum is just a spacer with a 1mm (0.040") wall?.... Any machining at the regulator end, or just flat?.... Is it thicker wall where the O-ring is?.... I get 41cc for the inside volume....

Bob

[YEMX]

So the plenum is just a spacer with a 1mm (0.040") wall?.... Any machining at the regulator end, or just flat?.... Is it thicker wall where the O-ring is?.... I get 41cc for the inside volume....

Bob

Yessir- essentially just a spacer tube.  The only machining is for an o-ring.  Nope- the o-ring groove is simply machined out of the wall thickness...  It's certainly thick enough!  LOL  So if it's 40+ cc's, this one SHOULD BE a .30 like I thought.  The .25 "XXL" plenum was listed as only 30cc on their website.  Right now, I bet both plenums are the same size, but I could be wrong.  AFAIK, these spacers are built so they don't crush under the pressure.  I think people did have some failures like that many years ago, seeing how far they could go w/the reg spacers/plenum thickness (i.e. how thin they could make the wall of the plenum tube).   

[rsterne]

So the inside diameter must be stepped to be thick enough for an O-ring at the end?.... What about the end that slides over the regulator?.... 1.056 - 0.976 = 0.080" / 2 = 0.040" wall....

Bob

[YEMX]

So the inside diameter must be stepped to be thick enough for an O-ring at the end?.... What about the end that slides over the regulator?.... 1.056 - 0.976 = 0.080" / 2 = 0.040" wall....

Bob

Nothing is sliding over the regulator- the regulator sits on top of the plenum.  The plenum sits on top of the gauge block (or the valve face, if you're able to eliminate the gauge block). I'm not sure where the o-ring sits- on the gauge block (or valve face), or on the end where the reg is.

[gauge block or valve][plenum][Huma Reg]--------//-------[front end cap/fill plug]

[rsterne]

So the plenum must seal at both ends?.... How is the regulator vented to atmosphere ?....

Bob

[HunterWhite]

Bob, it's kind of difficult to envision.  The gauge port is sealed to the gauge by a small o-ring at the bottom of the threaded part of the gauge block. The area where the threads go through the tube is not gas tight.

The gauge block itself has an O-ring at each end, but the area between the O-rings outside the gauge port but inside the tube is atmospheric pressure.

The Huma regulators that I have vent through a tiny hole in the regulator to that area between the big O-rings.

The Huma regulator for Marauders has an O-ring on the end that buts up against the valve face.

The muzzle end has the big O-ring just like the gauge port. The Huma with plenum replaces the gauge port.

 The area outside the plenum,  but inside the tube is atmospheric pressure.


Hunter

[HunterWhite]

Notes on the gauge port threads.
The early Marauders had the same thread on the gauge and gauge extention.
I guess that Benjamin Crosman couldn't get those gauges anymore.  The newer gauges have a European thread ( BSP?). Crosman changed the gauge extention to have a female European thread for the gauge to screw into, and the original thread (NPT?) To screw into the gauge port.
If you order a guage from Crosman, it will come with the adapter/extention that fits the Marauder gauge port.
I have ordered a couple of these.

The Huma regulator will have the European threads, and you need an extension that has European threads to screw into it. You will need a gauge with European threads also.
I have the gauge and gauge extention with the European threads for the Huma regulator.
I'm a little fuzzy on this now, but, that's what I remember.

Somewhere there is a thread that correctly identifies this much better than I have done here.

Hunter

[rsterne]

Hunter, thanks for the explanation.... I am familiar with the Gen 1 MRod setup, with the gauge mount, between its O-rings at atmospheric pressure, and I saw the tiny hole in the side of the Huma reg. on the back side of its O-ring, so I get that.... What I was missing was that when you install the Huma plenum, you remove the front gauge mount O-ring, so the outside of the plenum is at atmospheric pressure.... On the Gen 1 MRods, the gauge adapter was parallel threads into the mounting block, sealed with an O-ring, but NPT where the gauge threaded into it.... I'm not using a gauge, so it won't matter....

On my custom tube there will be no gauge port, or corresponding hole in the tube, the plenum will butt up against the front of the valve.... That means I should get the XXL, with the 30cc plenum, because I won't have any volume inside a gauge mount, just what is inside the valve itself.... If I use the XXL, I should end up with about 35 cc total, which is perfect for the power I'm looking for... However, I will have to drill a small vent hole in the tube beside the plenum, so that the regulator gets vented....

My gauge will be mounted on the front plug, which will be recessed so that the tube protects the gauge.... The fill fitting will be on the side or bottom of the front plug....

Bob

[HunterWhite]

Bob, wait until you get the Huma regulator to drill the hole. I think that it will read reservoir pressure id you get the same one that I have.


Hunter

[Mzq284]

Bob,
Could you please tell me where you found the angled front plug in your current grouse gun?

[rsterne]

Here is a sketch of what my new .25 Carbine should look like.... Click to enlarge....



As shown, that has a full 20" MRod barrel (no extended shroud, they are illegal in Canada).... LOA 34.5", I will probably shorten the barrel to 16-18".... As shown, with an XXL Plenum (about 35cc including the valve, no gauge block) the HP reservoir is about 168 cc.... At 3000 psi fill, 2000 psi setpoint, that should give me 730 std. CI of usable air.... If I can get 1.0 FPE/CI, I should be able to get 18 shots at 40 FPE.... with a shorter barrel my goal is at least 16 shots (2 mags) at 36 FPE (800 fps with 25 gr. Kings)....

Peter, I don't remember where I got it, maybe 20 years ago!....

Bob

[HunterWhite]

Hunter, thanks for the explanation.... I am familiar with the Gen 1 MRod setup, with the gauge mount, between its O-rings at atmospheric pressure, and I saw the tiny hole in the side of the Huma reg. on the . . .


What I was missing was that when you install the Huma plenum, you remove the front gauge mount O-ring, so the outside of the plenum is at atmospheric pressure.... On the Gen 1 MRods, the gauge adapter was parallel threads into the mounting block, sealed with an O-ring, but NPT where the gauge threaded into it.... I'm not using a gauge, so it won't matter....
Bob

Bob, the plenum screws onto the regulator on one end. The muzzle end O-ring is retained.  The breech end O-ring is not used because there is an O-ring at the end of the plenum that buts up against the valve. This means that the entire length of the plenum is atmospheric pressure until it gets to the valve O-ring.  The plenum wall thickness is doubled at the very end where it buts against the valve to allow for an O-ring to be placed in between the plenum and the valve.
This is how mine are. The ones that I have replace the gauge port.

If you have one that still uses the gauge port then the O-ring near the muzzle is not used, but there is an O-ring between the regulator and the gauge port.


Hunter