I may have done something silly...

[YEMX]

So...  I was perusing the interwebs, and came across a product that I've seen before and didn't give much thought to.  The price was right, so I said to myself: "Self...  order one up and lets take a look at it"...  Those conversations never end well...



So... I bought this raw AF extrusion a while back and haven't done anything with it yet.  I'm looking to do up a custom frame for a Texan project I'm "bench building" right now (meaning nothing major past the planning stage for now).  The idea:

-  Get a Texan, use everything but the frame
-  Machine the Raw Extrusion the way I want to get more rigidity from the frame

Now I'm kinda thinking of doing the above, but adding this bullpup adapter and a CF tank to the mix...  So I'd have a bullpup, custom framed Texan... 

I don't know what I'm doing with my life...   

The caliber would probably be 7mm

[YEMX]

Here's some guns that other people have done.  I'm not trying to copy, just showing what has been done in the past.







Just picture something like these builds, but with a Texan side lever set up.  Again, not sure if this will even pan out.  But I can dream, dang it!

[Hobbyman2007]

I’ve never really liked the airforce setup with the bottle position but with this reverse block that would shorten the rifle I think it’s great . 7mm sounds awesome. I wonder if the power will drop at all with the airflow path doing two hard 90* turns .

[YEMX]

...I wonder if the power will drop at all with the airflow path doing two hard 90* turns .

That's what I'm wondering too.  I want to see how big the orifices are, compared to the normal reg/tank set up.  If it's the same dia. all the way through, I guess it should be alright...  But again, we'll have to see.  I don't think anyone Stateside has actually bought one of these tank blocks and looked at/used it.  I could be wrong of course.  If anyone reading this post has/has used one of these blocks or a similar one, feel free to chime in!! 

This could be a cool project.  A kinda light weight comfortable to shoot bullpup Texan?  In 7mm?  Sign me up!    I wonder if I could swap the side lever to the left side?  That way I can have the loading port on the right side... Maybe the power wheel access on the right side as well...  Hhhmmmm...

*Takes a deep breath*  Okay, I'm getting ahead of myself... 

[KnifeMaker]

Tom, do you have a linky?


I like the BP's pictured, however, I would not trust the breach from the factory. I have had two blow up in my face on the .257!


Exciting times they were!

[dyotat100]

There is no plenum in there. So at best 22 with that volume and a high reg set pressure. Maybe 25 pellet.

If no reg it would need a way larger then poppit size hole to flow enough air that far. Pellet shooters are more forgiving. Once you start needing air for larger calibers it needs to flow free or you will not get the numbers you should.

[rsterne]

If the gun is regulated, there are no 90 deg. turns in the airflow between the valve and pellet.... However, as Doug points out, the plenum volume is non-existent unless you move the bottle and stock back.... like the first photo of the gun with the wood stock.... I'm not so crazy about the gauges being in the middle of the cheek weld.... If unregulated, then of course you are introducing those 90's.... but it looks like they are before the valve to me.... The whole thing looks a bit awkward, in any of those configurations, IMO....

Bob

[gabi.nechita]

I was used this reversed block in .223 with 41.5 grains RWS
Without block 320 m/s.
With block and all path enlarged to 7mm only 270 m/s.
Maybe you can enlarge to 8mm but no more than that becouse you have a cavity under the pressure gauge and your hole will respond there.
 You can also cancel the pressure gauge and put a plug instead and after this operation you can enlarge to 10mm.

[Kinetic45^]

On the 7mm / .284
The 14" twist is for longer heavier bullets and I have been in discussion with someone here on weights vs twists.
(Note that TJ's is closed till the 15th and there are discussions about me having a custom mandrel made to better suit the weights airguns can reasonably shoot)

Here's some of the calcs where bullet lengths vs twist where the speeds are 850 / 1100 using the twist calculator http://www.geoffrey-kolbe.com/barrel_twist.htm
Hint: Don't forget to select the correct Specific Gravity as it's default a jacketed bullet

.5 long   =    22" / 20.5"
.6            =   18.5 / 17
.65          =   17 /  15.5
.7            =   15.5  / 15
.8            =   13.75. / 13     [ this length is around 120gr slugs ]

Bullet specs I used, it's a gas check shanked as I've found those make a more accurate slug.
Dia: .284,  Nose L:.25,  Meplat: .15,  Base: .26,  Base L: .65

Most airguns are tube type and not as powerful as the Texan type guns so could not launch as heavy a bullet... so I want something for the Airgun community that will be possible in the 900 to 950 FPS most airguns can put out. Which means around the 85gr to 95gr weight bullet range.
85gr @ 950 FPS = 170 FPE
95gr @ 950 FPS = 190 FPE
I'm leaning towards having TJ make an 18" twist barrel to increase the choices airgunners have in this calibur instead of being limited to 14" but there are some arguments for a 16" twist being the 'one' we need most.

[Kinetic45^]

@ rsterne or @ dyotat100
I think what the readers would want to be asking:
How big (in cc or cuin) (so we can see how long a tube would be needed) a plenum would be needed to be inserted between the valve of the Texan and the  U adapter to provide enough 'instant' air before the flow restrictions of the OP's U adapter choked prrformance?
For say .257 or up to .308 medium weight slugs?
What is that formula you use Bob?

[rsterne]

If the restrictions between the reservoir and the valve are significant, in an unregulated PCP I would look for a "plenum" of about 1/2 cc per FPE.... If the restrictions are larger than bore-sized, I would think you would see little loss anyways.... it should be no worse than a conventional PCP where the airflow turns 180 deg. between valve and barrel....

For a regulated PCP, where the refill rate is much too slow to provide any pressure top up during the shot, I prefer 1 cc per FPE.... although you can get away with half that if you increase the setpoint pressure about 10%....

Regarding the barrel, since you can purchase TJ's 7mm barrels in 10" and 14" twist, if you want something slower I would do an 18".... I see little point in having both 14" and 16" twists.... JMO.... Remember, if you want to shoot boattails, they need a faster twist, and the 96 gr. LBT works well in the 14"....

Bob

[Kinetic45^]

Hummm
So quick trip to the calculator:
Most std air tube is very close to 1" ID and that calcs to .785 sqrin so that converts to 12.86 cc per inch.

Now for the most versatile plenum you want the biggest you might use so:  I went with a 120gr 7mm at 950 FPS = 240 FPE

Using 1cc per FPE you would get 240 ÷ 12.86 and so need an air tube plenum of 18.66 inches long.  Kind of long for a buttstock 😄

Could go with a larger OD tube but to have it short enough you are back to 2" which is the size of an aluminum or CF tank.... and I'm not counting the extra length of the end caps needed for attaching the valve or U adapter either.

Another rough calc then using a +- 2" std tube for the plenum
Not looking up exact wall thickness but a 2" tube has around 1.75 ID so that's .875 r
Works out to Area = 2.4 sqr in
So convert to cc = 39.33 cc per inch of a nominal 2" tube
Using the prior 240 FPE we get 6.1 inches long of internal plenum length needed

Now what I would like is a 2" tube plenum about 7.5" internal length long (295 FPE?) where the forward end cap contains the valve and the rear end is part of the REGULATED U adapter which mounts a 4500 PSI 350cc CF bottle (I have it all drawn out but have no one to machine the adapter block with built-in regulator. Jim at WAR has said he could but is too busy / booked)

[Nomadic Pirate]

Tom, do you have a linky?


I like the BP's pictured, however, I would not trust the breach from the factory. I have had two blow up in my face on the .257!


Exciting times they were!

That was exactly the first thing that came to my mind,.....no way I would want the Airforce non locking breach right by my face

[rsterne]

Don't forget that as you go larger in diameter, you must increase the wall thickness by the same proportion, to maintain the same MSWP.... If a 1.25" OD tube like an MRod needs to be 0.095" wall, than a 2" OD tube would need to be about 0.152" wall.... so the ID would only be about 1.7", and the volume about 37 cc per inch....

If you use 2024-T3 aluminum you may need a 3/16" wall, depending on the setpoint pressure you want to run.... Figure backwards from your FPE and barrel length to work out your required setpoint.... Do you really want to build a 295 FPE regulated (at 3600 psi) 7 mm with a VERY long barrel (32-36")?....

If you can find a shorter but large OD bottle, you may be able to build a buttstock with the regulator vertical inside the back, and the plenum becoming the comb.... Something like this?



Bob

[YEMX]

Tom, do you have a linky?


I like the BP's pictured, however, I would not trust the breach from the factory. I have had two blow up in my face on the .257!


Exciting times they were!

That was exactly the first thing that came to my mind,.....no way I would want the Airforce non locking breach right by my face

I agree with you both- that's why I want to see if this u adapter will work w/a Texan.  No way I would try with a standard breech.  Even if it is facing away from me...

I found it on ebay.  Price isn't terrible, and far cheaper than when they first appeared:
https://www.ebay.com/itm/High-Pressure-U-Adapter-Connector-for-Condor-Talon-Airforce-Airgun-CO2-Valve/183712759026?hash=item2ac6223cf2:g:gZEAAOSwsq1ce9~H

[Kinetic45^]

Don't forget that as you go larger in diameter, you must increase the wall thickness by the same proportion, to maintain the same MSWP.... If a 1.25" OD tube like an MRod needs to be 0.095" wall, than a 2" OD tube would need to be about 0.152" wall.... so the ID would only be about 1.7", and the volume about 37 cc per inch....

If you use 2024-T3 aluminum you may need a 3/16" wall, depending on the setpoint pressure you want to run.... Figure backwards from your FPE and barrel length to work out your required setpoint.... Do you really want to build a 295 FPE regulated (at 3600 psi) 7 mm with a VERY long barrel (32-36")?....

If you can find a shorter but large OD bottle, you may be able to build a buttstock with the regulator vertical inside the back, and the plenum becoming the comb.... Something like this?



Bob

Yes,, just like that, but using titanium tube.  Actually I have enough 2" for both a top and bottom tube instead of a CF bottle.
Of course the bottom bottle or tube would be shorter so as to not interfesr with the pistol grip. A 350cc CF works in mockup.

[YEMX]

Hmmmmm...

[rsterne]

Carl, how about three Titanium tubes.... the bottom two for the reservoir (at whatever length you need to fit behind the pistol grip to give the LoP you want)…. and the top one as the plenum.... Then install an adjustable regulator vertically in the buttstock....

Bob

[Kinetic45^]

Carl, how about three Titanium tubes.... the bottom two for the reservoir (at whatever length you need to fit behind the pistol grip to give the LoP you want)…. and the top one as the plenum.... Then install an adjustable regulator vertically in the buttstock....

Bob

Sounds like a winner idea that would not be that heavy but would put out good Texan performance and increase the shot count a lot.
I have the funds, just need a competent machinest.

[alamih]

Carl
maybe this device will solve your problem
Greg