Author Topic: .308 Texan titanium barrel tension  (Read 1431 times)

rcpro88

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.308 Texan titanium barrel tension
« on: March 07, 2018, 08:48:33 PM »
Hey guys,

I've been working on a barrel tensioning system for my .308 texan. I already have the muzzle brake/tensioning nut made as well as the retaining cup and all I have to do is cut the titanium tube to length and mount it in the Texan. I hope to have it finished by Friday.
I have an autocad drawing of what it kind of looks like. I tried attaching actual pictures of what I have done but the file size was too large and I was unable to post them.
The end of the barrel is threaded with 5/8-24 threads and the retaining cup that will hold the high pressure/thrust from the titanium tube is a stainless steel 316 split ring that gets held together when the retaining cup is press fitted over.
I hope this project will improve my Texans accuracy and maybe others can learn from my accomplishment/failure if anything.


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rcpro88

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Re: .308 Texan titanium barrel tension
« Reply #1 on: March 08, 2018, 12:20:42 AM »
Well I actually finished it sooner. I still need to finish up a few more small things but I shot the texan 3 times and none hit the muzzle brake exit, so I think all will be good to test soon.
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rcpro88

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Re: .308 Texan titanium barrel tension
« Reply #2 on: March 08, 2018, 12:41:06 AM »
Starting from top to bottom, these are picture descriptions.

1. Split retaining ring - I made a ring made out of 316 stainless steel on my lathe with an OD of 0.8" and an ID of 0.60 and a thickness of .170". I cut a groove to match in the barrel. After I turned the ring on the lathe, I used a dremel to cut it in half, didn't really cut straight but it'll get her done.

2. This is the retaining cup. It has an OD of 1.03" with a through hole of .671" and a .170" deep recess with a ID of .795" and a shoulder on the other of the retaining cup that has an OD of .895" that the titanium tube sits snugly on. It is crucial that when the split ring is on the barrel, the fit between the retaining cup and the split ring is press fit, this ensures no separation of the split ring halve when I go to torque this puppy down.

3. Threaded the last 1.5" or so of barrel with a 5/8-24 thread

4. This is the barrel assembly without the titanium tube. The retaining cup is pressed onto the split ring halves and the muzzle brake/tension nut is screwed on 7/10ths of the way.

5. This is the complete barrel assembly with the titanium tube in place.

6. This is the retaining cup when the titanium tube is in place. I had to sand the barrel down a tad. There were slight burs left from machining as well as those left from the set screws prevented my .671" ID retaining cup to slide on. I smoothed it out with some 320 grit on the lathe.

7. This is the muzzle brake/tension nut. I put a long hex key through the holes in the muzzle brake and crank it tight.

8. Another picture of the muzzle brake and barrel assembly.

9. The whole thing. Yes the .308 Texan fits in my hard case with tank and new barrel on with an inch or two to spare.   
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rcpro88

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Re: .308 Texan titanium barrel tension
« Reply #3 on: March 08, 2018, 01:12:57 AM »
A few things I need to do are:
1. The retaining cup needs holes drilled to allow the set screws to prevent lateral impact movement. Simply pressing up against the titanium tube/aluminum retaining cup will not be sufficient.
2. Make the barrel tension nut separate from the muzzle brake or silencer. This way if switching between an airstripper/silencer/muzzle brake you don't put more wear on the barrel threads.

A few things I've noticed:
1. The new barrel assembly has some weight. Not much, but its noticeable. I will say it seemed to have actually helped steadying when aiming the rifle off hand. The weight is added towards to center and front of the Texan. It could just be me as well.
2. The muzzle brake I made actually worked. When dryfiring a 3000psi fill, the felt recoil with and without the muzzle brake is what you'd expect from any other brake/shroud. The noise is reduced slightly but no where near the dB level of the Texan with an R&L shroud.

Why did i pick a titanium tube?
My local metal supply (Shapiro) stocks some titanium. When looking at buying options of carbon fiber online, I wasn't really sure if I would be getting the quality I'm expecting. Honestly, titanium was just the easier route as I also already knew how I could/can't machine it. 

Tension vs sleeves
When I see carbon fiber barrels, they are generally a carbon fiber tube, pressed and epoxied on and might be repeated a few times to layer up. In this application, you increasing the stiffness/rigidity of the barrel as you are adding more material to the outside as well as using a material that is much more stiff/rigid than the barrel, enhancing overall stiffness/rigidity of the barrel. The reason carbon fiber is often used here over titanium is because of its more stiff/rigid than titanium. However I do not mean to say that you cannot/will never see a carbon fiber tube used in a barrel tension setup.

When a barrel is placed under tension, it not only aids stiffness/rigidity to the barrel but reduces the amplitude of harmonic vibrations/increases their frequency. This is true for whenever you add/remove material/weight to and from your barrel. However this effect is amplified significantly when compared additional weight when internal forces are acting on the barrel/when it is under tension.
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rcpro88

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Re: .308 Texan titanium barrel tension
« Reply #4 on: March 08, 2018, 01:23:10 AM »
Why I think the Texan has a few hiccups:
When you can't free float a barrel you use a heavy one. The .308 Texan barrel is supported in the center of the rifle with 2 bushings which allow the barrel to vibrate like a tuning fork when shot.

The R&L shroud provides support to the end of the barrel from the body of the Texan. as you screw on the shroud you apply pressure to the end of the barrel which explains why it might be helping the .308 Texans accuracy issue.
Just one last thing, Matt @ R&L is the man to go to for all Airforce questions/ammo/advice. Matt has been a great help and source of GOOD information and advice.
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Christopher

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Re: .308 Texan titanium barrel tension
« Reply #5 on: March 08, 2018, 02:45:42 AM »
That looks great on your Texan..... looking forward to the accuracy results. How was your Texan shooting before the tensioning system?

Thanks,
Chris
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rsterne

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Re: .308 Texan titanium barrel tension
« Reply #6 on: March 08, 2018, 08:34:42 AM »
Very nice work.... If I understand your tensioner correctly, it tensions only the part of the barrel forward of the frame of the Texan?.... Is there any additional support for the assembly to reduce flex at that point?.... I would assume that the Texan barrel is either steel or CrMoly?.... Titanium has a thermal coefficient about 70% that of steel, so as you increase the temperature you will have less tension on the barrel.... This can have a large effect on the axial load (tension) on the barrel.... Are you torqueing the "nut" to provide a consistent tension, and if so, what is the approximate tension/torque you are using?....

Bob
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rcpro88

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Re: .308 Texan titanium barrel tension
« Reply #7 on: March 08, 2018, 02:17:46 PM »
The way the Texans are designed makes it very tricky to tension the whole barrel. I don't think having the whole barrel under tension would make much more difference as you mainly want to support/control the muzzle end of the barrel.
This barrel tension system tensions the barrel from where the original rear barrel bushing was to the muzzle of the barrel (about 21 inches).

As of right now the nut/muzzle brake that provide tension on the barrel when tightened is just one piece with no counter nut or beveled washers, it will be replaced soon.
As for the thermal properties of the titanium vs steel, this would matter significantly more if this was a firearm in which the barrel heats up after a few shots, from what I've experienced, I can shoot 50 shots through the Texan, grab the barrel and it won't feel any warmer than it did before the shots.

As far as barrel tension changing in various external temperature, such as tightening the tension nut inside then going outside in the cold to shoot - there might be a significant change in barrel tension, I haven't tested it out yet.

My plan is to use beveled washers to provide constant tension on the barrel regardless of temperature.

Last night I tightened the muzzle brake to around 90 ft-lbs which should put around 1200-1500 lbs of tension on the barrel.
 
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rcpro88

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Re: .308 Texan titanium barrel tension
« Reply #8 on: March 08, 2018, 02:40:05 PM »
Airforce claims the .308 Texan will shoot a 1" group at 100 yards out of the box. I would normally shoot 200 yards with the Texan, I would get anywhere from 4 inch - 6 inch groups at 200 yards with various types of pellets. I never really spent a lot of time shooting 100 yards with the Texan before so I can't really compare my results today with past results.

Today I took the Texan to a 100 yard range and it shot consistent 1.2" 10 shot groups BUT if you were to remove the 1-2 fliers the 8-9 other shots were all touching/overlapping with a group size right around 0.75". I believe the groups can get better and well I'm also not a perfect shot either, I would try to put the cross hairs on the bulls eye every time but there was a guy shooting next to me with a loud gun and honestly I would jump sometimes when he shot which would effect my shot of course.
This is gear I was using: 
.308 Texan
Primary Arms 4-16 zoom scope (should probably be upgraded)
$30 scope rings (no i did not lap them either, shame on me)
115 grain High Arc pellets

I used a scuba tank to refill the Texan. I don't have a regulator yet so I would fill to 3000psi, take two shots, refill to 3000 psi and so on.
I have the Texan power wheel all the way turned up. Matt at R&L told me the High Arc pellets like to be shot around 930-950 fps. I didn't chrono the gun yet with the High Arcs yet but I'm pretty sure a few of them cracked, so chances are they are moving around 1000-1100 fps.
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rsterne

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Re: .308 Texan titanium barrel tension
« Reply #9 on: March 08, 2018, 03:35:33 PM »
PCP barrels actually get colder when you shoot from the expanding air, but the effect is rather minor compared to the swings in external temperature between freezing conditions and desert heat.... The Belleville (cup/spring) washers will provide a huge reduction in the percent change in tension when the external temperature changes.... I use them in my tensioned barrels for that reason.... Your tensioning setup will greatly increase the stiffness of the barrel itself, reducing the harmonics.... However, the rather thin, weak area where the assembly meets the receiver will still allow the primary frequency a lot of freedom of movement.... by that I mean the end of the barrel moving relative to the receiver.... The only way around that is to take the loads from the shroud into the receiver somehow, like by sliding it over a turned stub on the receiver, and using the receiver as part of the tensioning system.... not an easy thing to retrofit....

Bob
« Last Edit: March 08, 2018, 03:42:05 PM by rsterne »
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rcpro88

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Re: .308 Texan titanium barrel tension
« Reply #10 on: March 08, 2018, 04:58:11 PM »
Bob,

The barrel assembly slides 8 or so inches into the receiver and 8 set screws hold it in place. The rear set of 4 set screws seat against the aluminum retaining cup and the other 4 seat against the titanium tube, there is actually a 9th screw which is on the bottom at the very end of the receiver. I plan on drilling a few holes in the retaining cup so the screws are actually seated in a 1/4" into the aluminum to prevent any impact type movement/shock.

I plan on swapping out the stock hammer with a brass one (I haven't settled on a weight for it but it will be heavier than stock)
I also plan on filling the gun to 3500 psi, if the heavy hammer has issues, I'll see about a slightly heavier spring.
Goal with the power increase is to shoot longer ranges/have a flatter trajectory as well as the ability to use spitzer rounds once pellets are flying mach.

I just ordered these pellets:
Hunters Supply - 154 grain SP
High Arc - TAWA 115 grain HP
Aeromagnum - Stinger 109 grain HP (these were $25 for 50! but reviews for the .357 version makes me optimistic)

I will also go ahead and order some belleville washers and machine a new tension nut and locking nut as well. I might even have time to make an airstripper attachment as well. I also want to build a titanium bipod (I have a good bit of titanium in my basement) as the one I currently have is a $45 POS.

Also I will probably get a new scope, this one seems like it will fit the bill without breaking the bank.
Mueller 8-32x44
https://www.amazon.com/Mueller-Tactical-8-32x44-Focus-Reticle/dp/B00MQUCBFE/ref=pd_sbs_200_1?_encoding=UTF8&pd_rd_i=B00MQUCBFE&pd_rd_r=BZMPX21JY3RQG0EK656N&pd_rd_w=fibXg&pd_rd_wg=j7ws1&psc=1&refRID=BZMPX21JY3RQG0EK656N
« Last Edit: March 08, 2018, 05:24:52 PM by rcpro88 »
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Steelhead

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Re: .308 Texan titanium barrel tension
« Reply #11 on: March 09, 2018, 07:11:35 AM »
As a .308 Texan owner I'm watching this thread very closely. While I am a Neanderthal among geniuses on this topic, I'm just glad that another person out there is working on the .308. Most info, mods,  and tuning is centered on the .45 so this is very cool. I'm learning a lot so thank you.
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Airforce Texan .308
Benjamin Armada .25
Benjamin Prowler .177
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rcpro88

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Re: .308 Texan titanium barrel tension
« Reply #12 on: March 09, 2018, 10:49:32 AM »
No problem! A big thing I've learned from reading things on forums is to take it with a grain of salt. I try to post accurate information but not everyone is like me.
Also if you need help with .308 Texan problems, give Matt at R&L a call, he personally owns a .308 Texan and has shot many, so if you have a problem, contact him as he has shot many .308 Texans so he knows what they should and shouldn't be doing.
Also keep in mind that a poor scope can leave you chasing your tail trying to find out what's wrong with your .308 Texan - either try a friends good scope or buy a good one, if the gun shoots the same then return the scope if you don't need it.
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Steelhead

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Re: .308 Texan titanium barrel tension
« Reply #13 on: March 09, 2018, 02:02:12 PM »
I've gotten mine to what I consider satisfactory. I've got 1"/touching groups at 100 and 2.5" to 3" groups at 200. But that took a lot of work and trial and error. Out of the box it sucks...it needs to be played with and IMO a Mad Dog stock to stiffen it up.
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Airforce Texan .308
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Benjamin Prowler .177
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rcpro88

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Re: .308 Texan titanium barrel tension
« Reply #14 on: March 09, 2018, 03:59:50 PM »
Did you get 1" groups at 100 yards before or after the mad dog stock? I don't see how it would tighten groups unless it aided in ergonomics which then would be dependent on the shooter.
Since people are viewing this page to checkout mods/tips with the .308 Texan, care to share what "tinkering" you did with yours to tighten group up?
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