Author Topic: .308 Texan titanium barrel tension (Read 3539 times)

Steelhead · « **Reply #15 on:** March 09, 2018, 05:50:25 PM »

The stock helped a lot...whether it be ergo or mechanical. Right now I'm running full power on the wheel. There's a thread in Big Bore I think about what my shot string was with a medium tune. I spent money on the AAO upgrade springs and they are still sitting in hobby room. Airforce told me to turn the tune all of the way up and then I started getting results. At one point I also had severe lead fouling.

So to get back to your original point of whether the stock helped or not, it was hard to tell what was helping what because I was solving multiple issues at the same time. But now if ain't broke, don't fix it.

rcpro88 · « **Reply #16 on:** March 10, 2018, 01:30:54 AM »

I polished up the titanium tube and muzzle brake...
I also drilled holes for the set screws to seat into.
I went ahead an made a new hammer out of brass, around 20 grams heavier than stock. After one shot with the tank filled to 2900 psi, the tank read 2400 psi...thats about 1/3 of a larger drop then I'd get with the stock hammer.
I plan on running the Texan at 3500 psi but i'll mess with pressure, ammo weight and see what speeds I can get out of it.

rsterne · « **Reply #17 on:** March 10, 2018, 08:15:55 AM »

A 500 psi drop in pressure in one shot indicates that most of the air is going out the muzzle long after the bullet has departed.... Just because it's loud, doesn't mean it has any more velocity.... I would strongly suggest that you try all possible power settings and do a simple graph of velocity vs. setting, like this....

You will note that once you reach a certain hammer spring preload (power setting) you don't get any more velocity, all you do is waste air.... You only need to shoot a shot or two at each power setting to find where the plateau starts.... There is no point in using even that power setting, as dropping the velocity back about 3% from there will use half the air....

Bob

rcpro88 · « **Reply #18 on:** March 11, 2018, 10:17:48 AM »

Bob,

The new hammer actually weighs 24.63 grams more than the stock hammer. It is also significantly longer which I believe is actually tad too long and I plan on trimming the new hammer to size once I get my scuba tank refilled/ammo comes in. I'm buying a chronograph today from cabelas and I understand the point of diminishing returns when it comes to how much air consumed per shot, I will test/tune accordingly. Keep in mind I plan on filling the Texan to 3500 psi, so it might not drop as much at higher psi.
I will of course take my chrono to the range when I test accuracy again with different ammo/modifications to barrel assembly and will post my findings on here.

Admittedly, I did make a mistake on my previous post. When I went to the range the first time with the new tension setup, the tension nut/muzzle brake was only torqued to 30 ft-lbs and not 90 ft-lbs. I now have it torqued to 90 ft-lbs, I'll double check it at the range before I start shooting.

rsterne · « **Reply #19 on:** March 11, 2018, 12:10:47 PM »

You might be interested in how much tension you are applying to the barrel at 90 ft.lb. (1080 in.lb.)... 8640 lbs....

https://www.engineersedge.com/calculators/torque_calc.htm

Tom at American Air Arms uses a tensioned barrel, and finds that the group size shrinks dramatically at about 800 lbs. of tension (80 in.lb. of torque), and after that changes little.... It will be interesting to see what you find.... You will not be able to find Belleville washers that don't compress completely flat at much over 1500 lbs. of force, unless you stack about 6 of them in parallel (nested).... My tensioned barrels use Bellevilles that go completely flat at 1300 lbs. of force, I use five in series to allow a larger temperature range without affecting the tension so much.... ~~Did you do a buckling calculation on the Titanium tube to see what happens at that huge compression load?.... Mind you, I guess it didn't collapse or you would have noticed it....~~ should not be an issue....

Bob

Monkeydad1969 · « **Reply #20 on:** March 11, 2018, 09:32:27 PM »

Wow rcpro88, all I can say is wow. Looks good.

Joe

rcpro88 · « **Reply #21 on:** March 12, 2018, 04:08:21 PM »

Monkeydad,

Thanks very much for the compliment!

An update on the chrono tests:
I got my order of 250 pellets from Hunters Supply yesterday (154 grain SP) but my scuba tank will be refilled by tomorrow. I'm getting more pellets in the mail tomorrow and Wednesday.
Mean while I've been using my "boosted" hand pump to top off the Texan, shoot 1-2 shots then repump for chrono testing only.
I bought a Caldwell "Ballistic Precision Chronograph" and I'm not sure if it was a defective unit but I either got and error or the shot just didn't register when I would use it. I tried 10 shots with the chronograph under direct florescent, then tried under direct LED and wasn't any better...So I took it back to the store for a refund.

Good News!: I bought a Magnetospeed Sporter, it has 3 sensitivity settings, with the sensitivity set to HI 2 (the highest) I was able to get the Megnetospeed to register the pellet/velocity. I plan on messing around with it a little to make sure the velocity readings are accurate and not all too low or too high.

I have many projects/ideas and always needed a chronograph to test velocity but never bought one. It is kind of a bummer than the Caldwell Chronograph didn't work out as it should typically register any projectile that travels between the gates regardless of the material the projectile is made of. The Magnetospeed chronograph only works on metal projectiles and they have to be large enough for the magnetic sensors to pick them up.

Monkeydad1969 · « **Reply #22 on:** March 12, 2018, 04:51:47 PM »

On the Caldwell, don't use florescent lights. LED or IR lights. But still looking forward to your results.

Joe

rcpro88 · « **Reply #23 on:** March 12, 2018, 06:05:45 PM »

I tried using my LED work lights and got nothing, half the time it would register the other half I'd get errors, tho I only gave it about 20 shots in total before boxing it back up.
I just got done with the first round with the chrono.

.308 Texan settings:
Stock hammer and 1 hammer weight, total 109 grams. (Matt at R&L told me his Texan and another .308 Texan in the shop had 2 hammer weights but I've sent my Texan into Airforce and it came back with 1 hammer weight just like it went in with.)
Ammo: Hunters Supply 154 grain SP, ACTUALLY 10.15 grams/156.68 grain - I weighed out about 50 rounds and used 20 that weighed in from 10.14 grams to 10.16 grams range.
Power setting: 100% max/can't turn up anymore.

Results:
3400 psi - 927
3300 psi - 926(clean barrel)
3250 psi - 895
3150 psi - 892, 895, 909(clean barrel)
3100 psi - 889, 891(clean barrel)
3000 psi - 881, 881, 876, 882(clean barrel)
2950 psi - 873
2850 psi - 868, 864, 860, 862(clean barrel)
2800 psi - 858
2750 psi - 849
2700 psi - 847, 845, 852(clean barrel)
2625 psi - 832
2600 psi - 828
2500 psi - 800

rcpro88 · « **Reply #24 on:** March 12, 2018, 06:18:53 PM »

Note that the above shot string was not all on one fill, there were 5 fills I believe.

Also my settings on the magnetospeed have changed, I use sensitivity setting "HI 1" which is the middle sensitivity setting - which also means that the magnetospeed should/might work with the .257's out there.

Daystate has an air rifle that has a built in chronograph in the moderator/silencer, Air Wolf MVT. However, on small calibers such as .177 and .22, there is little lead dust, which mean the chronographs optical "eyes" don't get very dirty/need to be cleaned very often. If the same concept would be used in a Texan shroud, every few shots you'd have to stop and clean off the lead dust from the "eyes". Since the magnetospeed uses magnetic detection to pick up the projectile/velocity, it seems having a magnetospeed built into a shroud/moderator for the Texan might not face the same problems or maybe will just need to be wiped off after every 100 shots or so.

rsterne · « **Reply #25 on:** March 13, 2018, 03:18:53 PM »

Some LEDs blink, just like a fluorescent, and that causes errors.... Other LEDs work fine.... Good old fashioned incandescent lights always work.... You need two, one over each sensor....

Bob

rcpro88 · « **Reply #26 on:** March 16, 2018, 07:48:51 PM »

Just an update on where things are at:
The barrel tension system showed significant improvement over the stock Texan barrel.
When shooting 130 yards at my friends farm, the Texan shot 0.8" groups. However the Texan was sitting in a bench rest type gun rest. It was my friends gun rest and I don't know the brand/model but the Texan was secured to the rest, aligned with the target then we used clamps to keep the gun rest from moving on the heavy table. All I had to do was touch the trigger. I only shot Aeromagnum 109 grain HP's during this test and I admit I did not clean the barrel previously after shooting a good bit of the Hunters Supply 154 grain ammo. The Aeromagnums chrono'd 1030 fps.

The next day I went to gun range to test the Texan some more. This time I was shooting both High Arc 111 grain ammo and the Aeromagnum 109 grain ammo and I was sitting on a bench and shooting off the bipod. Out of 10 shots, only 4 or so would group tight (sub .75") then the rest would form a 2" group. Needless to say I was very disappointed.
I came to the conclusion that the Texan is very hold sensitive. From the moment you pull the trigger to the time the pellet leaves the barrel is significantly longer than a power burner. It's similar to archery, after hitting the release, you need to make sure you follow through with the shot. Knowing this, I still was unable to get better groups. From what I've read about the .257's out there, the Airforce platform has significant flex and a stock such as the mad dog stock has been reviewed to fix the issue.

I've already made a custom barrel tension setup, so I think I will go with the route of making a light aluminum frame which will hopefully stiffen the Texan, replace the tank mounted butt plate and finally add a mono pod.

Monkeydad1969 · « **Reply #27 on:** March 16, 2018, 11:57:57 PM »

Eagerly awaiting your results.

Joe

Alan · « **Reply #28 on:** March 17, 2018, 03:47:55 AM »

Group consistency is highly reliant on the way a weapon is held.

Years ago, I was fairly good friends with Tom Brown, who used to do the ballistic and pressure testing for Hodgdons. From him, I learned that group size sometimes enlarged when shot from a machine rest which seems counterintuitive. And from my own experience of recent, my WAR Cobra shoots higher groups when supported by my bean bag rest, compared to using the bipod and rear rest—a reasonable and intuitive conclusion.

In your case, I suspect with more range time, you'll find a rest scenario when will maintain good groups. Of course, tuning is also as issue, just like it is for powder burners. And by the way, no matter how it was held, ≈3/4 inch groups at 130 yards is nothing to sneeze at!!

rsterne · « **Reply #29 on:** March 17, 2018, 08:19:57 AM »

Good results, but it sounds like possibly your rock solid barrel is moving around in the frame, relative to the scope....

Bob

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