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Author Topic: chrony numbers for .308 Texan  (Read 282 times)

Steelhead

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chrony numbers for .308 Texan
« on: December 18, 2017, 07:10:46 AM »
Interesting results when shooting yesterday with the chrony. I shot two different bullets, Hunter Supply 134 gr. s.p. and Nielson 122 h.p.  I began with a  2900 to 3000 fill on the gun for the first round with Nielson ammo and the first shots were horrifyingly slow (350 fps). I kept shooting through the chrony until the air pressure dropped enough to start getting decent shots and then I started recorded the numbers. I tried to read the psi of the gun as best as I could at each shot.

Nielson 122 gr. hollow point
544 @ 2600
585 @ 2550
606 @ 2500
625 @ 2450
638 @ 2400
650 @ 2350
656 @ 2300
706 @ 2250
768 @ 2200
742 @ 2150
788 @ 2100
772 @ 2050
787 @ 1950
740 @ 1900
749 @ 1850
722 @ 1750

Hunter Supply 134 gr. s.p.
609 @ 2500
660 @ 2450
673 @ 2400
695 @ 2350
723 @ 2300
764 @ 2200
754 @ 2100
763 @ 2000
768 @ 1950
768 @ 1900
744 @ 1800
731 @ 1700

I didn't run equal amount of shots because I wasn't interested in the WHOLE curve. Otherwise I'm just wasting bullets. Of course the beginning and end are somewhat useless and my goal was to find the optimum zone. I was quite surprised to find that zone was lower than I would have expected. Right at around 2000 psi is the happy zone for sure. I had previously thought that it was about 2400, so this was revealing. Once I adjusted my supply tank to 2000 and tethered it my results down range were nice and consistent with the best ballistics I've has yet.

At any rate, I'm always seeing 'fill it to the max', 'turn power wheel all the way up', etc., etc., and I'm wondering how these guns shoot at all set up like that. It's no wonder that guys say they have issues. I did clean my barrel after fifty or so shots and I could see a small amount of lead in the rifling (again using the borescope). Neither bullet seemed to make a big difference on lead deposits. Prior to the chrony, I had found that both of the bullets seem to perform the same as far as ballistics go, and the numbers confirmed that. Hopefully this helps anybody else looking for tuning info. That shooting session just advanced me a ton and should save me a lot of $'s in wasted bullets.



  • Petaluma, CA
Airforce Texan .308
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Alan

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Re: chrony numbers for .308 Texan
« Reply #1 on: December 18, 2017, 08:53:40 AM »
Did you note any accuracy during the runs?
  • Roswell, New Mexico
Alan

I have an Omega compressor. If you're a fellow Guild member, and you pass through Roswell, NM, I'll fill your portable tank as a courtesy.

Aceflier

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Re: chrony numbers for .308 Texan
« Reply #2 on: December 18, 2017, 02:22:27 PM »
Texan valve lock strikes again. Upgrade your spring and you should be over 1000fps with that weight at 3k fill. Or at least close to it.
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Steelhead

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Re: chrony numbers for .308 Texan
« Reply #3 on: December 18, 2017, 03:45:14 PM »
Alan,

Accuracy was impossible due to the huge difference in fps and also the logistics of chrony placement/range layout. However, after the run I tethered up at 2000 and the pics below are where I was at.

Aceflier,

I don't really know anything about the valve lock thing. I will look into it and feel free to offer some insight...I'd appreciate it. I will say this, at 3000 psi it sounds like the gun is busted. As the air pressure drops the shots get cleaner (and louder), the speed goes up, and the recoil increases.
  • Petaluma, CA
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rsterne

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Re: chrony numbers for .308 Texan
« Reply #4 on: December 18, 2017, 06:09:28 PM »
As tuned, the gun is obviously lacking in hammer strike.... If you can get the velocity peak to increase up to around the 2400-2500 psi point, you should get a significant increase in FPE.... roughly proportional to the pressure difference.... Right now you are getting a peak of  168-175 FPE at 1900-2100 psi.... If you could move the peak up to 2500, you should get about a 25% increase, to just over 200 FPE.... The bullet weights you are using are about right for 200-250 FPE.... With enough hammer strike you should be close to 250 FPE at 3000 psi, but that wouldn't be a usable string, tuned for that power the velocity would likely drop with each shot....

Bob
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Steelhead

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Re: chrony numbers for .308 Texan
« Reply #5 on: December 18, 2017, 06:20:57 PM »
I'm listening and absorbing. So Bob, with the hammer strike change would bring my FPE up. If I had a shot fall off in velocity with each shot I wouldn't be concerned because I'm bench shooting tethered. My velocity should stay consistent for a longer shot string.  Am I correct in that assumption? 
  • Petaluma, CA
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rsterne

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Re: chrony numbers for .308 Texan
« Reply #6 on: December 18, 2017, 07:23:46 PM »
When I tune a gun for shooting tethered, I tune it for a short bell-curve shot string starting at the tethered pressure with a very low ES, not more than 2%.... A 2% ES allows shooting up to 100 yards without having any concern about vertical stringing of the POI.... As an example, you might be able to tune for a string of 5 shots, starting at 3000 psi, with velocities of 850, 860, 865, 860 and 850 fps, and after that string the pressure might be down to, say 2400 psi.... When you tether at 3000, you will shoot consistently (usually within about a 1% ES) at about 850 fps.... If you remove the tether, then you can shoot 5 shots without worrying about the velocity dropping off....

The advantage to tuning this way is that when on tether you will be shooting at the most efficient part of the shot string.... Unregulated PCPs use less air per shot at the beginning of the string than at the end, because at the beginning you are using smaller sips of 3000 psi air, while at the end you are using larger gulps of 2400 psi air, to get the same power.... Tuning for a decreasing shot string is generally inefficient.... and if you really push the velocity to the maximum on the first shot, you may have the valve open after the bullet leaves the muzzle, and of course all the air released by the valve after that point create no extra power.... it is totally wasted.... As the pressure drops, and the velocity follows, you use even more air for the following shots, meaning that they are even less efficient, delivering less power from more air....

I had a DAQ Exile .308, which was the opposite of your gun, the first shot was the fastest, and it was a real air hog.... From a 3000 psi fill I got only 3 shots, of 836, 786 and 742 fps, and ended with only 1800 psi when stock.... I reduced the hammer strike and from the same 3000 psi fill I then got five shots of 824, 833, 832, 811 and 792 fps and ended with 2000 psi.... I used less air for 5 shots that it did stock for 3 shots.... and my 5th shot was faster than the 2nd shot in stock tune.... The efficiency increased from 0.38 FPE/CI to 0.88 FPE/CI....

Bob
  • Coalmont, BC

caniborrowsomeammo

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Re: chrony numbers for .308 Texan
« Reply #7 on: December 18, 2017, 09:11:47 PM »
Texan valve lock strikes again. Upgrade your spring and you should be over 1000fps with that weight at 3k fill. Or at least close to it.
Got any numbers on the spring that is needed, L x O.D. x Dia.?
Buncha BB guns that I don't get to shoot as much as I like.

Aceflier

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Re: chrony numbers for .308 Texan
« Reply #8 on: December 19, 2017, 06:22:55 AM »
I measured my stock spring awhile back and posted on the big bore section. I have the African air ordinance medium spring in my .45 but I’m not sure about the .308 size. Someone would have to take it out and measure.
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Steelhead

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Re: chrony numbers for .308 Texan
« Reply #9 on: December 31, 2017, 12:06:03 PM »
Thank you all for the great advice. It's helping a ton. Here's what I've done chronologically:

Ordered a set of two upgrade springs from AAO

Contacted Airforce about the power issue, warranty requirements, etc.  Their response was that the .308, .357, and .45 all behave differently tune-wise. Their suggestion was to use the tuning wheel at max power as according to them that's what the gun likes. I have to agree as my fps went from the high 700's at 2000psi to 927 at 3000psi. I have a fairly big drop off in fps each shot, but that's not the most important thing for me right now. Finding the power, greatly increasing my range, consistency, and accuracy is very gratifying. Staying on my barrel cleaning and getting the tune figured out has made shooting this thing a lot more fun. Now instead of wondering where my shot is going to go I'm picking apart 3" spinners at 200 yards.

Thanks again and Happy New Year!

Kevin

  • Petaluma, CA
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