Keeping gun in tune.

Beautiful Bryan, beautiful! Thank you from the bottom of my heart. :)

Well guys, there you have it; I rest my case. It is yours to decide what is true.

This thread has provided some priceless information and generated some great discussion.

I am especially grateful to Russel M., for initiating the thread and to Jim Borden and Bryan Litz for graciously sharing with us, their time, knowledge and experience. :)

Keep up the good work guys. Through sharing of knowledge, discussion of the issues in a civil manner and working toward a common goal for the benefit of all with no concern for who gets the credit, there is nothing we cannot accomplish. :D

It's been a great day! Good night to all.

Later,

Gene Beggs
 
Bryan I think you nailed something.

Could we hypothesize: "Variations in rifle tune are substantially proportional to the summed effects of supersonic drag"?

Any open-minded aero-physicist amongst us willing to work the math and attempt to account for approximately an X vertical point of impact change for a 10 degree temp differential at 100 yards and 2X change for 20 degrees, given the normal 6PPC parameters and match conditions?

If this holds, emperical evidence would suggest X is approximately .120".
 
Velocity verus Temperature

I have data on velocity versus temperature and it is about 1 fps per degree fahrenheit. That is why Hodgdon developed the Extreme powders. They still vary with temperature but not as much. That data is based on pre-loads and then shot in different temperatures over a chronograph.

there are other sources for photographs of the blow-by for center-fire rifles besides Vaughn's book and I will be looking those up to share here when I return.

My understanding of air flow and thermodynamics comes from more than my experiences with shooting and building rifles. I appreciate the information that Bryan has shared and will study it.

My concern has been that shooters are trying to become to "heady" about tune versus looking at some simple experiments of what helps us stay in tune. Jackie Schmidt has mastered that aspect and has been an inspiration of the use of tuners. He deals with the result versus the "why". It is important t for us to understand the "why". Yet getting into heady theories without backing up with data and calculations only provides a basis for us to visit further.

As for humidity effecting powder----try this if you have a loading trailer. set up the chronograph and load in trailer with air conditioner off on stable exterior temperature day. Chrono those loads. then turn up the air conditioner with your powder measure in the trailer and let the air conditioner suck the moisture out of the air. Load some more cartridges. then take them outdoors and let them come to the ambient temperature. Now chronograph those.

I will be gone until 10/13--will share more after that.

I have enjoyed this exchange--it has challenged thinking which is the way we learn

Jim
 
Tune Curve

CubCouper said:
Bryan I think you nailed something.

Could we hypothesize: "Variations in rifle tune are substantially proportional to the summed effects of supersonic drag"?

Any open-minded aero-physicist amongst us willing to work the math and attempt to account for approximately an X vertical point of impact change for a 10 degree temp differential at 100 yards and 2X change for 20 degrees, given the normal 6PPC parameters and match conditions?

If this holds, emperical evidence would suggest X is approximately .120".
Click to expand...

Might want to look at chapter 4 of Vaughn's book and look at the graph. 10 degree temp change can mean 10 FPS change in velocity and if you have the rifle "tuned" at the wrong place on that curve, you will get vertical.
 
Chisolm said:
Gene,
I am curious if the change in tune correlates with the Mach number of the bullet or is it just coincidence that both are affected by temp.

James
Click to expand...

James, my initial response to your question was, "I don't have a clue." I have since deleted that post and after studying the post by Bryan Litz, I can see that tune and the speed of sound are both very much affected by outside, ambient air temperature. I don't understand exactly how or why at this time, but I'm sure we will eventually get to the bottom of it.

Later,

Gene Beggs
 
My guess is, changing external air density not only affects a barrels ability to move, but its forced vibration pattern.
 
JJ-IA said:
My guess is, changing external air density not only affects a barrels ability to move, but its forced vibration pattern.
Click to expand...

JJ,

I disagree, I know for a fact that air density does not affect barrel vibration. Hopefully someone with more credibility will confirm this. :)

Gene Beggs
 
Gene Beggs said:
JJ,

I disagree, I know for a fact that air density does not affect barrel vibration. Hopefully someone with more credibility will confirm this. :)

Gene Beggs
Click to expand...

I’m just guessing out loud Gene.
It may not be a factor worth worrying about when it comes to tune. But I’m not sure anyone will convince me that air resistance and changing air density isn’t a factor when something moves in (against) air.
Wonder what an adjustable trim-tab would do at the end of the barrel, or a pair of mid-barrel wings?

Time to get the snow-blower out of storage. :D
Jim
 
JJ-IA said:
Wonder what an adjustable trim-tab would do at the end of the barrel, or a pair of mid-barrel wings?

Good idea Jim, I'll try it and let you know. ;) What do ya' think about adding an electric servo with a switch on the joystick of the Farley; DA goes up trim tab goes down. Of course this will all be tied in by computer to the Wind Probes. :cool:

Later,

Gene Beggs
Click to expand...
 
Gene Beggs said:
JJ-IA said:
Wonder what an adjustable trim-tab would do at the end of the barrel, or a pair of mid-barrel wings?

Good idea Jim, I'll try it and let you know. ;) What do ya' think about adding an electric servo with a switch on the joystick of the Farley; DA goes up trim tab goes down. Of course this will all be tied in by computer to the Wind Probes. :cool:

Later,

Gene Beggs
Click to expand...

I’m not trying to argue or be funny Gene.
I was just trying to give an example of how I‘m thinking, like making your existing tuner design twice the diameter while keeping it the same weight. If the tune changes compared to the original design, it should be because of increased air resistance during recoil.
IMHO, a countdown timer should be the only electronic device allowed on or in front of the bench.
I was hoping the electronic windflag thread from last year would give a better feel for how other people view electronics on the line, but the thread fizzled quickly. This is all way off topic so I’ll stop here and allow the thread to get back on topic.

Phoenix nationals,
I wish!!! I understand the dedication needed to be good at this game. But with only one registered CF/BR group match within traveling distance this year, my CF budget was diverted into shooting monthly club rimfire matches (every single month! :)) just for fun with my father and a couple of friends.
But don’t be surprised if you see me at the SS or Nationals in a few years, if I didn’t believe returning to CF/BR was a future possibility, I wouldn’t be reading this forum. ;)

Jim
Click to expand...
 
Last edited:
I'm going to Phoenix guys. I'm two days late and a hundred dollars short but I'll be there in time to do a little shooting and visiting. Visiting with friends and talking benchrest is the best part of it all anyway. Arnold Jewel was once heard to say, "The matches are just something you have to endure in order to justify playing with all the fun stuff." Do you remember saying that AJ?:D

Later,

Gene Beggs
 
Gene,
I cannot believe that the air density in a barrel is
never changing. After setting up, bolt open, no doubt the air
within the barrel is the same as outdoors. Ten or twelve shots
later the barrel is hotter. The column of air with in the barrel
will be heated rapidly. A typical barrel 22 " in 6mm will have
around 16 sq inches of surface area inside ,not counting the chamber.
With the chamber sealed as it would be with a close fitting
shell, the air escaping to the atmosphere is not fast, allowing
the air within that barrel to heat the barrel quickly.
Temperature and alltitude do affect the frequency that
most materials resonate at. Same as the speed of sound. All
metals are excited by a frequency. It is even likely that your
barrel is excited when your neighbors gun goes off. Or possibly
when a plane fly's over. That barrel being supported at one
end is subject to everthing that vibrates and to a lesser
degree harmonics
 
I want to appalagize for abandoning the thread I started. My German Sheppard had a stroke on wed. & I had to put him to sleep on fri. He was the son I never had.
I thank everyone for the help in understanding. Everone was a great help & was glad to see that everbody could agree to disagree. I know some have left for the Nationals. A special thanks to Genn,Jim & Bryan.
russell m
 
Russell,

russell m said:
I want to appalagize for abandoning the thread I started. My German Sheppard had a stroke on wed. & I had to put him to sleep on fri. He was the son I never had.
I thank everyone for the help in understanding. Everone was a great help & was glad to see that everbody could agree to disagree. I know some have left for the Nationals. A special thanks to Genn,Jim & Bryan.
russell m
Click to expand...

Sorry to hear about you beloved companion. Dogs are amazing, they become part of the family!

It is never easy.

And thank you for this wonderful thread, been one of the most interesting I have read on this sight and still more to come.

I have a surprise experiment I am doing in our tunnel Sunday that will hopefully show the air movement in the barrel as it heats up. I will post the pictures Sunday.

Best wishs,

Randy
 
Thank You Randy. He was my best friend.
The biggest thing I have learned that I can take to the range is that I am never more than .6 grains out of tune. Or 1 click on my Harrel's dispensor. Please correct me if I am wrong. Looking foreward to the pics. russell m
 
bsl135 said:
A few things that probably aren't the cause:
1. 'ambient' properties of the air inside the barrel. The air in the barrel is conditioned by the steel which becomes increasingly heated after each shot. The air inside the barrel is markedly different than the air outside the barrel, and tune is dependent on the air properties outside.
2. shifts in muzzle velocity due to ambient temperature. Gene has cited proof that the muzzle velocity remains constant as the ambient temp and tuner setting change.

Just a few back-of-the-envelope calculations to reinforce your point #1: the difference in mass of the air inside a 0.308” barrel 22’ long with a change in temperature from 20C to 30C is about 0.016 grains. Thus the increase in muzzle velocity and reduction in time that the bullet spends in the barrel, if the mass of the air is the dominant factor, should be about the same as that if the bullet were 0.016 grains lighter. That is 0.014% difference for the 118 grain bullets that I shoot. But Jim has observed a change of about 18 fps (converting C to F), which would be 0.6% difference starting from 3000 fps. That is a change 43 times larger than the difference estimated for the air mass, indicating that air mass is probably not the right parameter to focus on.

Another quick check: The density of air is 3.2% less at 30C than at 20C. Since dynamic backpressure is proportional to density, backpressure goes down by the same percentage, about 6.6e-6 psi. But the bullet is driven by the difference between the chamber pressure and back pressure. With chamber pressure at about 50,000 psi, the change in driving pressure is only about 1.3e-12%. Again, air mass is not the right parameter.

Incidentally, the effect of the air outside the barrel on backpressure is much less than the air inside the barrel, so that parameter isn’t the right one, either.

Gene’s contention that muzzle velocity does not change with ambient temperature is contrary to Jim’s data, and with the experiences of others, I would expect. This mechanism seems like the obvious one to cause the change in tune. The increased mean muzzle velocity and decreased mean barrel exit time due to an increase in temperature cause the group of shots to fall on a less favorable part of the barrel vibration curve, resulting in an out-of-tune condition. Moving the tuner alters the barrel vibration curve, moving the shots back to the favorable upward slope and restoring the tune.

Cheers,
Keith
Click to expand...
 
Keith,
Thanks for your thoughts.
I agree that a change in muzzle velocity with ambient temperature would very easily explain the adjustment in tune. Since I've never studied this phenomena first hand, my theories were based on the facts presented by Gene and the others. One of the assumptions was that muzzle velocity didn't change with ambient air temp, so I formulated my theories based on that. If muzzle velocity does change with air temp, then I completely agree with your description.

One thing to consider about muzzle velocity though...

If muzzle velocity does change with air temp, lets say 1 fps per degree F. Then you have a 20 fps difference in average muzzle with a 20 degree change in air temp (which is pretty extreme for a given day).
Here's my question: What's the extreme spread of muzzle velocity?
What I'm getting at is this:
Let's say the tuner is set 'properly' for a given ambient air temperature, which happens to correspond to 3400 fps mv. A +20 degree change in air temp would require a different tuner position, which corresponds to 3420 fps. BUT, even when the average mv is 3400 fps, you're likely to have shots as high as 3410-3415, and as low as 3385-3390 fps.
What I'm getting at is: the nominal variation in 'shot-to-shot' muzzle velocity is about the same amount as a shift in average muzzle velocity between two temperatures.
In other words, if the tuner setting is 'right' for 3400 fps, but 'wrong' for 3420 fps, what about the 'fast' shots that get up to ~3420?
Am I over stating the amount of velocity spread you guys achieve? If everyone is getting extreme spreads of 5-10 fps, then I can see how it works but that seems a bit unlikely to me.

Thanks again for your thoughts, this continues to be an interesting thread.
-Bryan
 
bsl135 said:
Am I over stating the amount of velocity spread you guys achieve? If everyone is getting extreme spreads of 5-10 fps, then I can see how it works but that seems a bit unlikely to me.
-Bryan
Click to expand...

Bryan,
The 6PPC shooters can chime in, but I have seen ES < 10 fps claimed. In some recent tests, my 30BR did ES = 34 fps for 15 shots. Compare this to the size of the sweet spot window in VarmitAl’s simulations of about 50 fps with a tuner. If one had the tuner tuned exactly to the middle of the window, it would be possible to forego tuner adjustment for up to a 20 fps change (20F temperature change with 1F = 1fps) with the 6PPC, whereas my 30BR would need an adjustment after anything greater than an 8 fps (8F) change.

All you tuner users out there, have you tried correlating your tuner settings to muzzle velocity windows? Do you need fewer adjustments with low ES? If there is a strong correspondence of velocity to ambient temperature, either one would work to gauge tune, but velocity may be the underlying cause.

The question could be answered by testing at the same velocity across different temperatures, and at the same temperature across different velocities.

Cheers,
Keith
 
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