Range Report Theoretical Look at POI vs. Barrel Temp

[denton]

There is an argument raging about the effect of barrel temperature on POI. That's taking place on a paid thread that I choose not to join. But maybe the following comments will add some light to the discussion:

Guns do what guns do. Sometimes we can do a pretty good job of predicting that with mathematical models. At least a little bit of modeling will tend to get us into the right ballpark, and cut down on the amount of bench time it takes to investigate an idea. It also helps us set reasonable expectations about how much work it will be to extract the information we want.

There are at least two mechanisms by which barrel temperature affects POI. One is that it affects the time the bullet is in the barrel, therefore changing the bullet's exit point vs. barrel whip. The other is that barrel temperature affects pressure and muzzle velocity, raising POI as temperature goes up, just by simple external ballistics.

The first of these is very hard to model. The second is fairly easy. So I'm going to neglect the first, and just talk about the second. In real life, what we'll see is the two effects acting combined.

Barrel temperature does affect pressure and muzzle velocity. I've very thoroughly demonstrated that in two published articles. Ambient temperature matters only in that it affects the temperature of steel, lead, and brass.

Using the RSI exterior ballistic software, we can estimate the trajectory of a projectile. It's well known that the simple G1 model isn't perfectly accurate, but it will give us a good estimate to start with. A Sierra 180 grain SBT with a BC of .407 and MV of 2800 FPS will impact at -45.43" at 500 yards.

Raising the barrel temperature will change the MV, and for a 20 degree F increase, 34 FPS is at least a decent estimate of the change we might see.

Adding 34 to 2800 FPS, we get a new MV of 2834 FPS. This projectile is estimated to impact at -44.21 FPS, 1.22" higher than before.

To me, the interesting question is, how hard would it be to detect this shift? The answer depends on how good the rifle is.

A 1 MOA system is going to scatter shots over about a 5" circle at 500 yards, assuming a perfect shooter and still air. The shots will have a standard deviation above and below the mean POI of about 2.5". Using the Power and Sample Size function in QuikSigma, we can determine that with a one-tailed T Test, we can detect a 1.22" shift with two samples of 73 shots each, one group shot at temperature A, and the other shot at temperature A + 20 F.

A 1/2 MOA system will have a vertical standard deviation of about 1.25", making the vertical shift detectable with two groups of only 19 shots each. A 1/4 MOA rifle gets us down to samples of only 6 shots each.

So does barrel temperature have an effect on POI? Absolutely. But it's not a huge effect, and you'd have to shoot a very carefully controlled experiment with quite a few shots in order to detect it, unless you're shooting benchrest competition at long distances.

Attempts to demonstrate the effect with samples of only 5-10 shots with a 1/2 MOA rifle are likely to end in frustration.

Also remember, we have considered only one of at least two effects at work here.

 

[Lowlight]

Re: Theoretical Look at POI vs. Barrel Temp

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Raising the barrel temperature will change the MV, and for a 20 degree F increase, 34 FPS is at least a decent estimate of the change we might see.</div></div>

Lindy on here experimented with temperature and found it to vary between .6fps and 5fps per 1 degree of temperature change... so you have to test your own loads and rifle to see, because a 20 degree change in Temp may only mean 10 to 12fps change in MV and not 34fps.

 

[Lowlight]

Re: Theoretical Look at POI vs. Barrel Temp

Also I would like to point out, chronograph results, I pulled these from a thread on here, to demonstrate.

But if temperature was linear, so that each degree "increased" MV as theorized, these results would not be possible.
1 - N/A
2 - 2578
3 - 2510
4 - 2579
5 - 2553

The 5th shot is slower than the 2nd, by all accounts the temp should be higher.

Some others pulled to demonstrate:

46.1grs 2804 2802 2796 AVG 2800

46.4grs 2821 2850 2819 AVG 2839

46.7grs 2844 2853 2844 AVG 2847

47.0grs 2861 2853 2851 AVG 2855

47.3grs 2890 2872 2870 AVG 2877

My own results have shown the same, as well as anyone who chronographs a rifle will know, it's never linear but fluctuates.

So, how do you map that... simply by your SD and ES, because linear methods don't hold true.

 

[Lindy]

Re: Theoretical Look at POI vs. Barrel Temp

The first post in this thread is by Denton Bramwell, who knows way more about this subject than I do.

Here's a link to some information about him, and some articles he's written:

http://www.shootingsoftware.com/tech.htm

The last word on this subject has not been written - but he's certainly been exploring the questions.

Oh, and Denton - the article "Powders, Primers, and Pressure, Part 2" is not linked on that site. I have a personal copy of the PDF file of it, which was sent to me by another Hide denizen. If there is a copy of it available on the web, perhaps you could post the link here, because it's highly relevant.

 

[Lowlight]

Re: Theoretical Look at POI vs. Barrel Temp

Well its nice for him to stop by, especially to explore the question of shift.

I don't think the question has been answered either...

 

[denton]

Re: Theoretical Look at POI vs. Barrel Temp

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">found it to vary between .6fps and 5fps per 1 degree of temperature change</div></div>

That agrees with my results. So I heartily agree that you have to test your individual load. I just used 1.7 FPS as an estimate to see what ballpark we're playing in.

 

[denton]

Re: Theoretical Look at POI vs. Barrel Temp

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">If there is a copy of it available on the web, perhaps you could post the link here, because it's highly relevant.</div></div>

At the moment, I don't have a convenient place to post those on the web, but I have no objection if someone else wants to. If someone has a spot to put them, I'll be glad to email the PDFs.

The last word has indeed not been written on the subject. Only The Almighty has pure truth. The rest of us have to put up with estimates.

 

[Lowlight]

Re: Theoretical Look at POI vs. Barrel Temp

I would be happy to host them for you, if you like...

If you like, I can add a blog entry and link the files. You can even write the entry and the intro.

 

[denton]

Re: Theoretical Look at POI vs. Barrel Temp

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">So, how do you map that... simply by your SD and ES, because linear methods don't hold true. </div></div>

That's a very good question.

The model you have to use is that the effect we want to see is mixed with a lot of statistical noise that comes from the normal random variation in MV, due to minor differences in neck tension, bullet weight, powder charge, etc.

The mathematical technique for extracting the effect from the background noise is called regression.

If your regression is successful, you'll see points randomly scattered above and below the central trend line. The trend line is the effect, and the scatter above and below is the normal random variation.

Same thing partly accounts for the variation between maximum published loads for the same components.

 

[The Mechanic]

Re: Theoretical Look at POI vs. Barrel Temp

How much of that has to do with the round warming the powder. Given that brass and or copper conducts heat well it seems that it would require mere seconds to elevate powder temp in a warm or hot chamber. It would be interesting to maybe on a test gun cool the chamber quickly with CO2 to get the chamber temp down and shoot while the barrel itself is still warm. Of course scientific info only has no real life use. just curious.

 

[Lowlight]

Re: Theoretical Look at POI vs. Barrel Temp

Nice try Brown Dog...
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Brown Dog</div><div class="ubbcode-body">I don't join these scientist vs flat earth ballistician debates any more.

I can't even bring myself to address the contention that charge temp doesn't have a linear relationship with burn rate

</div></div>

But here you go again, bringing yourself to the table...

by the way, I agree with what you are saying, my problem has always been the "rule of thumb" crowds as well the guys who "model" things that can't be verified in the field. Especially stuff that doesn't hold true across the board, how 1 person is using 1.5 MOA for Spin Drift and another is using nothing but both hit the same target the same amount... so clearly SD is not the issue, at least in my opinion. However that is another post.

Then again, youre not in this thread discussing this topic or any others are you.

 

[Brown Dog]

Re: Theoretical Look at POI vs. Barrel Temp

Good keyboard drills. You posted before I even had a chance to delete the post -it can only have been up for a couple of minutes before I decided to withdraw it

Well, since I got pinged, I'll put back in the main part of what I offered:

Terminology: You're all saying "barrel temp" when you actually mean "charge temp". Increasing barrel temp alone actually <span style="font-style: italic">decreases</span> mv.

As before, have fun

 

[The Mechanic]

Re: Theoretical Look at POI vs. Barrel Temp

Maybe one of them are using controlled spin bullets?

little joke, OK not that funny, I'll stop


<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lowlight</div><div class="ubbcode-body">
how 1 person is using 1.5 MOA for Spin Drift and another is using nothing but both hit the same target the same amount... so clearly SD is not the issue, at least in my opinion. </div></div>

 

[Lowlight]

Re: Theoretical Look at POI vs. Barrel Temp

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Brown Dog</div><div class="ubbcode-body">

Good keyboard drills. You posted before I even had a chance to delete the post -it can only have been up for a couple of minutes before I decided to withdraw it

Well, since I got pinged, I'll put back in the main part of what I offered:

Terminology: You're all saying "barrel temp" when you actually mean "charge temp". Increasing barrel temp alone actually <span style="font-style: italic">decreases</span> mv.

As before, have fun

</div></div>

ya, you have to be quick, I have a big screen with a lot of real estate to keep the browser open, plus tabs.

As I said, i agree with you on your point, not that you need me to agree with anything you say, I'm just saying.

 

[The Mechanic]

Re: Theoretical Look at POI vs. Barrel Temp

explain? I can see both ways. Barrel temp goes up barrel expands gets bigger, less drag, but that also means less sealing? unless internal dimension decreases but that means sealing would go up? Just seems like way to many variables to account for to accurately predict.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Brown Dog</div><div class="ubbcode-body">
Terminology: You're all saying "barrel temp" when you actually mean "charge temp". Increasing barrel temp alone actually <span style="font-style: italic">decreases</span> mv.

As before, have fun

</div></div>

 

[denton]

Re: Theoretical Look at POI vs. Barrel Temp

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">How much of that has to do with the round warming the powder.</div></div>

Apparently not much, even though much of the literature assumes this.

The full articles will be available by link pretty quick, I think. They explain it in more detail than I can here.

The short version is that if you stick a thermocouple up the primer hole of a cartridge, and watch the internal temperature change, you see that it takes a matter of minutes for the internal temperature of the powder to change. Whatever the mechanism is that changes MV in response to temperature, the change happens much, much faster than that.

This physical observation is inconsistent with powder temperature being the main mechanism. So powder temperature is likely ruled out.

Another candidate is primer temperature. So I squirted the rear end of cartridges with freeze mist, and very quickly closed the bolt and fired. The effect on MV was negligible.

That pretty well rules out primer temperature as the main actor.

Barrel temperature near the receiver tests as the strongest variable influencing MV, with ammo temperature being second in importance.

Looking at all the possible mechanisms, the best fit to the available data is this: Cold barrels, cold brass, and cold lead rob heat out of the propulsion gas more than their warm counterparts. Even though the gas is in contact with the metal for only a very short time, it is also true that fairly small amounts of heat transfer are enough to make a measurable difference in MV.

 

[Lowlight]

Re: Theoretical Look at POI vs. Barrel Temp

here is the link to the articles, I will clean up the page later, but this will get you started

Denton Bramwell Articles

 

[denton]

Re: Theoretical Look at POI vs. Barrel Temp

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">here is the link to the articles, I will clean up the page later, but this will get you started

Denton Bramwell Articles </div></div>

Thank you. That makes things much better.

 

[The Mechanic]

Re: Theoretical Look at POI vs. Barrel Temp

Seems odd that powder manufacturers seem to believe that powder temp means changes in velocity. They do have a lot of money to test these things.

thanks for the .pdf's Frank I have some reading to do

 

[Lowlight]

Re: Theoretical Look at POI vs. Barrel Temp

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: The Mechanic</div><div class="ubbcode-body">Seems odd that powder manufacturers seem to believe that powder temp means changes in velocity. They do have a lot of money to test these things.

thanks for the .pdf's Frank I have some reading to do</div></div>

it is what Brown Dog is saying, that temperature differences effect the burn rate of the powders, so hypothetically if you you say the powder burns at 100 degrees, and the powder is already 90 degrees it will burn at a different rate than a powder that is at 20 degrees which needs to build to that 100, or something like that... the linear part BD was saying above.

 

[The Mechanic]

Re: Theoretical Look at POI vs. Barrel Temp

Group therapy is a great read. Couldn't agree more.

 

[denton]

Re: Theoretical Look at POI vs. Barrel Temp

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Group therapy is a great read. Couldn't agree more.</div></div>

Thank you for the kind words.

 

[Supersubes]

Re: Theoretical Look at POI vs. Barrel Temp

Anybody measured how much a bullet swells if you heat it to two or 300 degrees? If it grew while sitting in the freebore that would cause more pressure

 

[Supersubes]

Re: Theoretical Look at POI vs. Barrel Temp

A friend and I had a circumstance at 600yds the other day with his 7wsm. The load and rifle were working great. Anyway he chambered a round and then proceeded to mess with his bipod. About a minute later he fired and the round landed atop the berm about two feet high. Solid gun and a very solid shooter who hadn't missed the target with any of the previous shots. He was shooting factory ammo which had proven accurate and was later confirmed to have an ES of 12 fps for five shots. Anecdotal, but graffic for us at the time.

 

[McCrazy]

Re: Theoretical Look at POI vs. Barrel Temp

I was the shooter that Supersubes is referring to in his previous post.

Last week we were shooting my 7WSM and I put a 10 round string through it in fairly quick succession with all rounds having good POI vs. POA, but building a fair amount of heat in the rifle. I then chambered a cartridge and let it heat-soak in the chamber for about a minute or a little more. When I squeezed off that round it went about 2 feet over the top of the target which was at 600 yards. About 15 seconds after that I fired another round which impacted on target with the initial 10 bullets.

Seems to me that if barrel heat was the issue then the bullet that heat-soaked in the chamber for a minute+ while the barrel had a chance to cool should not have impacted so high (more than likely due to an increase in MV). Instead, the quick follow-up shot that wasn't heat-soaked but went down a warmer barrel and returned to a normal POI vs. POA seems to indicate it is a heat-soak issue at the chamber and not a barrel heat issue.

If it can be proven as Denton has mentioned that it takes several minutes of heat-soaking for the powder and primer to significantly heat up then that points to what Supersubes suggests with possibly the copper jacket and lead core of the projectile itself heat-soaking more quickly in the hotter leade/throat area and expanding against the bore and thereby raising the pressure and MV.

Thoughts, theories, suggestions????

 

[Brown Dog]

Re: Theoretical Look at POI vs. Barrel Temp

<span style="font-weight: bold">If you hear hoof beats; think horses not zebras.</span>

Denton,

Just read the first 2 articles. Well written and interesting stuff

.

Good use of statistical analysis but, I suspect, it may give a veneer of validation to the results that -perhaps- distracts from the home workshop level of scientific process in the underlying data sources (or testing processes).

I say that because some of the findings run counter to conventional scientific wisdom on the subject; and there isn't much room for alchemical-type debate on these matters.

<span style="font-weight: bold">I highlight in particular the 'finding' that rising barrel temp alone increases pressure. </span>

<span style="font-style: italic">Because this is the direct opposite of conventional learning on the subject; I would offer one observation :</span>

I believe that the experimental pressure 'measurement' you obtained is actually a mathematical inference drawn from the electrically-measured elastic behaviour of a polymer strip applied to the exterior of the barrel (correct ?).

I strongly suspect that the results obtained are actually showing the effects of the barrel temp on the physical characteristics of the polymer -and the resultant skewing of the mathematical inferences.

And thus, I suspect the graph could effectively be retitled "the effect of temperature on polymer elasticity"

It would be worth seeing whether you can find the same apparent correlation if you were to repeat the experiment and directly measure MV (rather than inferred strain) data.


Peer-review; not pissing

 

[CoryT]

Re: Theoretical Look at POI vs. Barrel Temp

McCrazy, two feet of elevation change at 600 yards is NOT a result of a change in muzzle velocity, unless you think you got an extra 500 fps or so. Bullet lead alloys melt at around 550 to 600 F. I rather doubt the gun got THAT hot. It's possible the bullet did finally get that hot during the firing event, and resulted in some core slippage, which could cause such an errent shot.

 

[denton]

Re: Theoretical Look at POI vs. Barrel Temp

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">I strongly suspect that the results obtained are actually showing the effects of the barrel temp on the physical characteristics of the polymer -and the resultant skewing of the mathematical inferences.</div></div>

Skepticism! A wonderful thing!!

You are quite right to question the thermal sensitivity of the strain gauge. I did exactly the same thing myself.

Strain gauges are commonly made to have temperature coefficients matched to either steel or aluminum. If you use one that is matched to steel, over the temperature range I've been working in the error is around 150 PSI.

So I think we can rule that out as the source of the rising pressure.

I've never heard it asserted that rising barrel temperature decreases pressure. In my conversations with Ken Oehler and with the head of Hodgdon's pressure lab, both clearly think rising temperature raises pressure. That is also what all my data indicate.

If you think about it from basic physics, we know that increased barrel temperature results in higher muzzle velocities. That increased muzzle velocity has to come from somewhere, else Conservation of Energy is violated. Increased pressure is the logical source.

 

[McCrazy]

Re: Theoretical Look at POI vs. Barrel Temp

Good idea Cory, but I was not trying to imply the bullet melted. Just possibly was allowed to heat up to the point of expanding enough to increase resistance and therefore pressure and MV going down the barrel.

Did you happen to plug the data into a ballistic calculator to arrive at the conclusion it would take a 500 fps increase in MV to get those results? Not sharpshooting, just wondering. We chronoed the load out of my rifle and have the BC of the projectile if someone wants to do it.

 

[Brown Dog]

Re: Theoretical Look at POI vs. Barrel Temp

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: denton</div><div class="ubbcode-body">Strain gauges are commonly made to have temperature coefficients matched to either steel or aluminum. If you use one that is matched to steel, over the temperature range I've been working in the error is around 150 PSI.</div></div>

I'm confused

the temperature coefficient of 'what' is matched? Resistance? Elasticity? Expansion? How are you measuring the 150 psi error -further mathematical inference?!

The only way to take the variable out is to repeat the experiment with something you can measure directly -and to measure it in a way that absolutely cannot be affected by the variable whose effect you are attempting to quantify (ie MV!)

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: denton</div><div class="ubbcode-body">If you think about it from basic physics, we know that increased barrel temperature results in higher muzzle velocities. .</div></div>

Not my understanding at all. I think the confusion comes from the incorrect but frequently freely interchanged civilian use of the term 'barrel temp' with 'charge temp'.
The basic physical change of increased barrel temp is to increase internal volume of chamber and bore. All other things being equal (and as long as we're clear we're talking barrel temp <span style="font-weight: bold">not</span> charge temp!)this brings a concomitant decrease in all things pressure related:ie burn rate, rate of pressure increase and therefore mv.

 

[denton]

Re: Theoretical Look at POI vs. Barrel Temp

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">the temperature coefficient of 'what' is matched?</div></div>

Well, that's an honest question that deserves an answer.

The temperature coefficient of the gauge factor of the gauge, i.e., the gauge's output vs. applied strain. You don't have to worry about the metal expanding or cooling and stretching or compressing the gauge because the electronics usually automatically re-balance the Wheatstone Bridge connected to the strain gauge, or signal you to do that manually.

A strain gauge is a long ribbon of metal film arranged on a small plastic substrate. By manipulating the characteristics of the metal film, you can match the gauge factor so that it is almost constant vs. temperature for various things you might want to attach it to.

The manufacturer takes care of the design and verification, and each batch of strain gauges has the information on the accompanying data sheet. The Oehler strain gauge system and the PressureTrace system both use the same model of gauge, which is matched to steel. If you're really curious, I could post a scan of the manufacturer's specification.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">The basic physical change of increased barrel temp is to increase internal volume of chamber and bore. All other things being equal (and as long as we're clear we're talking barrel temp not charge temp!)this brings a concomitant decrease in all things pressure related:ie burn rate, rate of pressure increase and therefore mv.</div></div>

That sounds plausible enough, until you put some actual numbers to it. The coefficient of linear expansion of steel is in the neighborhood of 12 x 10^-6. A half inch diameter chamber will therefore expand about 60 millionths of an inch for a 10 degree C increase in temperature. That's not enough to make any easily detected difference. At the same time, the bullet is expanding too, providing more base area for the propellant gas to work against, and hence higher MV. But once again, the effect is far to small to be of interest.

I have run many, many tests over the years with a thermocouple attached to a barrel just forward of the receiver. In all cases, hotter barrels produced higher muzzle velocities so long as I was careful to account for normal random variation. The chronograph does have its own temperature coefficient, as the quartz crystal in it warms and cools. But even a really cheap crystal has a tempco around 10 ppm/degree C, so, again, the effect is too small to worry about. That's .3 FPS out of 3,000 FPS for 10 degrees C if I haven't misplaced a decimal somewhere.

 

[CoryT]

Re: Theoretical Look at POI vs. Barrel Temp

McCrazy, that's exactly what I did, just estimated for the 7mm. I didn't mean you implied the bullet melted, I'm just offering a possible explanation in the form of core slippage.

 

[The Mechanic]

Re: Theoretical Look at POI vs. Barrel Temp

It would seem to me that any of the projo problems would give either a lower POI or a left or right POI. Correct me if I am wrong but seems that any failure to fly would make the bullet less aerodynamic causing lower bc causing lower POI. Many other factors can cause a high POI like higher MV, BC, or positional shooting failure, upwind, tailwind etc.
Flame away.

 

[Brown Dog]

Re: Theoretical Look at POI vs. Barrel Temp

Denton, thanks for the detail.

Might want to revisit your maths on this: <div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: denton</div><div class="ubbcode-body">

That sounds plausible enough, until you put some actual numbers to it. The coefficient of linear expansion of steel is in the neighborhood of 12 x 10^-6. A half inch diameter chamber will therefore expand about 60 millionths of an inch for a 10 degree C increase in temperature. </div></div> I'd SWAG a significantly bigger chamber diameter change than in your example....and in a world where we don't fire 0.311 bullets down 0.308 bores; the small dimensions are everything

..think 'shot start' pressure.

I'm still gaining the strong impression that you're including conductive heating of the round in your consideration of the effects of barrel warming on MV; eg this: <div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: denton</div><div class="ubbcode-body"> At the same time, the bullet is expanding too, providing more base area for the propellant gas to work against, and hence higher MV. But once again, the effect is far to small to be of interest. </div></div> Why do you consider the bullet to be expanding? in-chamber heating? That's not the effect I believe we're discussing. I'm talking of the effect of the barrel alone. If you're including conductive heating of the ammo in the equation then we're talking around each other!

Visualise a machine gun that has a quick change barrel and fires from an open bolt -such as a Bren gun. With ammo at a constant temp and firing a single round from the magazine in each instance, when will the highest chamber pressure be achieved -when the barrel has been removed, chilled to minus30degC and fired on refitting or removed, heated to 200degC and fired on refitting?
ie when is the highest chamber pressure -when the barrel has contracted to minimum internal dimensions or expanded to maximum?

As a common-sense check, worth noting the artillery maxim that cold guns 'fire long'. An effect known as 'cold gun effect' (not a particularly imaginative title!) and the process of subsequent rounds falling shorter on the gun-target line is known as the 'warmer effect'



That's enough from me on this

I enjoyed reading your articles.

 

[Greg Langelius *]

Re: Theoretical Look at POI vs. Barrel Temp

My take on all of this is that I'm very pleased to see Denton and others whom I respect weighing in on this.

IMHO, the crux if this discussion, and many similar to it, is the dichotomy between calculable and expected results, and the starkly noncooperative aspects of reality.

When it happens to me, my first response is to remind myself how big the world is and how small my grasp on its complexities. I know that when my expectations fail, it is my failure; Ma Nature is just doing Her things, same as always. Put differently, the Devil is in the details; and clearly, I'm missing at least some of the more subtle ones.

POI's can't be used reliably as an indicator of any single aspect of ballistics, like temperatures (of what? Barrel, propellant, the air; again..., what?). As bore transit speeds alter, so does dynamic harmonic response. Counting on the barrel to send the bullet in precisely the same direction (relative to the sight axis) every time, regardless of the conseqeunces of harmonic swing, is flatly ludicrous. It doesn't even occur when there are NO temperature variances (again, which temperatures?).

When you consider that the amount of normal dispersion under load variances that do NOT correspond to accuracy nodes can be several MOA, it is perhaps easier to see flies in the ointment. Combine this with the absolute certainty that propellant temp variances can drive cartidge peformance beyond the limits that correspond to an accuracy node, and maybe you begin to see my central point..., the one behind much of what I propose on this site.

If hitting the target is your goal, then be reasonable about how small a target you choose to defeat. If you manage to stay within reason, the task is simpler, and the really esoteric nuances need not be considered in great detail.

In other words, your world continues to turn in greased grooves.

If, OTOH, you demand what I might consider less reasonable goals; you condemn yourself to chasing goals whose attainment becomes irrevocably intertwined with hard to compute, harder to define, and perhaps hopelessly random factors.

Your grooves contain more than pleasantly sloppy grease; there be rocks and shoals down there, Matey...

And that is the point where we part intellectual company.

Greg

 

[denton]

Re: Theoretical Look at POI vs. Barrel Temp

Brown Dog...

I hope you understand that I'm having great fun with this, and am not being personal. Challenge and discussion are the tools of creation. It isn't necessary that we agree, but it is a lot of fun to discuss. Thank you for your comments.

Yes, I am specifically saying that the temperature of the steel in the barrel is a key influence on muzzle velocity, and that the hotter the steel gets, the greater MV will be. I am also saying, contrary to conventional wisdom, that the temperature of the powder is a minor factor. My tests in this area cover sporting centerfire rifles over a temperature range of about 40 F to 95 F. What demons live outside that space, I don't know. I can't speak to the effects on artillery, machine guns, or magnetic rail guns in frigid conditions or branding iron hot.

From what you've said, I think you and I would agree that making sure you can trust your measurement equipment is critical before doing tests. You went straight to questioning the gauge, and I think that is a very proper inquiry. I spent quite a bit of effort making sure that what I'm seeing is not a measurement artifact. The manufacturer's specs (which get routinely field tested by a bunch of customers) and a large body of my own data say that what I'm seeing is not a thermal error.

All my tests, and all that I've read, support the notions that loads developed in January will produce higher pressure and MV if they are fired in an August prairie dog hunt, and that in general higher temperatures lead to higher pressures and velocities, all other factors constant.

So far, I am unable to reconcile your statement with that widely observed and measured outcome, and I can't build a theoretical model that supports it either.

Thermal expansion of the barrel creates less additional volume than than a slight polishing would.

If powder temperature were the main actor, the change in MV would take place only after a cold cartridge had a few minutes in a hot chamber to reach equilibrium. What we actually see is a practically instantaneous change, before the powder has time to warm.

The only explanation I can come up with that explains everything I've observed is the one I've put forward in the article. If someone has a better explanation than mine, I will cheerfully kick out old notions for better ones.

In any event, I popped into this board because someone pointed out that someone was quoting my article, and had only the first half. I thought it would be helpful to offer something additional. I've enjoyed my stay, and thank you for your hospitality, but I probably won't be here long. My email is on file... glad to hear from any of you.

 

[The Mechanic]

Re: Theoretical Look at POI vs. Barrel Temp

My thought on that is as compensated for in the Paris gun that had progressively larger diameter rounds to compensate for wear. Not taking into account the fouling possibility of black powder in the "warm" artillery piece.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Brown Dog</div><div class="ubbcode-body">As a common-sense check, worth noting the artillery maxim that cold guns 'fire long'. An effect known as 'cold gun effect' (not a particularly imaginative title!) and the process of subsequent rounds falling shorter on the gun-target line is known as the 'warmer effect'



That's enough from me on this

I enjoyed reading your articles.</div></div>

 

[Brown Dog]

Re: Theoretical Look at POI vs. Barrel Temp

Denton,

Thanks too.

I'm still confused as to what you think you're isolating in your experiments. This statement is all about charge temp: <div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: denton</div><div class="ubbcode-body">All my tests, and all that I've read, support the notions that loads developed in January will produce higher pressure and MV if they are fired in an August prairie dog hunt, and that in general higher temperatures lead to higher pressures and velocities, all other factors constant.</div></div>

We'll both bow out gracefully.

 

[denton]

Re: Theoretical Look at POI vs. Barrel Temp

Separation of the powder variable takes place here:

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">If powder temperature were the main actor, the change in MV would take place only after a cold cartridge had a few minutes in a hot chamber to reach equilibrium. What we actually see is a practically instantaneous change, before the powder has time to warm.
</div></div>

It's been a pleasure!

 

[RoughNeck182]

Re: Theoretical Look at POI vs. Barrel Temp

Hey just curious what sort of affect heat would have on the stiffness of the barrel and if that would affect the amplitude of barrel harmonics by any significant amount. I've heard it said (can't remember where) that the best loads have small velocity variations and a barrel time which has the bullet leave just before top of the swing such that a slightly slow leaving bullet has a slightly higher angle and a faster leaving bullet is slightly low on the swing so that they are offset. Maybe a hotter barrel wags more at a lower frequency the way a guitar string under less tension does (higher amplitude, lower freq.). That would throw the barrel/bullet exit timing out (loss of accuracy) even further than the changes in pressure from temp effects alone as discussed above, as well as changing the point of impact.
Not sure if that made any sense at all... but I thought I'd throw that out there for some feedback. Thanks!

 

[Greg Langelius *]

Re: Theoretical Look at POI vs. Barrel Temp

It makes sense, and I believe it is true to at least some degree. But the variations, in my opinion, are comparatively small over the range of temperatures that most of us operate our barrels. Small is a relative term, and its significance will vary with viewpoints. That's opinion, and I'm prepared and quite willing to be mistaken about that.

I haven't actually done any quantitative testing, and all I can say is that tuning a load for a cold, clean barrel makes sense if you intend most of your shooting to be done with a cold clean barrel. Likewise if your primary shooting regime involves longer strings of fire and hotter barrels, then these are the conditions under which you should be doing your load testing.

Your comments regarding loads, velocities, and barrel swing extremes are, very generally, correct; but people have a multitude of explanations about the details. Figuring it all out will likely be one of the more enjoyable aspects of your shooting quest.

Greg

 

[RoughNeck182]

Re: Theoretical Look at POI vs. Barrel Temp

Thanks Greg... about the variations being very small... that was sort of what led me to that line of thinking was that how could a few really small effects add up to a larger (noticeable) one. I don't have nearly a fraction of the experience of most of you guys but I have seen my groups open up by about a good half inch or more if I shoot more than a couple 5 shot groups with very little time between (through a factory Rem 700 .243 with varmint barrel, not free floated (yet)) - with all else being equal (as far as I know - but it could be me too). A guy at the range saw my targets and the widening groups and told me accuracy goes out when the barrel gets hot so I was wondering why and how it relates to this POI change (I haven't observed that myself, I tend to play with the POA alot while shooting).

 

[Greg Langelius *]

Re: Theoretical Look at POI vs. Barrel Temp

Small? Yes. Insignificant? Nothing you can see is insignificant; and yes, you will usually see accuracy differences between warm and overheated barrels. It's one of the reasons I concluded that despite its excellent accuracy, the .22-250 isn't so well suited as a match round.

 

[Sig685]

Re: Theoretical Look at POI vs. Barrel Temp

I thought this was a very interesting thread but it raised a lot more questions than it answered, at least it seemed that way to me.

So, I do a lot of target shooting and because of the courses of fire, I actually get to shoot a lot of rounds at the same targets and over the years I have noticed that with a good barrel, POI doesn't seem to change because of the heat in the barrel or ammo; with a good barrel the POI remains as is. My more recent match barrels certainly act this way, but I do remember some barrels that were not as good. The barrel in my match AR-15 was a Wilson before I switched to Kriegers. What I found was that as the barrel heated up , the shots would start dropping down. At 300 yards, after shot 13 or 14 in a 22 shot string, the POI would go down from the 10-ring into the 9 ring and then the 8-ring. I would start to aim progressively higher after the first shot went low. My current Krieger is rock solid throughout the 22 round match.

I had a Douglas in my .308 before this new Broughton was installed. After 5-6 rounds, the Douglas was done, it would start flinging the bullets all over the target. The Broughton is also rock solid. It never wavers.

Earlier in the discussion, somethign triggered my memory and I went checking. The M1 Abrams tank has something called an MRS, muzzle reference sensor. I understand this to be a laser that is aimed at a mirror sitting on top of the barrel at the muzzle. The MRS is used to detect barrel droop due to heating and this amount of droop in used in the firing solution calculation along with the temperature of the propellant.