Load Tuning for temperature

[mks]

Keith,

My thought was the rate of the chemical reaction (the powder burning). Thoughts?

Greg J.

Greg,
Hatcher gives figures for a Browning machine gun. 49.2% of the energy in the powder is lost as heat to the case, barrel and gases. As these things heat up, less energy is lost, so the bullet achieves a higher muzzle velocity. The largest part, 23.7%, is heat transfer to the barrel. The reason ambient temperature is important in tuning is that it affects barrel, case and powder temperature and, therefore, the heat losses during combustion.

Cheers,
Keith

 

[Vern]

Keith your right at least according to Beggs.
I know he and I have talked several times, even on the phone for quite a spell once.
He did express his frustration about how people seem to be obsessed with the column of air in the barrel and he specifically stated (if I remember correctly) that the column of air in the barrel is not what he is addressing.

 

[SGJennings]

Read post #2 in this thread carefully. You'll see where I'm confused. I just don't see how air density can have enough to do with it. It has to be something bigger.

Way back when I was in college, we talked about rates of reaction. The general rule of thumb was that, at standard temperature and pressure, the rate of reaction doubled for each 10 degrees C. Now, that's a *very* broad rule of thumb. Don't take it as anything like exact.

Bottom line, it's one candidate for something big enough to cause the kinds of swings that we see.

 

[Vern]

Jennings as I read what you are saying "the rate of reaction doubled for each 10 degrees C" this would affect the powder in a case regardless or any other factors?

FYI where Gene draws some of his conclusions from is as I remember being a pilot for some decades and effects of altitude density on planes and flight. I am not trying to speak for Gene and I do hope at some point he will chime in with his thoughts.

Just as an aside thought in this whole thing, without going to the expense of buying a tuner and having it put on a barrel couldnt we perform the same basic experiment by just hanging a weight from a wire on the end of the barrel and moving it back and forth... just for experimentation purposes. I know you couldnt compete with it that way.

 

[SGJennings]

Vern:

Below the dotted line below is an excerpt from post #2 in this thread. One assumes that the "Q:" was from Russel M. and the "A:" is from Gene. It would seem to say that Gene is explaining, at least casually, that

Disclaimer: I don't really *know* Gene. I've read a great many of his posts, traded emails with him and just now bought a 220 Beggs FL die from him. I hold Gene in the *highest* possible regard. He is an innovator, is generous with his time and hard-won knowledge and, most important to me, is a gentleman of the first order.

With that said, I'm a scientist to the bone. I'm more interested in the science and engineering of benchrest than the actual shooting, though I like that too.

When I see things that don't add up to me, I can't turn it loose. I lay awake at night thinking about it.

I know from experience that tuning by powder charge works. I know from experience that tuning with tuners works.

What I can't get to add up is the exact why. If I knew the why, I could create a formula or tables that would reasonably predict tuning behavior for a given cartridge and conditions.

1. If air density is the "why", why would 30s stay in tune better than 22s and 6s.

2. If air density is the "why", why would top shooters get the idea that some powders stay in tune better than others. Anyone remember Speedy writing about "T"? Tony hints around at some of this in his book, too.

3. Related to #2, why would Hodgdon claim that a powder is less temperature sensitive? I know, I know, the same reason that some quack claims Acai berries cure cancer. But, we're a skeptical bunch and we don't accept things like that. I don't think Hodgdon would just say it without some kind of science in the background.

4. If it's air density is the "why", why can our rimfire friends get a rifle in tune and stay in tune throughout the day? Their air column is longer, than a 22 CF and their bullet and charge much, much lighter. Air density would seem to be more critical to them. We all know that our rimfire friends get a barrel in tune, then they, by any means fair or foul, obtain ammo at that same velocity.

5. If it is heat loss, see question 2 and 3?

6. I know from work back in college that rates of chemical reaction can vary dramatically by temperature. See "Arrhenius Equation" and "Eyring Equation". Is the temperature difference of the chemical reaction we're interested in, i.e., the powder charge burning, so small that it isn't a factor?

I'll quit now. I have no ego tied up in these questions or answers to them. I'm just posting observations.

------------

Here is an excerpt from a question posed by russel m. and the answer provided by Gene Beggs that I pulled from my Benchrest file. Hope this helps:

Q: 6ppc using v 133 powder. Is it just humidity or is humidity & temp that affect tune? When the Humidity goes down I go up on the powder to keep my gun in tune? Is it that the powder kernals weigh less with lower humidity so I have to go up on the charge? Is it that the conditions change the harmonics of the barrel. Is it that the bullet travels through the air differently with the changes in conditions? russell m

A: Changes in air density makes our rifles to go out of tune.

 

[Vern]

Keep in mind as you have read anything I have said, I have neither agreed nor disagreed with the air density theory.
My personal thoughts have, at least the last year that it is, the humidity and heat issue of affecting powder as it sits at the range, experiencing the climate changes and is then loaded into a case. My contention that those factors are eliminated by loading everything under controlled conditions.

My questions then become what affects do humidity and temperature have on powder that is loaded and not subject to varying temperature and humidity BEFORE it is put into the case.
You mentioned the ""the rate of reaction doubled for each 10 degrees C" and if that is an affect that is present even though the powder all contained the same humidity sealed in a case throughout the day then that is a piece of information I need to consider as I watch what happens with preloaded rounds fired through out the day as the temp increases. Could be that the T powders were not subject to absorbing moisture as much as other powders are.

I certainly understand how it can affect the POI as snipers use these things in their calculations for long range shots. But for short range group BR shooting Unless the POI will greatly shift during the 7 minute match the POI is not that important as long as all the shots went into the same hole. I would think that maybe some tunnel tests could solve part of this if the different variables were grouped and explored.
Such as some tunnels are climate controlled. Have enough rounds preloaded so as to be able to shoot say 10 groups. Set them aside. Have a regular loading set up the same as you would have at the range set up outside the tunnel in the elements and load 5 shoot a group in the climate control of the tunnel then follow it by a group of the preloaded rounds. Do this through out the day. At the end of the day we might have an idea of whether the temp of the rounds loaded outside under varying temp and humidity conditions group exactly the same as those that were pre loaded.
Understand that the preloaded rounds would be loaded based on finding tune based on a load that is in tune and loaded at a particular temp and humidity level under a controlled environment.

 

[K Hope]

The reason ambient temperature is important in tuning is that it affects barrel, case and powder temperature and, therefore, the heat losses during combustion.Cheers,
Keith

Not sure I follow; I see energy losses as the variable (operations) in the equation and start temperature (powder) as the coefficient.

Ken

 

[Bench 1]

Tuning for Temp,Hummidity and so on

I have been reading along with all the post to this debate. Let me say I in no way claim to the expert on this subject but have done alot of testing in this area. And from what I have read it brings on alot head scratching.

#1 If temp is the only thing that matters then why is it that a rifle in tune at 5:00pm temp.75 not shoot 2 days later at 8:00am temp 75 with the exact same load. It has never worked for me anyway. At this point the only difference we have is at 5:00pm humidity is 42% and at 8:00am it is 93% ?

#2 If preloading at home is the answer so the temp and humidity are totaly controlled during loading. And we placed our loaded rounds in some type of container that would match and maintain the same temp and humidity untill you are ready to go to the line then our rifles would always be in tune ?

I am not saying that that temp. is not a factor. I am saying that that it is only 1 factor in keeping a rifle in tune. The air in barrel is not the the only factor either its my opinion it all starts at the lands of the gun and ends at the target and what the powder has to do to get it there. Its no secret that dense heavy air requires more powder to push a bullet at the same velocity it does in dry thin air. When the temp rises the the humidity drops in most cases placing less resistance on the bullet. I say this because if you take powder out of the equation and do your test with a air rifle. You will find it takes more air in a high humidity condition to make the rifle push the pellet the same speed and group than it does when the humidity level drops. Also the heavier the pellet is the less effect the temp and humidity has on the velocity at a certain range. This is why we see that the 30s stay in tune much better.

 

[Boyd Allen]

A little story,told to me by Jim Borden, that seems to be on point for this discussion:

Some time back, he was preparing for a big match (I forget which one.) by preloading about 300 rounds of ammunition. (Again, the number may be inaccurate, but you get the idea.) He did this by loading at the range, on several different occasions, on which the weather, (temperature and humidity) varied greatly. Since it is his view that for a given rifle/barrel combination, that accuracy is velocity specific, he would set up his chronograph and adjust his load for the specific velocity that he wanted. He did this at each of a 5 or 6 sessions (same disclaimer). When he had finished the total spread of charge weights was (I believe) something like a grain and a half. At that point, as much to quiet the objections of his shooting buddy (Bill Gebhardt?) who thought that loads that varied so much in charge weight would not shoot into the same group, he selected cartridges from each group, mixed them and shot a few test groups....that were just fine. If you want more accurate details, Call Jim.

One of the reasons that T powder has been valued so highly is that evidently (I don't have any.) it is not very sensitive to changes in temperature and humidity insofar as what velocity is produced by a given volume, and perhaps weight of powder. I say perhaps, because weighing charges for short range benchrest has been a relatively recent development that has mostly come about as the supplies of T were drying up. It is this property that caused Lou Murdica to embark on an expensive eight year journey that has given us LT32, made in the same factory, as the original batch of 8208 surplus powder that became known as T powder.

Added later: I forgot to mention that the powder that Jim loaded was 133.

 

[Andy Cross]

Yep it's a lot bigger

Read post #2 in this thread carefully. You'll see where I'm confused. I just don't see how air density can have enough to do with it. It has to be something bigger.

Way back when I was in college, we talked about rates of reaction. The general rule of thumb was that, at standard temperature and pressure, the rate of reaction doubled for each 10 degrees C. Now, that's a *very* broad rule of thumb. Don't take it as anything like exact.

Bottom line, it's one candidate for something big enough to cause the kinds of swings that we see.

The air density has an almost immeasureable effect. The real culprit is that the pre ignition temperature increases the rate of the reaction. That in turn alters the frequency of the burn and thus the tune the barrel plays whilst the bullet is in the barrel. This comes from an elderly guy Ted White who was a chemical engineer in charge of quality control at an explosives plant. Ted may be old but as he said the laws of physics don't age.
Andy.

 

[K Hope]

Andy,

If it was me Mr. White would be my best new friend.

Ken

 

[mks]

Not sure I follow; I see energy losses as the variable (operations) in the equation and start temperature (powder) as the coefficient.

Ken

Ken,
Quite the opposite, energy losses are deterministic from temperatures and the other characteristics of the system. Or according to the conventions of dimensional analysis, initial temperatures are independent variables, and energy losses are dependent variables.

I hope I interpreted your question correctly. If not, let me know.

Cheers,
Keith

 

[mks]

6. ... Is the temperature difference of the chemical reaction we're interested in, i.e., the powder charge burning, so small that it isn't a factor?

Greg,
You have lots of good questions. The reaction rate is a factor. It increases with temperature and pressure (see Fig. 10 in http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA425264). But if all the powder gets burned before the bullet exits the barrel, it matters less whether it burns quickly or slowly than how much of the energy gets transferred to the bullet. There is a significant simplification here, because the completeness of conversion of nitrocellulose and nitroglycerine to gases also varies somewhat with temperature, thus the total amount of energy available from the reaction varies. But since only about 30% of the energy typically gets transferred to the bullet, anything that effects the other 70% becomes pretty important. Heat losses are the largest part of the other 70%.

Cheers,
Keith

 

[K Hope]

Ken,
Quite the opposite, energy losses are deterministic from temperatures and the other characteristics of the system. Or according to the conventions of dimensional analysis, initial temperatures are independent variables, and energy losses are dependent variables.

I hope I interpreted your question correctly. If not, let me know.

Cheers,
Keith

Keith,

Sorry, I will try again. Does the dependent variable (energy losses) not respond to the independent variable ? (powder start temperature)

My understanding of NC temperature sensitivity is that there is a burn rate Arrhenius type dependency on temperature whereby the rate of energy release increases with increasing powder temperature. E.g. - Vihtavuori powders have an experimentally defined temperature gradient of approximately 3% change in chamber pressure and 1% change in muzzle velocity per 10 deg. C change in powder temperature.

No doubt there is unavoidable heat loss but I not sure how much net value there is shot to shot (apart from a cooked round) and in the end I think the Arrhenius values will be higher?

Ken

 

[mks]

Keith,

Sorry, I will try again. Does the dependent variable (energy losses) not respond to the independent variable ? (powder start temperature)

My understanding of NC temperature sensitivity is that there is a burn rate Arrhenius type dependency on temperature whereby the rate of energy release increases with increasing powder temperature. E.g. - Vihtavuori powders have an experimentally defined temperature gradient of approximately 3% change in chamber pressure and 1% change in muzzle velocity per 10 deg. C change in powder temperature.

No doubt there is unavoidable heat loss but I not sure how much net value there is shot to shot (apart from a cooked round) and in the end I think the Arrhenius values will be higher?

Ken

Ken,
Yes, the heat losses do depend on temperature. Convective heat losses to the barrel depend on the temperature difference between the combustion gases and the bore of the barrel. Radiative heat losses to the barrel depend on the difference of the fourth powers of these temperatures (Tg^4 - Tb^4), where Tg is the gas temperature and Tb is the bore temperature.

Shot to shot, a round that sits in a warm chamber does affect heat losses to the case and the energy available from the powder, but the greater factor is that Tb increases, which reduces heat loss to the barrel.

Ambient temperature effects muzzle velocity because the initial Tb is equal to ambient temperature. Then Tb warms up with each shot.

 

[K Hope]

Ken,
Yes, the heat losses do depend on temperature. Convective heat losses to the barrel depend on the temperature difference between the combustion gases and the bore of the barrel. Radiative heat losses to the barrel depend on the difference of the fourth powers of these temperatures (Tg^4 - Tb^4), where Tg is the gas temperature and Tb is the bore temperature.

Shot to shot, a round that sits in a warm chamber does affect heat losses to the case and the energy available from the powder, but the greater factor is that Tb increases, which reduces heat loss to the barrel.

Ambient temperature effects muzzle velocity because the initial Tb is equal to ambient temperature. Then Tb warms up with each shot.

Keith, when I went to school the teachers I appreciated the most were the ones that made me think. I appreciate your input, thanks. A couple of follow up questions:

• Values for this type of temperature dependence must vary by gun design / construction and by how quickly the rounds are repeatedly fired?

• How does this type of temperature dependence respond to less temperature sensitive powders?


Ken

 

[mks]

• Values for this type of temperature dependence must vary by gun design / construction and by how quickly the rounds are repeatedly fired?

• How does this type of temperature dependence respond to less temperature sensitive powders?


Ken

These are good questions. The burning rate of the powder depends on both temperature and pressure, which are related to each other by the ideal gas law (pressure is proportional to temperature for constant volume). This compound effect is a reason why a barrel temperature change that is relatively small compared to the flame temperature of smokeless powder can cause too-hot loads. Shooting a lot of rounds quickly can make a difference in the bore temperature. I don't think gun design makes much difference, at least for BR rifles with heavy barrels. It takes a while for the heat to get to the outside of the barrel. It is mostly soaking into the steel and not being lost to outside air during the typical 10 minute relay. I have never measured temperatures more than about 10 F above ambient on my barrels after 10-15 shots in 10 min. Maybe a metal stock would get heat out of the barrel more quickly, but not much.

Every little bit helps, but even if burning rate were entirely unaffected by temperature, the change in heat losses with bore temperature would still influence muzzle velocity. That is the biggest point I am trying to make here.

Cheers,
keith

 

[SGJennings]

Keith,

So, if I'm reading this correctly, far and away the largest component of "tune" is barrel temperature? The rate of reaction of powder is in play, but isn't really significant?

Changing subtropics...

What is your thought on barrel profiles and tuners/muzzle weights? Am I going to see you down in KY toting a 26" LV with a bigazz barrel weight?

 

[tim in tx]

The temp has the most effect but barometric press is secondary to a lesser degree

I have mainly used temp but could not measure air density but could measure barometric pressure which manditory to match up the drop charts. We did some testing in Oklahoma at the Badlands Tactical Training Facility shooting exactly 1 mile ,recorded all conditions ,shot well[16 -17 inch groups nice and round ]and went home .The scope was left at the 1 mile zero , 2 weekends later we did it again to the exact same time of morning, same ambient temp , same wind speeds and angles, same steady 2 moa push to the left BUT not the same Baro,the Baro was a full inch higher then 2 weekends before.The first thing we noticed that we had to go up on elevation 9.25 moa to get back on, with the higher Baro the calculated drop[ 1.7 moa]more was only a fraction of what we observed .With the same temp the barrel was actually pointing them lower due to change in air density alone from Baro change changing the exit timing just as Gene has said it would do .And the change in air density due to temp only would be miniscule just as Keith said. The second thing we noticed was the rifle was out of tune the groups were stringing vertical at 25 inches and more with as little as 6 inches of windage at times at the target. We repeated it a few times to confirm and just could not squeeze them inside of 25 inches . Once .5 of a grain was added and life was good but just starting to crater the primer so press was not far behind but the rifle was shooting well at the high Baro. There are times when a big Baro swing happens in a short time and it is not linier with temp and it can and wi;ll change tune .This is without a tuner .if the baro would rise your bullets exit later and the tuner should be moved out or forward slightly with large baro swings this is in addition to your normal temp considerations. Good luck gents .Hope this helps.Great thread.

Tim in Tx

 

[mks]

There are times when a big Baro swing happens in a short time and it is not linier with temp and it can and wi;ll change tune .
Tim in Tx

Tim,
Thanks for the thought-provoking data from long range. Gene and I have discussed air density and back pressure on the bullet before on this forum. Barometric pressure is a bit different factor. The min and max world records for barometric pressure are 25.69 and 32.06" Hg or 12.62 and 15.75 psi (from wikipedia), so the largest change in pressure we would ever expect to see on the range is 3.13 psi. When the trigger is pulled we have 50k - 60k psi pushing on the back of the bullet and 14.7 +/- 3.13 psi pushing back of the front. The 3.13 psi change is just 0.006% of 50k psi. That seems too small to affect tune as much as you observed. I don't think barometric pressure affects bullet exit time significantly. The change in the force on the bullet is just too small.

On the other hand, we know that barometric pressure affects external ballistics. Higher pressure increases air density and, therefore, drag on the bullet. Both bullets have more drag from the muzzle to the target, causing more drop for both. The difference in drop also increases, which increases the necessary difference in launch angle for tune. The slow bullets need to be launched at an even higher angle to hit the same hole as the fast bullets. Tune changes similar to increasing yardage.

So anyway, my idea is that barometric pressure does not affect tune by altering barrel harmonics, bullet acceleration, or anything else related to internal ballistics, but rather it changes the tuning condition, determined from external ballistics, that we need to match. What do you think?

Cheers,
Keith