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RAG2
04-15-2013, 02:45 PM
I've been out of the game for a while (not sure I was ever really in it :)...but I was hoping some could share their personal experience.

How much do you find that you need to adjust (PPC, N133, 66gr) your powder for a 20 degree tempature swing.

I am going to be doing some pre-loading (not for match). I seem to recall backing off my Harrell's 2 baby clicks (2/6ths of a full click) for a 10 degree temp increase...but it's been a while. Just your personal findings/opinions would be great. Thanks!

abintx
04-15-2013, 10:01 PM
I've been out of the game for a while (not sure I was ever really in it :)...but I was hoping some could share their personal experience.

How much do you find that you need to adjust (PPC, N133, 66gr) your powder for a 20 degree temperature swing.

I am going to be doing some pre-loading (not for match). I seem to recall backing off my Harrell's 2 baby clicks (2/6ths of a full click) for a 10 degree temp increase...but it's been a while. Just your personal findings/opinions would be great. Thanks!

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. If the bullets are exiting when the barrel is stopped at either the top or bottom of the swing, the rifle is in tune. If the atmosphere becomes less dense due to an increase in temperature, the bullet encounters less resistance as it travels down the bore and exits early before the barrel comes to a complete stop, in which case, we can either reduce the load or adjust the tuner to compensate. I think it's much simpler and easier to use a tuner. You NEVER have to change the load and can make your adjustments at the line.

Atmospheric density is the result of pressure altitude, temperature and to a far lesser degree, moisture content. Contrary to what many believe, dry air is heavier. Steam rises and it's 100 percent saturated.

So .. altitude, temp and humidity in that order. What do we mean by altitude? Range elevation above sea level. The Midland Shooters Association range is 2800 feet above sea level; that never changes so we can forget about it. Yes, slight variations in barametric pressure can change the pressure altitude but for all practical purposes, we can forget about it.

****Temperature? This is the big one! It's actually the only thing you must concern yourself with. The temperature range from being perfectly in tune and being completely out is 20 degrees F.****

Yep, if your rifle is perfectly in tune during the first match of the day when temperature is 70 it will be completely out of tune when the temp reaches 90 if you do nothing to compensate.

To keep the rifle in tune throughout the day, reduce the velocity 30 fps for each five degree increase in temp. With most powders, N133 being the classic example, this equates to one half click per five degrees. For example, let's say your rifle was in tune for the first match; temp was 70. Your Culver type measure was set on 54 clicks. When temp reaches 75 the proper click value is 53.5, for 80, 53, 75, 52.5 and for 90F, 52 clicks. Would you have ever believed it would take two full numbers to stay in tune with a twenty degree spread?

"Now wait a minute Beggs; are you telling me relative humidity has nothing to do with it?" Yep, that's right; very little, so little that for all practical purposes we can ignore it

RAG2
04-16-2013, 08:44 AM
Wow, that's sorta depressing. Makes pre-loading (for match or varmint blasting) pretty difficult to stay in tune. I never would have thought such large adjustments were needed. Thanks.

Vern
04-17-2013, 10:56 AM
Rag I have been working with this for a little while now.
I use to do a lot of reading and question asking about temp and humidity in regard to loading HOWEVER one thing that you have to consider is that 99% of the people you are asking are loading at the range in the pool of temp and humidity change.
In the past some (not all) of the notable shooters reference the effects of temp and humidity as being on the powder. They said that in the morning the powder has more humidity and is larger and later in the day when it gets hot the powder loses some of that and becomes smaller. Since most use throwers of some sort this would have a distinctive effect.

Having said all of that last year I gave some thought to the fact that if I wanted to find tune I needed to have everything done under the same conditions so that as the day wore on while tuning there wouldnt be an issue with it.
I preloaded my ammo for tuning at home at a certain temp and humidity.
Later I took that same thought to loading the best tune load at home under same temp and humidity condition.
The last match of the year last year I went preloaded. It worked out pretty well. My barrel is worn out but the first 2 matches of this year I did the same thing. It seems to be working real well. I still get some of the small group pots and I still pull the trigger when I shouldnt have. But it seem to shoot better for me when I have eliminated that variable.
The only thing that can be left is how the external temp can affect the powder burn inside a sealed case and so far no one can positively give evidence of that because of loading powder exposed to some of the elements.
One thing about it Humidity can not have any effect on the burning of the powder once sealed inside a case. Still out on what affect the external temp can have.

I also noticed a month or so ago that one of the new possible world records was shot with preloaded ammo.
One thing about it theres a lot less to carry to a match and a lot more time to study the wind flags.

Boyd Allen
04-17-2013, 11:19 AM
So Vern, reading what you have done, would you say that for his first trip, in addition to a larger amount of best guess loads that he should take a variety of test loads, so that he can improve his results for the following year?

Hunter
04-17-2013, 11:35 AM
Vern, referencing post # 2 above, are you preloading to gve effect to temp changes during the day? If so, can you describe the details?

Vern
04-17-2013, 11:51 AM
Boyd what I am saying in regard to tuning is that when I go to tune I load all of my loads at the house.
based on 3-4 different powder weights and seating depths.
If you are trying to find tune at the range while loading at the range you are trying to find tune while tune is actually changing due to conditions under which you load.
There are many that do tuning like this.
I have just taken it one step further. When I load for tune I load at 76 degrees and 40% humidity.
Since I have started preloading I create the same conditions in the house/loading room prior to trying to load.

Hunter I have not seen any changes in my groups due to temp or humidity while at the range when preloaded.
The only thing I have seen are errors in my flag reading or gun handling at the bench.
I am still testing the preloading theory at this point and with a worn out barrel at that.

Example: I was doing pretty good at 200 last sunday then in the last match I got caught in a let up and blew the group into a 2" agg killing group.
I was frustrated to say the least. HOWEVER I know that I am running short on bullets and time for future matches so my current load was 62gr flat base cheek. I used the same powder load and die setting and loaded up 10 65 gr bart boat tails just to try at the end of the day. After the 2" blowout with the time remaining I saw the condition sort of holding and I blew all 10 rounds down into the sighter as fast as I could send them.
They measured .413 for the 10 shot group.
It seems to be working for me but it may be different for others.

It has always been said that a large part of Benchrest is mental. For me I am hoping to at least take the worry out of what to do with the powder if I think something is going wrong.
Now I dont see the changes I used to but time will tell.

Edit: Hunter in re reading your question I dont see the groups grow or change nor do I see any difference in impact as the temp or humidity change throughout the day.
I believe sunday started at the commence fire with temps in the mid 60's maybe lower didnt check for sure, over cast and almost a mist but not quite... very high humidity. By the end of the day it was sunny windy and in the mid 80s, much lower humidity. POI was still the same except for the wind. Groups were still good if I did my part.

RAG2
04-17-2013, 12:05 PM
Yeah, I always loaded at the match/range too.

Gosh, I might have to look into a barrel tuner at some point. Seems to me, if someone had to pre-load (for whatever reason), a barrel tuner would best allow a shooter to adjust according to conditions. Would this potentially be the case? Of course, that's assuming you knew where/how to adjust for various conditions.

abintx
04-17-2013, 12:50 PM
Yeah, I always loaded at the match/range too. Gosh, I might have to look into a barrel tuner at some point. Seems to me, if someone had to pre-load (for whatever reason), a barrel tuner would best allow a shooter to adjust according to conditions. Would this potentially be the case? Of course, that's assuming you knew where/how to adjust for various conditions.

Read through this. Again, I took it from my Benchrest file that I created from replies to various threads posted here on Benchrest Central:

TUNE with TUNERS or POWDER by Gene Beggs

With all the talk of barrel harmonics, stopped muzzles, density altitude, water in the bore, this, that and everything else, it's enough to drive you crazy; isn't it? When the subject of tuners comes up, many just say, "Oh, to heck with it; it's all too complicated." If that's what you have been thinking, don't despair; it's not that hard.

BARREL HARMONICS

Sounds pretty impressive; doesn't it? But in my opinion the word harmonics does not accurately describe what happens to a rifle barrel when fired.

Webster defines the word harmonic as, "of or relating to musical harmony; pleasing to hear."

Rifle barrels are anything but pleasing to the ear, and they don't harmonize with anything. When fired, they whip up and down like an old cane fishing pole, mostly in the vertical plane and the muzzle comes to a complete stop at the top and bottom of the swing. It is at one or the other of these stops that our bullets should exit, as small variations in muzzle velocity result in the least dispersion. That's it; that's all there is to "tuning." You can time the bullet's exit with the powder charge, or you can vary the vibration frequency of the barrel by moving a weight fore and aft on the barrel, whichever you feel is most convenient. Me? I'll use a tuner. I have tried it both ways and with the tuners of today, adjustment is quick and easy. I never have to adjust the load; it's as easy as focusing a scope.

DENSITY ALTITUDE

This has caused a lot of confusion. Yes, a change in air density is the culprit that causes our rifles to go out of tune if we do nothing to compensate. You can monitor barometric pressure, relative humidity, temperature, dew point, wet bulb temperature, compute density altitude, etc., etc.; but do you know what the single most important thing is? Temperature!! Yes, changes in temperature have more affect on atmospheric density than all others combined. You can keep your rifle perfectly in tune throughout the day with nothing but an accurate thermometer and the simple formula that follows. Please remember that we are talking about short range benchrest group shooting with the typical 22 and 6mm cartridges in use today.

If you tune with the powder charge, decrease velocity 30 fps for each five degree increase in temperature and vice versa. With most powders, N133 being the most common, .3 grain equals 30 fps. With most Culver type measures, one full number equals .6 grains. For example, if your favorite load is 54 clicks, and your rifle is perfectly in tune for the first match of the day when temp is 60 degrees, you will decrease the load to 53.5 clicks when temp reaches 65 degrees, 53 clicks at 70 degrees and so on.

Me, I prefer to use a tuner. I never have to change the powder charge and I find it much easier to remove the bolt from my rifle, slide it back so I can reach the tuner easily and make a small adjustment with the two little three inch Tommy bars I carry around. Takes about fifteen seconds and I'm ready to go. No hassle, no pulling bullets and redoing a dozen cartridges, no cussing and fussing'. "But how much do you move the tuner, and which way?" you ask. Simple,

If the rifle is perfectly in tune, make a note of the temperature. Turn the tuner "IN" an *eighth of a turn for each five degree* increase in temp and vice versa. The tuner is marked with a single reference line. This line is placed at 12, 1:30, 3:00 4:30 etc. If the rifle dialed in for the first match of the day at 6:00 o'clock and temp was 60 degrees, move the tuner to the 4:30 position when temp reaches 65, 3:00 o'clock at 70 and so on. Understand? It's the simplest thing in the world but some try to complicate it. ... GB

It is a well known fact that the 30's stay in tune better than the 22's and 6's so you may not have to move your tuner much to keep the rifle in tune.

One good thing about behind-the-muzzle tuners is that installation is not critical. You can place the collars anywhere from flush with the muzzle to as much as two inches aft and they work exactly the same. Regardless of where you start, you will never be more than a half turn in either direction out of tune.

Let's say to begin with you lock the collars with the reference mark at 12:00 o'clock and the rifle shows a full amount of vertical on the sighter. This indicates that the bullets are exiting right out in the middle of the barrel's swing, halfway between a positive peak stop and a negative. You can get the rifle in tune by rotating the tuner a half turn in either direction. One way will tune the rifle to a positive peak, the other to a negative. It's the simplest thing in the world. Wish I had known all this stuff twenty years ago.

Some say it is better to tune the rifle to a positive peak rather than negative, but I cannot see any difference in group size.

Boyd Allen
04-17-2013, 01:28 PM
I have several tuners, all of different designs that I have tried. One was used for a sanctioned match, and, over the course of that weekend, I was impressed with the improvement that it gave, over the same barrel BT (before tuner). Having said that (mostly to let you know that I have some personal experience with tuners), the most successful tuner user that I know of is Gene Buckys, and he uses his quite a bit differently than most. There is an article in the current issue of Precision Rifleman on this subject that you might want to take a look at. As always, there is more than one way to skin a cat. Just because two shooters take different approaches does not mean that one of them is wrong. I just wanted to point out that there are radical differences in how shooters use tuners.

RAG2
04-17-2013, 02:05 PM
Thank you guys! And wow! I wish I had more time to play. For now, I just want my varmint guns (which are HV benchrest guns) to peform at peak in as many conditions as possible...so I think getting a barrel tuner will be the ticket in this regard!

SGJennings
04-17-2013, 03:04 PM
OK, just thinking out loud here. I just can't everything to make sense to me. Maybe you guys can help.

A. Air density in the barrel is the dominant factor that causes our tune to change.
B. The 22s and 6s are about the same. 30 fps / 5 degrees F
C. The 30's stay in tune better.

OK, so in the 30 barrel there is more air. It seems to me like air density would have a larger effect than the 6s and the 6s would be more than the 22s, etc.

What am I missing?

Chism G
04-17-2013, 03:36 PM
Thank you guys! And wow! I wish I had more time to play. For now, I just want my varmint guns (which are HV benchrest guns) to peform at peak in as many conditions as possible...so I think getting a barrel tuner will be the ticket in this regard!



That's the spirit. Nothing ventured,nothing gained. The browning BOSS tuner design sure worked great on their varmint rifles and it came with setting instructions for a variety of factory loads.




Glenn

RAG2
04-17-2013, 05:09 PM
This doesn't really matter for the Benchrest "game" I know, but I'm wondering if a tuner would help maintain a consistent point of impact too? I would think it would, logically, but I don't know. In theory, temp changes would lead to velocity changes which lease to changes in bullet release within the barrel's harmonics, so adjusting the tuner (if done accurately) would return the bullet release point to the "sweet spot" again...typically at the top of the swing at least movement. So it would not only help shoot smaller groups, but also help maintain a consistent point of impact for score shooter or varmint blasters, Right?

Except the velocity would be different since the powder isn't being adjusted. But still, for pre-loading, I would have to think it would help POI as well as grouping.

Boyd Allen
04-17-2013, 07:38 PM
In my experience, with several tuners, tuner adjustments can, and usually do change point of impact. If the temperature requires a different tuner setting, and point of impact is critical, best go to the sighter.

mks
04-17-2013, 08:50 PM
A. Air density in the barrel is the dominant factor that causes our tune to change.


Greg,
The mass of the air in the barrel is NOT the reason for changes in tune with temperature. The change in air mass is miniscule. For instance, 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. That is only a 0.014% difference compared to a 118 grain bullet, which is negligible.

Cheers,
Keith

abintx
04-17-2013, 11:51 PM
This doesn't really matter for the Benchrest "game" I know, but I'm wondering if a tuner would help maintain a consistent point of impact too? I would think it would, logically, but I don't know. In theory, temp changes would lead to velocity changes which lease to changes in bullet release within the barrel's harmonics, so adjusting the tuner (if done accurately) would return the bullet release point to the "sweet spot" again...typically at the top of the swing at least movement. So it would not only help shoot smaller groups, but also help maintain a consistent point of impact for score shooter or varmint blasters, Right?

Yes. That's right.

Boyd Allen
04-17-2013, 11:58 PM
Rereading, I see that I missed what you were asking. If you mean by consistent point of impact you mean that at a given setting, that has the rifle in tune, shots will group better, then yes, that is true. Sorry for getting it wrong the first time. On the other hand, adjusting the tuner may move the center of the group, relative to the point of aim, and if the position that is to bring the rifle into tune in one set of conditions, is different than that required by another, then the location of the group will likely not be the same. For this reason, I would never adjust a tuner in the middle of a record group without going to the sighter to both confirm that the adjustment had improved the tune of the system, and also to see where the adjustment had moved my point of impact to, so that I could hold off the correct amount. All in all, not a good idea.

Rob Carnell
04-18-2013, 06:36 AM
I have been using tuners on all my barrels for the past 4 years. I have 2 x Beggs, 2 x Shadetree, and 1 of my own design. This has been going quite well, including a Top 10 in the World Championships.

They all perform similarly, they work.

I now try to load at home, and I shoot the same cases, load and seating depth in all my barrels. I just adjust the tuner to get in dialled in.

I also shoot moly coated bullets, and I only clean every 3-4 groups, but that is another story.

I fire one shot between the bottom bulls to foul the barrel. Then I shoot 2 quick shots onto one of the bulls in similar flag conditions, but no need to be identical. If there is any vertical, I adjust the tuner, and move to another aiming point for another 2 shots until it is dialled in with no vertical. Then I wait for a condition I like and shoot the record. I keep the large sighter bull in case I need to go back to the sighter.

Yes, point of impact does change with tuner setting. Up to maybe 1" of vertical and maybe 3/8" of horizontal at 200 yds, so I would never consider changing tuner setting during a group.

I follow Gene Beggs advice about winding the tuner in as the day warms up. Makes it easier to at least know which way to move the thing.

Hope this is of interest.

Rob Carnell
Sydney, Australia

SGJennings
04-18-2013, 01:31 PM
Keith,

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

Greg J.



Greg,
The mass of the air in the barrel is NOT the reason for changes in tune with temperature. The change in air mass is miniscule. For instance, 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. That is only a 0.014% difference compared to a 118 grain bullet, which is negligible.

Cheers,
Keith

mks
04-18-2013, 02:04 PM
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
04-19-2013, 10:08 AM
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
04-19-2013, 10:20 AM
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
04-19-2013, 10:27 AM
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
04-19-2013, 11:55 AM
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
04-19-2013, 12:20 PM
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
04-20-2013, 01:51 PM
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
04-21-2013, 09:55 AM
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
04-21-2013, 11:50 AM
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
04-23-2013, 12:00 AM
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
04-23-2013, 06:06 PM
Andy,

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

Ken

mks
04-24-2013, 09:21 AM
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
04-24-2013, 10:13 AM
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
04-24-2013, 07:25 PM
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
04-24-2013, 10:38 PM
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
04-27-2013, 08:42 AM
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
04-27-2013, 12:52 PM
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
04-28-2013, 10:29 PM
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
04-29-2013, 01:52 AM
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
04-29-2013, 10:01 AM
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

mks
04-29-2013, 10:32 AM
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?

Greg,
I am not so sure about "far and away the largest." That depends on the temperature/pressure sensitivity of the powder. I don't have burning rate curves for modern powders, but if older military research papers are any indication, good powders have a plateau of reduced sensitivity to temperature/pressure. That plateau pressure seems to be pretty high for 133 (best accuracy at really high pressure), but lower for 8208XBR (best accuracy at lower pressure). So my guess is heat losses are the largest factor, but reaction rate is not insignificant.

The long barrels are needed at long range to get the muzzle to flip fast enough for tuning. Tim in tx has done a lot of good work on this. At short range, I think we need barrels and/or stocks that are a bit less stiff in the vertical plane, and as stiff as possible in the horizontal plane. I haven't made an asymmetrical (wider than tall) barrel yet, but think this holds some promise, particularly for the new NBRSA sporter class where barrel contour is not limited. My new stock is much stiffer in the horizontal plane, and it shot OK last summer. I will be playing with a stock more flexible in the vertical plane this summer.

Cheers,
Keith

tim in tx
04-29-2013, 03:26 PM
Thanks for the complements Keith, You have a really good way of explaining things.I think the shockwave forming in the barrel can have some aditional pressure that would be hard to calculate as a constantor not?The extra drop and tune change was not the only reason I think it affects in bore time but when I am tuned for 100% compensation with higher baro at 1 mile there is little to no poi change as a matter of fact the drop is completely negated and the bullets are automaticly aimed higher remaining at the same poi and the only way I can see it aimed higher is that the muzzle is in a differing position at launch because the bullet is slowed in the bore enough to be aditionally compensated in a vertical respect with just a baro change.So the drop that is normally seen with regular muzzled rifles with higher baro is way more than the calculated drop and with 100% vertical compensation the poi is the exact same or even higher is some cases but not all. I have never seen a poi remain a constant on a standard muzzled rifle with baro change.I have some pictures of the air being compressed in front of the bullet and will try to find them,but by the time the bullet gets to the end of the barrel it is at higher pressure in front of the bullet and a lower pressure behind the bullet.I can certainly be wrong but some of this vertical compensation starts to show some things I have never seen before.That is why I am thinking what I am.

Tim in Tx

mks
04-30-2013, 07:32 AM
Thanks for the complements Keith, You have a really good way of explaining things.I think the shockwave forming in the barrel can have some aditional pressure that would be hard to calculate as a constantor not?The extra drop and tune change was not the only reason I think it affects in bore time but when I am tuned for 100% compensation with higher baro at 1 mile there is little to no poi change as a matter of fact the drop is completely negated and the bullets are automaticly aimed higher remaining at the same poi and the only way I can see it aimed higher is that the muzzle is in a differing position at launch because the bullet is slowed in the bore enough to be aditionally compensated in a vertical respect with just a baro change.So the drop that is normally seen with regular muzzled rifles with higher baro is way more than the calculated drop and with 100% vertical compensation the poi is the exact same or even higher is some cases but not all. I have never seen a poi remain a constant on a standard muzzled rifle with baro change.I have some pictures of the air being compressed in front of the bullet and will try to find them,but by the time the bullet gets to the end of the barrel it is at higher pressure in front of the bullet and a lower pressure behind the bullet.I can certainly be wrong but some of this vertical compensation starts to show some things I have never seen before.That is why I am thinking what I am.

Tim in Tx

Tim,
It is curious that poi does not shift with an increase in barometric pressure, and does suggest that internal ballistics is affected. Otherwise, the denser air should cause more drop. Considering for a moment that the greater pressure on the nose of the bullet slows it down, the bullet should exit later. In Al's simulations of Lynn's long range rifle, bullets exited just before the peak in muzzle projection. So if the bullets slow down and exit closer to the peak, then this would decrease poi shift. But compensation for muzzle velocity differences would decrease, because the rate of muzzle angle change decreases as the peak is approached. Is this what you are seeing - no poi shift, and an increase in vertical dispersion?

Thanks,
Keith

tim in tx
04-30-2013, 08:38 AM
Hi Keith ,Only when I am fully tuned for 100% compensation then I see no shift or drop due to baro,if I have vertical I have poi changes as well.I think I am still away from the peak or taper off of compensation ,I am actually at the bottom of the slope with the ammo from the mile test session. The reason is the faster velocites are at the bottom of the slope [2960]and the slower velocities [2820] is when the compensation starts to taper off at 1000yds.I normally try to keep stuff for the mile at 2930 fps or exit at the bottom end of the slope .If possible I want the bullet exit to be in the middle of the velocity range within the compensation swing , Unfortunately high frequency likes to hang at the muzzle when the bullet exits in that area so slightly before the peak or right off the bottom to the middle is probably a good thing . :)


Tim in Tx