Barrel vibration and linear powder adjustments

U

upandcoming

Guest
OK, I admit it, I might be complicating this too much now, however when theory like this enters my mind I have an urge to understand why. So, here it goes.

The way I understand it the barrel vibration goes like a sine curve. Using Gene Beggs information it should be around 1.2 gr. of N133 between two separate nodes, more or less. I have been able to test this in my gun, and how found a distance of about 1.07-1.14 gr, slightly dependent on the size of the sweet point. Also I have found that at around a difference of 20 degree F the sweet point repeat itself. So far, so good.

And then my mind enters an area which make me understand less than I like: since the barrel vibration obviously far from linear, we are able to quantify the difference between nodes how can it be that a change of 1 degree F always can be adjusted my the same amount of powder regardless on how far from the node the bullets is when released? I mean, a change of x degree(s) F can be adjusted for by adding/subtracting y gr. N133. Is it always so, of is the theory just simplified by using a piecewise linear approach?

I hope I was able to present what I am thinking in an understandable way, but can anyone explain this to be? I assume Gene Begg has some input to come with?:)
 
I would assume the amplitude of barrel oscillation is a function of many variables, including

temp
powder (or velocity?)
mechanical characteristics of barrel (density, stiffness [or something similar like elastic modulus] )
bullet mass and configuration.

A graph in multi-variable space would be a complicated drawing.

However, for small changes in one variable, it probably can be approximated by a (nearly linear) function of only how variable. However, if temperature changes significantly, there may be a shifting of the curve as well as a change in its variation with any other variable (such as powder or velocity)

gordon
 
According to the information in the VihtaVuori manual DTD 1-99, a 10 degree F change in powder temperature should produce the same change in muzzle velocity as a 0.14 grain change in powder charge - if you are loading around 28 grains. If one can hold bore time constant by holding muzzle velocity constant, and barrel density and elastic modulus don't vary much with a 10 degree change in temperature, one should be able to preserve "tune" by adjusting charge accordingly. (more powder at lower temperatures).
 
Graph of "Tune".

Harold M said:
According to the information in the VihtaVuori manual DTD 1-99, a 10 degree F change in powder temperature should produce the same change in muzzle velocity as a 0.14 grain change in powder charge - if you are loading around 28 grains. If one can hold bore time constant by holding muzzle velocity constant, and barrel density and elastic modulus don't vary much with a 10 degree change in temperature, one should be able to preserve "tune" by adjusting charge accordingly. (more powder at lower temperatures).
Click to expand...

Harold:

If we equate "Tune" to a fixed velocity, i.e. 2800, we should be able to make a graph. For sake of argument let us say X axis = Temperature 0-120 Deg. F. Y axis powder charge 26-30 gr n 133. Assuming 6MM BR Norma, 30" barrel, 107 SMK, and N-133, 28.0 gr, (Quickload says 2799 fps I do not know assumed temperature). However for discussion lets assume that the temperature is 72 F. This gives a point on the graph. If we go up 20 deg F. from the original point and decrease the charge by .28 gr we should get another point. If we go down 20 deg F. from the original point and increase the charge by .28 gr we should have another point. This should give us three points to determine a line. I now have to go and make the graph. The graph turned out like I thought it would. If we go in with a temperature and hit the line we have drawn we should be able to go down to a charge weight for that temperature.

The same graph could be made by two in tune loads shot in the cool and hot parts of the day. They would probably be a much more accurate graph. If this were done at the three nodes one would have three lines on the graph for the three seperate nodes. This would apply to one rifle. I think with the measures that work on clicks that the numbers derived from the graph would not work well. I would think keeping a range note book would be more helpfull. I am a newbie so this is just my opinion.

Jeffrey Tooker
 
I've tried to take some data collected at several shoots over the last few months, and using 2007 N133 it looks to me like the number should be more like .26 grains per 10 degrees. This is based on my memory since I don't have my record book with me at the moment.
 
I have an Excel file that does this both in grains and in "clicks" and have the graph taped to my loading box (for 6ppc.) One needs only to benchmark a particular rifle by firing over a chronograph, input this velocity and corresponding charge for the "sweet spot" (or "spots") and all is done. It should work for any cartridge and any velocity and charge combination. I can't say it's helped my match performance, but the physics appears sound. I tried to attach the file to my previous post, but couldn't figure out how, if indeed such is possible.
 
Why not just have precision measured loads in 3 increments of .2gr. You should ALWAYS be able to find a node. I have tried to figure this game out from an analysis perspective, but in the end I''ll just let the target tell me what the tune is. fwiw
Russ
 
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Splitting frog hairs

upandcoming said:
OK, I admit it, I might be complicating this too much now, however when theory like this enters my mind I have an urge to understand why. So, here it goes.

The way I understand it the barrel vibration goes like a sine curve. Using Gene Beggs information it should be around 1.2 gr. of N133 between two separate nodes, more or less. I have been able to test this in my gun, and how found a distance of about 1.07-1.14 gr, slightly dependent on the size of the sweet point. Also I have found that at around a difference of 20 degree F the sweet point repeat itself. So far, so good.

And then my mind enters an area which make me understand less than I like: since the barrel vibration obviously far from linear, we are able to quantify the difference between nodes how can it be that a change of 1 degree F always can be adjusted my the same amount of powder regardless on how far from the node the bullets is when released? I mean, a change of x degree(s) F can be adjusted for by adding/subtracting y gr. N133. Is it always so, of is the theory just simplified by using a piecewise linear approach?

I hope I was able to present what I am thinking in an understandable way, but can anyone explain this to be? I assume Gene Begg has some input to come with?:)
Click to expand...


Upandcoming, (I wish everyone would use their real name) in my opinion, you are trying to make this tune thing too complicated. You began your post with,

"The way I understand it the barrel vibration goes like a sine curve. Using Gene Beggs information it should be around 1.2 gr. of N133 between two separate nodes, more or less. Also I have found that at around a difference of 20 degree F the sweet point repeat itself. So far, so good."


All true and if you had stopped right there, you would have been just right, but you continued on to the point of trying to split frog hairs. :rolleyes: :D


You continued by saying, "Since the barrel vibration obviously is far from linear, we are able to quantify the difference between nodes how can it be that a change of 1 degree F,,,,"

I'm not sure what you mean by 'barrel vibration is far from linear' but I will say that when you start trying to quantify changes in temperature to the nearest degree and powder charges to the nearest hundredth of a grain, you have gone much farther than necessary; you're trying to split frog hairs. ;) :D

Hope this helps. Please understand, I'm not trying to be condescending with some of my comments. I'm just trying to get you to lighten up a little and realize that most of this stuff is not rocket science. :) Here it is in a nutshell;

When fired, the barrels vibrate; the muzzle whips up and down in the vertical plane as the bullet travels down the bore. The muzzle slows and comes to a complete stop at the top and bottom of its swing. You want your bullets to exit at either the top or bottom stop. You can accomplish this with the powder charge or tuner. If using a tuner, NEVER change the powder charge, neck tension or seating depth; make all adjustments with the tuner. :)

There it is; that's all there is to it. :)

Later

Gene Beggs
 
Gene

You continued by saying, "Since the barrel vibration obviously is far from linear, we are able to quantify the difference between nodes how can it be that a change of 1 degree F,,,,"

I'm not sure what you mean by 'barrel vibration is far from linear' but I will say that when you start trying to quantify changes in temperature to the nearest degree and powder charges to the nearest hundredth of a grain, you have gone much farther than necessary; you're trying to split frog hairs.
Click to expand...

OK, I understand this was not very good explained so let me elaborate just a bit by example.

For the example, let's assume a temperature of 50 degree F and the gun is in perfect tune. The temperature increases with 5 degrees F and is suddenly out of tune. To compansate one would need to adjust by (as an example) Y gr. of powder to get back in tune. However, lets assume that we do not adjust anything but the temperature increases by another 5 degree F and gun is completely out of tune. I now assume that we can agree on that the movement of POI was bigger on the table with the last 5 degrees F than the first 5 degrees F? This is where my unlinearity comes into the picure: two equal shifts in the temperature makes the POI shift more the second time, however by adjust (as an example) with 0.3 gr the first time an another 0.3 gr the second time the gun should be back in tune.

So, non-linear movement when we move far enough away from the tune, but this can be adjusted for by linear (equal) amounts of powder.

Do you understand where I am going?

I appreatiate your answer, an honest answer is always the best in my opinion. I do agree that I might be making things a thouch too complicated now, but that's kind of one of the intersthing sides of BR-shooting from my point of view. Understanding the complicated parts makes, hopefully, the easier parts even more understandable... :D

I am looking into tuners right now, actually I am waiting for one of yours and it will be interesthing to test it in front of next season and perhaps through that season as well.


Roy
 
upandcoming said:
OK, I understand this was not very good explained so let me elaborate just a bit by example.

For the example, let's assume a temperature of 50 degree F and the gun is in perfect tune. The temperature increases with 5 degrees F and is suddenly out of tune. To compansate one would need to adjust by (as an example) Y gr. of powder to get back in tune. However, lets assume that we do not adjust anything but the temperature increases by another 5 degree F and gun is completely out of tune. I now assume that we can agree on that the movement of POI was bigger on the table with the last 5 degrees F than the first 5 degrees F? This is where my unlinearity comes into the picure: two equal shifts in the temperature makes the POI shift more the second time, however by adjust (as an example) with 0.3 gr the first time an another 0.3 gr the second time the gun should be back in tune.

So, non-linear movement when we move far enough away from the tune, but this can be adjusted for by linear (equal) amounts of powder.

Do you understand where I am going?

I appreatiate your answer, an honest answer is always the best in my opinion. I do agree that I might be making things a thouch too complicated now, but that's kind of one of the intersthing sides of BR-shooting from my point of view. Understanding the complicated parts makes, hopefully, the easier parts even more understandable... :D

I am looking into tuners right now, actually I am waiting for one of yours and it will be interesthing to test it in front of next season and perhaps through that season as well.


Roy
Click to expand...


Roy, a 5 degree increase in temperature does not throw a rifle completely out of tune, nor will a 10 degree increase. With the typical 6mm BR rifle, it takes a 20 degree increase in temp to throw the rifle completely out of tune assuming we do nothing to compensate. A full out of tune condition is indicated by two bullet holes of vertical dispersion at 100 yards.

When tuning with the powder charge a 5 degree increase in temp calls for a .15 grain reduction in weight, 10 degrees would call for .3, 15 degrees .45 and 20 degrees .6 If I were tuning with the powder charge, I would not attempt to make a change in weight of less than .3 grains which is equal to half a number on most Culver-type powder measures.

You mention changes in Point of impact (POI) in regard to tune saying, "The movement of POI was bigger on the table with the last 5 degrees F than the first 5 degrees F? This is where my unlinearity comes into the picure: two equal shifts in the temperature makes the POI shift more the second time."

I understand what you are talking about; it is due to the rate of acceleration as the barrel transitions throughout its swing. At mid swing, the barrel is moving the fastest, hence, greater change in POI. But this is nothing to worry about. Yes, the greater our understanding the better, but to keep our rifles in tune, we need only understand that a full out of tune condition at 100 yards is indicated by two bullet holes of vertical; a half out of tune results in only one bullet hole of vertical. As I said earlier, if I were you I would make only .3 grain changes when tuning with the powder charge.

Gene Beggs
 
I get what you are saying, things get clearer for me as what I did have in mind kind of appears as I thought they would. I guess it all adds up to being piecewise linear when it all comes to small shifts from the current status.

But just for the record, what you are saying is that with a 6ppc it takes a change of 40 degrees F to find that sweet point reappear, or to get back in tune? A change of 20 degrees F is as far out of tune you get when nothing is done to compensate?
 
upandcoming said:
I get what you are saying, things get clearer for me as what I did have in mind kind of appears as I thought they would. I guess it all adds up to being piecewise linear when it all comes to small shifts from the current status.

But just for the record, what you are saying is that with a 6ppc it takes a change of 40 degrees F to find that sweet point reappear, or to get back in tune? A change of 20 degrees F is as far out of tune you get when nothing is done to compensate?
Click to expand...



Roy, you pose an interesting scenario. I don't think I have ever seen a temperature differential of more than 25 degrees from sunup to sundown but I suppose it could happen. If so, you would be correct.

If the rifle was perfectly in tune at say 60 degrees and we did nothing to compensate, it would slowly go out of tune and be completely out of tune when the temp reached 80 degrees. If the temperature continued to climb, the rifle would slowly come back in tune and would again be right on when the temp reached 100 degrees. :eek:

You are also correct in saying,

"A change of 20 degrees F is as far out of tune as you get when nothing is done to compensate."

Hope this helps.

Later

Gene Beggs
 
Gene Beggs said:
Roy, you pose an interesting scenario. I don't think I have ever seen a temperature differential of more than 25 degrees from sunup to sundown but I suppose it could happen. If so, you would be correct.

If the rifle was perfectly in tune at say 60 degrees and we did nothing to compensate, it would slowly go out of tune and be completely out of tune when the temp reached 80 degrees. If the temperature continued to climb, the rifle would slowly come back in tune and would again be right on when the temp reached 100 degrees. :eek:

You are also correct in saying,

"A change of 20 degrees F is as far out of tune as you get when nothing is done to compensate."

Hope this helps.

Later

Gene Beggs
Click to expand...


Helps a lot, thank you.

As for the temp change, I live in northern Scandinavia and I have done matches (non BR) where the temp rose by more than 45 degrees F during the day (53 F in the morning, close to 100 F in the afternoon). So there are reasons why I am extra interested in this subject.... :eek:
 
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