rifle accuracy nodes and how they are affected?

mulemania

mulemania

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If you have a rifle that likes shooting at a certain speed with a certain weight bullet, IMO that is an accuracy node/harmonic peak for that bullet weight. NOW my question is, from all of your experience, how little or much does it take to alter the node of a barrel, can it be as little as taking .25" off the barrel? my rifle is bedded 1 3/4 inch past the action, could bedding it 2.5" make a difference?? In theory it should make a difference, but how much?
also if anyone has experimented with it, any details from there experiences would be appreciated
 
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When you get to speaking of barrel harmonic nodes, you need to consider barrel twist rate as well. There are those who will swear that they somehow tune to the barrel length to harmonics but that is so hard to validate when you consider all the other things that are going on. What I’m getting at is that each bullet will fly best when the velocity is matched to the barrel twist rate.

An example might be a 6mm running 115 DTACs out of a 9 inch twist vs an 7.5 inch twist. You will find the accuracy will be best for the 9 inch twist if you run the bullets at above 3050, but you will get best accuracy with the 7.5 twist at around 2900.

Run a 108 grain bullet and you can drop the velocities for both of the above examples simply because the lighter and shorter 108 will stabilize with less twist at a given velocity than the longer 115.

These are not barrel harmonics at work, just the mechanical realities of twist rate relative to the bullet length and velocity.

I'm sure there is some guy out there who will swear he gets “great” accuracy out of his 6mm using 115s with a 6.5 twist at 3200 fps, but the reality is that would be a fast twist for 3200 fps and he will not be as accurate (side by side) as the same rifle and load with a slower twist rate.

Having stated the above, if a person was to venture on by removing barrel length and shooting groups until they tighten up, how would you know when to stop? Groups got better, so you take off another ¼ until what… the group got bigger? You can’t put it back on once you cut it back.

Besides, everyone knows a shorter barrel is more stiff than a longer one, so when does it end?
20 inches?
18?
Cut it short enough and your velocities wll drop - see above.

If you want to focus on harmonics, then try a barrel tuner, that way you don’t cut the barrel just adjust the device.

Check this out
http://bulletin.accurateshooter.com/tag/barrel-tuner/
 
Nodes are a pandora's box, look out.

Anything which will change the natural frequencies of a barrel- length, diameter, contact points, wrapping it in carbon fiber- will 'make a difference'. What difference is made, however, is a difficult thing to identify. The speed of sound (vibration) in steel is 20,000 fps. Therefore, in the 0.0015 seconds that the bullet is in the barrel for a highpower rifle, a shockwave (ie the impulse of the firing pin hitting the primer, the bullet jumping to the lands) will travel up and down the barrel 30 times. In that 0.0015 seconds, many of the natural frequencies above 700 Hz will be active. For the Sako TRG barrel, it has natural frequencies at 860, 1610, 2250, 6600 and higher Hz. What reloading yields is an acceleration rate where the bullet leaves the barrel at the same point in its vibration cycle every time. This is similar to how a rimfire tuner works. On that thought, your first statement needs clarification in that the accuracy node may not hold for a bullet weight, as different bullets of the same mass will have different bearing surfaces, leading to different acceleration rates, leading to release occurring at different positions in the vibration cycle. The nodes may be very close, but they will not be the same. This is why two powders may shoot to the same muzzle velocity, but one will shoot well and the other won't- the variation comes in the acceleration rate, not the velocity. I'd bet that the 'bad' powder would be a shooter when it yields a different muzzle velocity (ie different acceleration rate).

So, after all that... don't chase accuracy by fiddling with the mechanics of vibration. The barrel will vibrate and there is nothing anyone can do to stop that. A fat barrel will change and spread out the natural frequencies, but who's going to carry a ten lb BARREL into the woods? Chase the load and then practice practice practice.

Variance due to heat expansion will be for another day...
 
Barrel harmonics is a far more complex subject than you could imagine. There is no simple "formula", in practice, it's all trial and error.

A great, visual example of barrel harmonics is provided on the following website. Take a look at all the various modes of vibration further down the page, and consider that all of those modes of vibration are occurring simultaneously.

The most important aspect of accuracy is consistency, that the bullet leaves the barrel at exactly the same time as the barrel is vibrating and the barrel is in exactly the same place when the bullet leaves the barrel. Since there is always some variation in velocity and acceleration of the bullet, the bullet leaves the barrel at slight different times (from ignition). The key is to find a time/node of vibration when the barrel is moving/vibrating slower through a node when the bullet exits the barrel. Meaning the barrel hasn't moved much during the plus or minus average exit time of the bullet.

http://www.varmintal.com/amode.htm



Effects of adding a barrel tuner

http://www.varmintal.com/atune.htm


There are a lot of other links on the menu on the left side with descriptions and analysis of different combinations and barrels. Is it complex? Yep, but you did ask the question, and those Web pages are by far the best to explore barrel harmonics rather than relying on speculation . :)

Not sure if it's discussed on the Varmint Al web pages, but with accuracy people typically consider only velocity. Part of finding the best accuracy is also experimenting with different powders. Even if the velocity is the same, some powders might be more accurate than others. Why? Because different powders introduce another variable, time. Different powders have different burn rates, so accelerate the bullet at different rates. The time the bullet exits the muzzle is different, so the barrel is vibrating at a different location. So even if velocity is the same, the time (in, milliseconds) the bullet exits the barrel is different. But that's another topic altogether, internal ballistics.
 
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To supplement SR's info via the varmintal page- look at modes 3, 5, 7- these are the criticalities. Modes 1 and 2 fall below the frequency range within the time spectrum of the bullet in the barrel so 1 will not be affecting, and 2 will be negligible compared to 3, 5, 7.
 
rimfiremac said:
To supplement SR's info via the varmintal page- look at modes 3, 5, 7- these are the criticalities. Modes 1 and 2 fall below the frequency range within the time spectrum of the bullet in the barrel so 1 will not be affecting, and 2 will be negligible compared to 3, 5, 7.
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Thank you for filling in details. It has been many years since I went through those pages in detail.

I've also cleaned up my post/description above to hopefully make it a little more clear.

If anyone wants more information, they have both calculations and actual tests and accuracy results of a barrel with a barrel tuner. Interesting and even more technical.

http://www.varmintal.com/aeste.htm
 
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thanks a lot guys, this is very good information, here is the reason I am pursuing it. I have a 300 rum 28 inch gillaird barrel with 1:10 it currently shoots 210g bergers with 90g of retumbo at 2980 with a es of 30. It groups quite well usually at .5 inch at 100 yards. I have tried a lot of things with that powder but cannot seem to get my es lower than 30. I have also tried h50 bmg which produces es ranges in the mid teens and sometimes single digits. but it is not as accurate (.65)av. I would like the bullets to go a little bit faster in the 3100 range. I was thinking by changing the guns bedding, it may change the way the bullets would react at that speed range. the H50 bmg powder sure does react a lot differently, I can achieve significantly higher speed with way less pressure signs on the primer or brass
 
Similar to what's coming out about chronographs in another thread here, don't worry about the statistics (ES) the chronograph gives you- chrony's are good for ballpark velocity and the rest is entertainment. If it prints good on paper, at distance, then it's good.
 
mulemania said:
thanks a lot guys, this is very good information, here is the reason I am pursuing it. I have a 300 rum 28 inch gillaird barrel with 1:10 it currently shoots 210g bergers with 90g of retumbo at 2980 with a es of 30. It groups quite well usually at .5 inch at 100 yards. I have tried a lot of things with that powder but cannot seem to get my es lower than 30. I have also tried h50 bmg which produces es ranges in the mid teens and sometimes single digits. but it is not as accurate (.65)av. I would like the bullets to go a little bit faster in the 3100 range. I was thinking by changing the guns bedding, it may change the way the bullets would react at that speed range. the H50 bmg powder sure does react a lot differently, I can achieve significantly higher speed with way less pressure signs on the primer or brass
Click to expand...

If you give me a COAL, and choice of powder, I can tell you what velocity you need to achieve to reach your optimum node. Nodes are predicatble, within reason. If you have a reliable and accurate chronograph, it can be proven. Nodes have nothing to do with twist rates and other voodoo, they are simple harmonics based on barrel length, nothing more.

R.
 
Rman said:
If you give me a COAL, and choice of powder, I can tell you what velocity you need to achieve to reach your optimum node. Nodes are predicatble, within reason. If you have a reliable and accurate chronograph, it can be proven. Nodes have nothing to do with twist rates and other voodoo, they are simple harmonics based on barrel length, nothing more.

R.
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Did you read the links, including the last one where they actually ran experiments with the barrel tuner? I think not. I think you talking about this? http://www.the-long-family.com/OBT_paper.htm and http://www.shootingsoftware.com/barrel.htm

Harmonics are anything but simple. And that's why barrel tuners work. Otherwise the barrel tuners wouldn't have any effect.

Yes Long's work does work to an extent, but all it does is get you in the ballpark, load development is still necessary around those nodes.



But hey, convince me, I'm open.
 
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SandRoad said:
Did you read the links, including the last one where they actually ran experiments with the barrel tuner? I think not. I think you talking about this? http://www.the-long-family.com/OBT_paper.htm and http://www.shootingsoftware.com/barrel.htm

Harmonics are anything but simple. And that's why barrel tuners work. Otherwise the barrel tuners wouldn't have any effect.

Yes Long's work does work to an extent, but all it does is get you in the ballpark, load development is still necessary around those nodes.



But hey, convince me, I'm open.
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I did not say that load devopment isn't required. I also didn't say that harmonic tuners didn't work. What I did say is that harmonics, and nodes, are predicatble, to a point, and with the proper equipment that point can get very sharp, and the ballpark very tiny.
Why would you run a barrel tuner if it isn't required? If they were required, you'd see lots of doughnuts on lots of barrels. Tune the amunition to the rifle, not the rilfe to the amunition.
I am not sure what I am supposed to convince you of?
Mathematics is mathematics. It is truth, nothing more, nothing less.

R.
 
Rman said:
I did not say that load devopment isn't required. I also didn't say that harmonic tuners didn't work. What I did say is that harmonics, and nodes, are predicatble, to a point, and with the proper equipment that point can get very sharp, and the ballpark very tiny.
Why would you run a barrel tuner if it isn't required? If they were required, you'd see lots of doughnuts on lots of barrels. Tune the amunition to the rifle, not the rilfe to the amunition.
I am not sure what I am supposed to convince you of?
Mathematics is mathematics. It is truth, nothing more, nothing less.

R.
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I don't doubt you at all. Just you haven't said how or what you are using, so how can anyone comment? That's all I'm asking, show us. :).


If you are using Long's method, it only looks at one single mode of vibration, when there are 3 significant modes. That's because only that single mode, P wave (primary or pressure wave) reflecting longitudinally along the axis of the barrel, is the only one that can be easily calculated. The other modes require barrel stiffness calculations and are very complex. However Long's methodology does work as it optimizes for one mode which seems to be significant, and will give best results with stiff barrels, as they minimise the other modes (whipping). And obviously, it will vary rifle to rifle.


Also, tuners don't just "tune the rifle to the ammo" they take the very best load and make it better. Tuners go beyond that single mode, and in fact don't change P waves. And as mentioned, in Long's methodology, a tuner is irrelevant, but in fact they do work. (And as a side note, tuners are often used in rimfire where there is no ability to tailor loads) However they are ugly and most people can get good results without them. Their greatest benefit appears to be on thinner barrels. I won't use one, at least not right now.
 
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I am using a modified version of Long's method, and have developed a program to run it. It has proven to be very accurate, and very predictable. So much so, that I have started a business using this program to help hand loaders, long range shooters, and hunters get the very best out of their ammunition and rifle combination.
While you are correct that this method only addresses the primary wave, you are also correct in that this is the only wave that can be easily calculated. I have used my program on thin, thick, long and short barrels, and have been extremely accurate in predicting the node, and the velocity at which that node exits for a given set up. It is not perfect, and the load development aspect is still very important, but it does provide a starting point, and a clear objective with respect to a target velocity. This eliminates the need for "ladder testing" and many other time and money consuming factors spent on finding a great load in a given rifle.
I am aware of what tuners are, and what they are used for. They have been used successfully by rim fire shooters for many years. Again, this addresses the issue of being unable to hand load rim fire ammo. I will disagree with your statement about tuning the rifle to the ammo. Perhaps we are saying the same thing, just differently? The whole point of the tuner is to tune the rifle, as it has zero effect on the ammunition being used, hence the love of tuners by rim fire competitors. While I realize that the tuner and hand load combination can work, it seems unnecessary when the ammunition variable can reduced greatly by targeted load development.

R.
 
Thanks, and we have the same ideas about barrel tuners, just approaching them differently.

For those who would like to explore the method of Optimal Barrel Time, as originally developed by Long, there is a good writeup on his website. Note that you will need a chronograph and Quickload software, and a good understanding of how Quickload works. Quickload basics are straight forward to grasp, the advanced functions take a wee bit more time. For putting it all together, a good head for math is advisable.

http://www.the-long-family.com/optimal barrel time.htm


Or you can get in touch with Rman :)
 
Rman said:
I am using a modified version of Long's method, and have developed a program to run it. It has proven to be very accurate, and very predictable. So much so, that I have started a business using this program to help hand loaders, long range shooters, and hunters get the very best out of their ammunition and rifle combination.
While you are correct that this method only addresses the primary wave, you are also correct in that this is the only wave that can be easily calculated. I have used my program on thin, thick, long and short barrels, and have been extremely accurate in predicting the node, and the velocity at which that node exits for a given set up. It is not perfect, and the load development aspect is still very important, but it does provide a starting point, and a clear objective with respect to a target velocity. This eliminates the need for "ladder testing" and many other time and money consuming factors spent on finding a great load in a given rifle.
I am aware of what tuners are, and what they are used for. They have been used successfully by rim fire shooters for many years. Again, this addresses the issue of being unable to hand load rim fire ammo. I will disagree with your statement about tuning the rifle to the ammo. Perhaps we are saying the same thing, just differently? The whole point of the tuner is to tune the rifle, as it has zero effect on the ammunition being used, hence the love of tuners by rim fire competitors. While I realize that the tuner and hand load combination can work, it seems unnecessary when the ammunition variable can reduced greatly by targeted load development.

R.
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okay if your theory is correct im using a 210 berger 3.775 coal 28 inch barrel and h50bmg or retumbo? I have a fairly good idea how much powder I need for my nodes tell what you come up if u don't mind
 
mulemania said:
okay if your theory is correct im using a 210 berger 3.775 coal 28 inch barrel and h50bmg or retumbo? I have a fairly good idea how much powder I need for my nodes tell what you come up if u don't mind
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I do know that the theroy is correct. As mentioned above, the theory is not mine. Also, I run a business based on the program I developed. That means I charge money for the information you are asking for.
Also, there is a little more to it than me just telling what velocity your node is at.

R.
 
Vibration theory opens up a huge range of cause/effect possibilities!
Very interesting indeed!
I wonder if us shooters will ever reach a point where we dont need to obsess/tinker till it drives us crazy! Lol!

I sure hope NOT! This stuff is way too thought provoking!

Vibration analysis for rotating equipment is part of my job as a millwright, and is a constant challenge to read/track/ and reduce/eliminate.
 
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