Barrel length and width of accuracy nodes.

[BoydAllen]

There is one more issue regarding tuner weight that can be very important, rifle balance. Short range group rifles are most commonly built to 10.5# weight limits with a single rifle being used for LV, HV, and Sporter. Many shooters prefer to shoot free recoil, and in those cases rifle balance is critical to performance, as are stock and bag design. The weight in front of the point where the front bag supports the rifle acts to lessen the down force on the rear bag, and even without a tuner, this can be an important issue for 10.5# rifles. For these rifles balance is the limiting factor for tuner weight. One of the more interesting things that is overlooked is the relationship between barrel length and stiffness. People generally understand that for a given contour that shorter barrels are stiffer but I am pretty sure that they do not appreciate just how fast stiffness increases as length decreases. Dan Lilja has a barrel stiffness program on his web site that can be instructive to play with. My point is that if a barrel is stiff enough at a longer length and there is some room to remove weight by decreasing diameter slightly, if you shorten a barrel, you might be able to make it smaller and still have the same stiffness as before you cut it off. This weigh bonus could be put into sorting out rifle balance, with or without a tuner. Here is a link to the page on the new Lilja website. http://riflebarrels.com/computer-software/
When I say room to remove weight by decreasing diameter I am talking about the minimum diameter that is required so that the number of shots fired in a match will not heat the barrel to the point where accuracy suffers. Of course fluting can also be done to deal with weight issues but am primarily concerned with what can be done without fluting given the cost and accuracy life of competition barrels. I do not have any solid information about how slim a barrel may be at a given length, without compromising performance. I have spoken with a shooter who has worked with barrels that were lighter than are considered "normal" for short range group work, and he told me that those barrels performed very well. The reason that he used them was because of stock availability issues when he was stationed overseas. A heavier stock required a lighter barrel.

 

[dcali]

It's not a simple yes/no answer. You need to think in terms of the rifle's frequency of vibration. When the rifle fries, the whole thing flops around. You can't really see it, but it does. The frequencies at which it does that, and the shapes those frequencies take are called "modes" of vibration. Every rifle has multiple resonant frequencies, or modes, that take vibrate in different manners (side to side, up and down, back and forth, etc. The combination of all of those modes is what determines how a rifle will respond to a shot.

If that's confusing, you're on the right track. You can, however, make some general statements:

Longer barrels have lower frequency
thicker barrels have higher frequency
lighter barrels have higher frequency
weights on the end (like tuners) lower frequency
The stock, rests, and scope are significant, both in terms of stiffness, and the weight distribution.
How firmly the rifle is held matters. A bolted down rest vs free recoil will react differently.

Tuners work by lengthening/shortening the barrel and by moving the mass forward and back. (It turns out frequency is very sensitive to length).

So all told, my best guess would be longer, thinner barrels would exhibit the least sensitivity. That's not necessarily what you want, though. You want a rifle that's sensitive enough that you can tune it without it being too finicky. Milage may vary, though, because weird things happen that aren't easy to predict, and the window we care about is so small that it's not uncommon to just chalk it up to "well, the rifle likes that" (which is BS, of course - rifles don't have brains.) But that's how hard it is to predict this stuff with certainty.

But at the end of the day, this is not a matter of some theoretical simplifications - you have to test this stuff to find out exactly how a rifle behaves. I used to model the structural dynamics of spacecraft. An amazing amount of effort and software power went into solving this exact same problem in that world, and at the end of the day, we still had to go test. We'd literally shake the spacecraft and see how it responded to different frequency drivers. Sometimes the software was right, sometimes it was a little off. But in no case was "well, the model said this" good enough. This is even more true for precision rifle dynamics. The errors we tolerate are so small that traditional engineering math will not get you close enough to not follow up with testing. The models are hugely helpful, and will get you close, but we can kind of skip that step because we have decades of trial and error to guide us to "close enough".

 

[dcali]

Just daydreaming aloud here: What if you had an untapered barrel, and a tuner weight which could be positioned anywhere along its length? Been tried?
-

This would work, but there's a reason they put them at the muzzle (aside from practicalities of how you'd actually build such a thing). The distance from the front face of the action to the tuner's center of mass is what matters. The further out you put it, the more sensitive it gets. Depending on barrel length, contour, and tuner weight, it would not surprise me to find combinations that have an optimal position (in terms of ease of adjustment) that is a little back from (or ahead of) the actual muzzle, but you'd really have to play around to find out if that's true.

 

[gunsandgunsmithing]

I was just quoting Brian Lits at Applied Balistics when I said heavier barrel produce wider nodes. Do you think his research is flawed?

I'm not familiar with any study that says that, as I'm interpreting it. It is counter to physics. As I said earlier, I may be reading more or less into that statement than is intended or some other misinterpretation, but simply stating that a heavier barrel has a wider node, I would disagree with..yes. Now, if by a heavier barrel, he means adding mass, such as a tuner at the muzzle..different story, as that will lower the frequency and widen the node.

 

[Grimstod]

I'm not familiar with any study that says that, as I'm interpreting it. It is counter to physics. As I said earlier, I may be reading more or less into that statement than is intended or some other misinterpretation, but simply stating that a heavier barrel has a wider node, I would disagree with..yes. Now, if by a heavier barrel, he means adding mass, such as a tuner at the muzzle..different story, as that will lower the frequency and widen the node.

Buy his book and read it. I would post a screen shot of the page but a friend is reading it at the moment. Which reminds me he has had it a while now. Should ask him if he is done with it.

 

[gunsandgunsmithing]

Buy his book and read it. I would post a screen shot of the page but a friend is reading it at the moment. Which reminds me he has had it a while now. Should ask him if he is done with it.

What test equipment was used to measure with? I would like to see a screen shot. I'm confident that his intent was not for it to be interpreted the way you seem to be presenting it here. Again, it depends on the context of "heavier" or "weight".
Bottom line is, stiffer will vibrate at a higher frequency. More mass will dampen it faster, though. Perhaps that's what he was referring to.

 

[dcali]

It's not either/or. Both weight and stiffness have an impact on vibration frequency, mathematically speaking. More specifically, it's the ratio of stiffness to weight that matters.

 

[gunsandgunsmithing]

It's not either/or. Both weight and stiffness have an impact on vibration frequency, mathematically speaking. More specifically, it's the ratio of stiffness to weight that matters.

Not sure what you mean. I get that, if we were talking carbon fiber vs 416ss, it would matter. Please explain, but assume 416ss and identical contours, so we're comparing apples to apples. I like to measure stiffness by hanging a weight from the end of a barrel and measure the amount of deflection, or sag. In this context, an 1.250 straight is less stiff than a HV contour, both being typical and same barrel lengths. Is this an example of what you're referring to.

 

[DirtySteve]

Very good reading and discussion here. I know little about the subject but was contemplating what I have read and trying to decide how this all works. I think that short stiff heavy barrels may have an advantage when it comes to overall accuracy. By that I mean accurate over a wider velocity range. The reason I believe this is it would take more energy to impart a vibration in the barrel due to it mass, and the vibration would be less than that of a barrel of equal length and less mass. By less vibration I am referring to the distance from the peak at the high point to the bottom of the low point. A longer barrel would (By my thinking) have a wider accuracy node due to the longer frequency of the vibration compared to a shorter barrel with a higher frequency of the vibration. Just a thought. I did drop out of an Engineering school, so I may not be on the mark with this. (Damn higher math got me).

 

[dcali]

Not sure what you mean. I get that, if we were talking carbon fiber vs 416ss, it would matter. Please explain, but assume 416ss and identical contours, so we're comparing apples to apples. I like to measure stiffness by hanging a weight from the end of a barrel and measure the amount of deflection, or sag. In this context, an 1.250 straight is less stiff than a HV contour, both being typical and same barrel lengths. Is this an example of what you're referring to.

Let me clarify. There are two physical quantities that matter here. One is stiffness, which is dependent on material properties, the loading you care about, and the geometry of the part. The other is weight, or more exactly, the distribution of weight. *All else equal*, increasing stiffness increases frequency, and increasing weight decreases frequency. The ratio of the two is the simplest way to think about it - if stiffness per ounce goes up, then frequency does too.

Let's also be precise about what "stiffness"means. The ratio of muzzle deflection to force (in your example of hanging a weight off the muzzle) is one example. the amount of length contraction per pound of force if you were to squeeze the barrel longitudinally would be another, as would the change in circumference of the barrel per psi of pressure. All of these various stiffness definitions are relevant to different vibrational modes. But they all follow the basic relationship that frequency is proportional to stiffness divided by weight. That's a bit of an oversimplification, but it's a high level picture of what's going on.

When comparing contours, you must, must, must also think about length. Length has a much larger impact on barrel frequency than contour (longer length means less stiffness). Nobody really thinks about this for some reason, but it's the absolutely the case.

In real life, of course, these things are all connected from a design perspective - swapping materials (from, stainless to titanium, for example) would change weight *and* stiffness - titanium is about half as stiff as steel and also half the weight. Changing a barrel contour would do the same - removing material reduces both weight and stiffness. But the two fundamental, independent quantities are weight and stiffness, and they both play a role in the physics.

The much harder question is what is the best frequency? That, I can't tell you, but it could be figured out for a specific rifle configuration if someone had a lot of time and money on their hands.

 

[gunsandgunsmithing]

Let me clarify. There are two physical quantities that matter here. One is stiffness, which is dependent on material properties, the loading you care about, and the geometry of the part. The other is weight, or more exactly, the distribution of weight. *All else equal*, increasing stiffness increases frequency, and increasing weight decreases frequency. The ratio of the two is the simplest way to think about it - if stiffness per ounce goes up, then frequency does too.

Let's also be precise about what "stiffness"means. The ratio of muzzle deflection to force (in your example of hanging a weight off the muzzle) is one example. the amount of length contraction per pound of force if you were to squeeze the barrel longitudinally would be another, as would the change in circumference of the barrel per psi of pressure. All of these various stiffness definitions are relevant to different vibrational modes. But they all follow the basic relationship that frequency is proportional to stiffness divided by weight. That's a bit of an oversimplification, but it's a high level picture of what's going on.

When comparing contours, you must, must, must also think about length. Length has a much larger impact on barrel frequency than contour (longer length means less stiffness). Nobody really thinks about this for some reason, but it's the absolutely the case.

In real life, of course, these things are all connected from a design perspective - swapping materials (from, stainless to titanium, for example) would change weight *and* stiffness - titanium is about half as stiff as steel and also half the weight. Changing a barrel contour would do the same - removing material reduces both weight and stiffness. But the two fundamental, independent quantities are weight and stiffness, and they both play a role in the physics.

The much harder question is what is the best frequency? That, I can't tell you, but it could be figured out for a specific rifle configuration if someone had a lot of time and money on their hands.

Ok, then we agree.
As to the "best" frequency, you have to remember that we're chasing a moving target with powder characteristics that change with temps. That's a big part of why guns go in and out of tune with conditions and why we must change the load or move the tuner to time bullet release with optimal muzzle position.
Thankfully, it's much, much easier in practice than understanding all of the whys and hows. I don't think there is a "best" frequency, for this reason. That's also why I do not believe in setting a tuner and forgetting about it, any more than setting a load and never changing it...unless you have a tuner, of course.

 

[dcali]

Ok, then we agree.
As to the "best" frequency, you have to remember that we're chasing a moving target with powder characteristics that change with temps. That's a big part of why guns go in and out of tune with conditions and why we must change the load or move the tuner to time bullet release with optimal muzzle position.
Thankfully, it's much, much easier in practice than understanding all of the whys and hows. I don't think there is a "best" frequency, for this reason. That's also why I do not believe in setting a tuner and forgetting about it, any more than setting a load and never changing it...unless you have a tuner, of course.

As with any "best" you have to define what that means. I think that a rifle that allows for a good load to be found at max pressure and without too much sensitivity to hard to control factors is the goal. In that regard, I think you could probably optimize the system to make this easier. I suspect that years of trial and error and adherence to various competitive rulebooks have resulted in some fairly optimal setups, but you never know.

 

[gunsandgunsmithing]

As with any "best" you have to define what that means. I think that a rifle that allows for a good load to be found at max pressure and without too much sensitivity to hard to control factors is the goal. In that regard, I think you could probably optimize the system to make this easier. I suspect that years of trial and error and adherence to various competitive rulebooks have resulted in some fairly optimal setups, but you never know.

Yes, I agree with that too. But, as conditions change, tune changes. That's fact..or at least an experience based conclusion that most all serious shooters will agree with. I do think that can very often tuners allow one to use a proven load for a given cartridge and tweak the gun to the load.
As for the wide tune window or possibly even improving accuracy with a barrel that gives more deflection at the muzzle...Savage come to mind. Their barrels aren't as big at the breach as most. Not saying that this is why they shoot so well for a factory rifle, but it is something to consider and is an example of a gun thats barrel is less stiff than what is typically seen. It's hard to argue with how most of them perform, for what they are. Railroad tracks and all. Just a thought along the lines of this thread and barrel stiffness. May mean zilch, but I don't think it's all due to the floating bolt head either.

 

[carlsbad]

Remember gentlemen, we're talking 2 concepts here guys: inherent "accuracy", and width of node when tuning.

A stiffer barrel will generally be more accurate in raw form (untuned). However, a stiffer barrel will have a narrower node because of it's higher frequency. Higher frequency generally means higher velocity so the node is narrower.

A floppy barrel will not generally be as accurate but it is moving slower so it will have a wider node.

So if we're designing the ultimate barrel, we would want to determine how wide a tuning node we need and design the stiffest barrel that gives us that.

So I just put my first tuner on a barrel, on a 30" extra heavy varmint (1.020" muzzle) and the width of the tuning node was plenty wide. Tuning with load change is more challenging.

--Jerry

 

[gunsandgunsmithing]

Remember gentlemen, we're talking 2 concepts here guys: inherent "accuracy", and width of node when tuning.

A stiffer barrel will generally be more accurate in raw form (untuned). However, a stiffer barrel will have a narrower node because of it's higher frequency. Higher frequency generally means higher velocity so the node is narrower.

A floppy barrel will not generally be as accurate but it is moving slower so it will have a wider node.

So if we're designing the ultimate barrel, we would want to determine how wide a tuning node we need and design the stiffest barrel that gives us that.

So I just put my first tuner on a barrel, on a 30" extra heavy varmint (1.020" muzzle) and the width of the tuning node was plenty wide. Tuning with load change is more challenging.

--Jerry

Remember gentlemen, we're talking 2 concepts here guys: inherent "accuracy", and width of node when tuning.

A stiffer barrel will generally be more accurate in raw form (untuned). However, a stiffer barrel will have a narrower node because of it's higher frequency. Higher frequency generally means higher velocity so the node is narrower.

A floppy barrel will not generally be as accurate but it is moving slower so it will have a wider node.

So if we're designing the ultimate barrel, we would want to determine how wide a tuning node we need and design the stiffest barrel that gives us that.

So I just put my first tuner on a barrel, on a 30" extra heavy varmint (1.020" muzzle) and the width of the tuning node was plenty wide. Tuning with load change is more challenging.

--Jerry

Well said Jerry

 

[BenPerfected]

For some, barrel stiffness is more of a goal

 

[BartsBullets]

It seems to me that this is all just polite conversation!

Any barrel configuration can exhibit a small tuning window (node) or a large one! It doesn't matter if it's long, short, fat or skinny. That's just how it is in the real world.

The type competition you shoot in may dictate or limit the type contour you can use. For short range benchrest to make 10 1/2lbs you are pretty much limited to a LV tapered. The world opens up a little more with a 13 1/2 lb HV. In 600 or 1000 yard bench the weight class jumps to 17Lbs. So there are lots of HV taper and 1.200 straights.

For practical purposes I want the longest possible barrel that I can get. From there if it doesn't shoot I can have the barrel cut and recrowed and try again. I've seen barrels go from junk to jems by taking off 1/4 to 1/2 inch.

Bottom line is you have to take it out and shoot it to see what you have.

Bart

 

[chop house]

Many shooters prefer to shoot free recoil, and in those cases rifle balance is critical to performance

Boyd, can you elaborate on what is optimal in terms of balance, or point me to any previous threads or articles that address the subject. Is vertical stringing a typical symptom of improper balance? thx

 

[BoydAllen]

Chop House,
First tell me what sorts of rifles you shoot from the bench what sort of bench equipment you use to support them and if you shoot any of them real free recoil (nothing touching your rifle except your trigger finger until your shoulder stops the rifle during recoil). The reason that I want to know this stuff is so that I can do a better job of answering your question. Before you do that, little tiny groups can be shot holding a rifle, including trigger hand contact shoulder and cheek. A lot of shooters seem not to know this, but there are rules that apply and if you do not follow them you may come to the conclusion that it cannot be done. My reason for bringing this up is that balance becomes much less important if you are holding a rifle, rather than barely touching it or shooting real free recoil. Finally, I prefer to write from my own experiences unless there is a strong reason to cite a source. I can give examples of what I have seen others do as well. One time just after finishing explaining some rather tricky aspect of reloading to a friend, something that for all of his years of experience he had not learned, that I had figured out entirely on my own and had never read, he asked me what my reference was. I really had to restrain myself. I wanted to ask him why he thought I would need to have found the information in a book or magazine, and tell him that no one that I knew of could have given him a better explanation than I just had. Most of what I know I have figured out on my own, learned by experimentation, or from conversations with record holding shooters who have been kind enough to have spent some time educating me. Of course I have also read every single thing that I have been able to lay my hands on that related to accuracy, for about 35 years.

 

[chop house]

what sorts of rifles

Mainly varmint profiles, wt 12+/-#, 24 to 26", 204 to 243, triggers mostlly in the 2# range, beavertail forends or b&c provarmint stocks. Also some sporter wt 700's up to 7mm 08.

bench equipment

hoppes-like adjustable front with protecktor owl ears, rear leather stacked on either a baseplate or flat sandbag for elevation adjustment.

real free recoil

not as described. triggers are all too heavy at 2#

barely touching

i try . the best results are when i get the rifle to ride straight back. no horizontal problems then. when rifle rotates up or twists ccw i feel i am losing control and target shows it.

thanks!