Tuning for barrel harmonics

[scottmahr]

Hey all, I was watching a video on youtube where the guy was using different muzzle devices and shooting groups to try to understand barrel harmonics. He basically got a bunch of confusing data out of that, and didn't know what to do. He had previous experience that adding his favorite suppressor usually improved group size on many rifles, and his thought was that heavier suppressors would do better than lighter ones, which didn't turn out to be the case.

He has a lot of good reloading videos, so I wanted to send him some thoughts on what he was seeing, based on a bit of physics intuition, and also this and this video from Erik Cortina.

I have zero practical experience in this and could very easily be completely wrong. Wanted to see how this group thought about this topic.

Maybe this is obvious to people already, but it helped me to draw it out. It shows that:

1. You need to either lock down your powder charge or seating depth before trying to fine tune the other.
2. As long as you have consistent charge and seating depth, you should be able to tune your barrel to match.
3. You probably want to make tiny changes, so you don't jump between sine waves and get confused.

My biggest questions now are:

• What is the ballpark frequency of most barrels? How many times do they oscillate before the bullet leaves the barrel? (I read somewhere it was like 4, but I have no source for that)
• In which direction does the barrel oscillate? What does that depend on?

 

[Straightshooter1]

Hey all, I was watching a video on youtube where the guy was using different muzzle devices and shooting groups to try to understand barrel harmonics. He basically got a bunch of confusing data out of that, and didn't know what to do. He had previous experience that adding his favorite suppressor usually improved group size on many rifles, and his thought was that heavier suppressors would do better than lighter ones, which didn't turn out to be the case.

He has a lot of good reloading videos, so I wanted to send him some thoughts on what he was seeing, based on a bit of physics intuition, and also this and this video from Erik Cortina.

I have zero practical experience in this and could very easily be completely wrong. Wanted to see how this group thought about this topic.

Maybe this is obvious to people already, but it helped me to draw it out. It shows that:

1. You need to either lock down your powder charge or seating depth before trying to fine tune the other.
2. As long as you have consistent charge and seating depth, you should be able to tune your barrel to match.
3. You probably want to make tiny changes, so you don't jump between sine waves and get confused.

View attachment 1224630

My biggest questions now are:

• What is the ballpark frequency of most barrels? How many times do they oscillate before the bullet leaves the barrel? (I read somewhere it was like 4, but I have no source for that)
• In which direction does the barrel oscillate? What does that depend on?

Based on my limited knowledge of resonance, it's a lot to do with the material, length, contour and thickness of a barrel and how it reacts to a particular cartridge charge and even how what's touching it effects the barrel, where they react much like a tuning fork.

Maybe this might help understand how a barrel's resonance nodes can work (see the nodes in the flames, start at 6:05) . . .

 

[CharlieNC]

Visit Varmint Al website for a number of good evaluations on the topic of harmonics, these are the most comprehensive I have seen in one spot.

In terms of "locking down" powder charge and seating depth one at a time, as with any experiment this is a widely misunderstood topic. For example in his book Tony Boyer shows examples of a grid where he shoots all combinations of something like 5 charges x 5 depths = 25 combinations ( I may have the exact number wrong but you get the drift) in order to find the sweet spot because the best charge can depend on the seating depth (and visa versa). In statistics this is called a full factorial design. But there are other valid, more efficient approaches whereby the 5 levels of each can be evaluated in only 9 combinations. In both approaches the focal point is recognizing that the effect of one variable depends upon the value of the other variable, which you will not learn by simply doing each one individually. This is not the so called "change one thing at a time scientific method", and is actually much more comprehensive. While the methods are more than a five minute discussion, I bet a good overview could be found on youtube. Probably more than you wanted to know on the topic.

 

[scottmahr]

Damn, I had a very naive, wrong understanding of this, is anyone gets to here, check out

Using barrel vibrations to tune a barrel

and

Using barrel vibrations to tune a barrel

for great analysis.

Scott

 

[RegionRat]

Both of those links are the same, so if you meant to point to different ones, you may want to edit your second one.

ETA: maybe this one?
http://www.geoffrey-kolbe.com/articles/rimfire_accuracy/modelling_barrel_vibrations.htm

The idea that a tuner weight will affect a group performance is valid and nothing new. The exact mechanics of that behavior and the angle transducer used are not widely accepted or peer reviewed. My only comments on that work, is to take it with a grain of salt until such time that the results have been independently reproduced and reviewed. Both papers are good work, but limited resources contribute to issues.

ETA2: I like JRB on YouTube. He does honest work and presents it at face value.
In that video, he only made one error but it was a big one. The jumps are way too large and he passes over several tuning nodes. For example, if he only focused on the Harrel's tuner, the size of the steps when that is adjusted are much, much smaller than the jumps he makes in all those tests. Even the Limbsaver damper showed it can have a dramatic affect on group size just based on being slid from the muzzle to near the stock. If he had gone back to tune any single one in fractions of a thread turn, he would have seen completely different results, including better ones and worse ones.

When experimenters make assumptions or propose hypothetical explanations for the mechanics of bbl harmonics, and then try to explain only the ones in their theory, it is a form of experimental confirmation bias. Some harmonics are due to what we will call structural natural frequencies, with the emphasis on it being plural because there are multiple modes at work all at the same time, and some are related only to the speed of sound in the steels used. If any experiment is designed to focus on just one, without accounting for all the other simultaneous harmonics, it dooms itself when it is constructed with a typical gun. There are also harmonics within the gas column behind the bullet as it travels down the bbl, and favorable results depend on getting those waves right at the same time. You could build a bbl and receiver with such a heavy section that the natural frequency of any structural mode was off the charts, but you would still catch harmonic behavior due to the internal ballistics and the speed of sound in the metals.

 

[memilanuk]

If you are referring to the YouTube channel "Johnny's Reloading Bench"... while I do enjoy the channel and the content... Tuners aren't magic. The gun, gear, ammo, etc. have to be all working together very well *before* a tuner will be anything more than an exercise in frustration.

 

[DocBII]

scottmahr,

You can play around with the different nodes with this tool:

Link to download Quickload OBT Tool

I've had decent results just using the first node.

HTH,
DocBII

 

[Uncle Ed]

You can tune the ammunition to the rifle or tune the rifle to the ammunition.

Meaning if the barrel is free floating you can tune the ammo. Or like many rifles you can change the up pressure at the forend tip. For example the older Remington 700 had 7 to 9 pounds of up pressure at the foreend tip.

You can also try changing the action screw torque settings that effects barrel vibrations.

Savage Action Screw Torque Tuning​

https://www.accurateshooter.com/technical-articles/savage-action-screw-torque-tuning/

 

[bsekf]

You can tune the ammunition to the rifle or tune the rifle to the ammunition.

Isn't this the reason I fool with CBTO?

 

[Webster]

Hey all, I was watching a video on youtube where the guy was using different muzzle devices and shooting groups to try to understand barrel harmonics. He basically got a bunch of confusing data out of that, and didn't know what to do. He had previous experience that adding his favorite suppressor usually improved group size on many rifles, and his thought was that heavier suppressors would do better than lighter ones, which didn't turn out to be the case.

He has a lot of good reloading videos, so I wanted to send him some thoughts on what he was seeing, based on a bit of physics intuition, and also this and this video from Erik Cortina.

I have zero practical experience in this and could very easily be completely wrong. Wanted to see how this group thought about this topic.

Maybe this is obvious to people already, but it helped me to draw it out. It shows that:

1. You need to either lock down your powder charge or seating depth before trying to fine tune the other.
2. As long as you have consistent charge and seating depth, you should be able to tune your barrel to match.
3. You probably want to make tiny changes, so you don't jump between sine waves and get confused.

View attachment 1224630

My biggest questions now are:

• What is the ballpark frequency of most barrels? How many times do they oscillate before the bullet leaves the barrel? (I read somewhere it was like 4, but I have no source for that)
• In which direction does the barrel oscillate? What does that depend on?

A lot of assumptions here. You are assuming the barrel only vibrates in a single plane. The vibrations may be in all directions. Maybe like a figure 8. You could draw an almost straight line thru the data for the top line in the chart. It's sine shaped because you drew it that way. I'll always go by group size. Am I missing something here? Pointing away from the target does it mean any direction? I don't always see a change in impact location with powder changes? Accurrate varmint rifle 6BR Kreiger barrel.

 

[KY_Windage]

I was developing my 115-gr. cast lead 30-cal load yesterday because I got a new barrel. I do my seating at the range for that, so I can easily play with different seat depths and powder weights. I spent all day doing that.

The amazing thing to me was that with one particular powder-weight I was getting consistent 1 moa groups, but if I varied it even a 10th of a grain up or down I was back to 3 moa.

The seat depth I had worked out earlier, but who knows, maybe now that I have nailed the powder weight I could improve it even more by playing with the seat depth again.

I also have to wonder if I don't need to start setting my beam scale to my (apparently) optimum setting, but then trickling until I am, say, ONE-THIRD of the way between the "zero" line and the next line up! And then 75% of the way to the next line, etc.



It's funny how many guys walk by and think I'm nuts for "loading at the range," but I think I'd have to be nuts not to.

 

[Uncle Ed]

Isn't this the reason I fool with CBTO?

With many military rifles, you use issued ammunition that you can not tweak, so the rifle bedding is used to tune the rifle. There are many ways to tune a rifle and the simplest is to vary the torque on the action screws. This also controls how the barrel vibrates along with other methods.

I had a Winchester model 70 with BOSS and with just two adjustments the rifle was shooting 1/2 inch groups. And with milsurp rifles, all I had to do was change the fore end up pressure.

With CBTO you are changing the start pressure and its effect on the barrel vibrations.

Or you can use a bayonet and a flashlight to tune your rifle.

No koala bears were hurt or injured during the filming of this posting.

 

[normmatzen]

Scott Mahr's articles (both) were written by Dr. Kolbe. The same author of Region Rat's article.
He used to own Borders Barrels located close to the Scotland-England border.
He is one smart cookie! The yellow instruments used in the first article were designed and built by him.
Unfortunately, he is a small bore shooter and seems to have a disdain for center fire rifles.
Consequentl, I have not seen articles by him for long range tuning. I contacted him once and that was the impression I got.

 

[Shooter13]

Can't believe @gunsandgunsmithing hasn't responded yet, hope he is working on my gun!