Understanding barrel harmonics...cause I don't

[SS427]

This is more thinking out loud than anything else, so take it with that in mind.

I read about barrel harmonics and a load being in tune, but don't fully understand how this is suppose to work. When the load is in tune, supposedly it leaves the muzzle either at the top of the vibration or the bottom of the vibration, or somewhere during the slack moment when the barrel reverses direction.

This is what I don't understand. I don't think a barrel vibrates like an ocean wave, up and down. Why would it behave that way? The barrel doesn't know up from down from side to side. I know the weak force of gravity isn't over coming the forces of the explosion in the chamber to make the vibration go up and down. I imagine a barrel vibrating like how a wet dog shakes there at the end, when he is going in all directions at once. You know that last shudder when he is finishing up.

Does anyone have an explaination that barrels really do only vibrate up and down?

And if they vibrate like a shuddering wet dog, how the heck can you ever find a load that is tuned to accomodate all that harmonic direction? Or does the ideal load produce no harmonic vibrations? Or does it create vibrations that cancel each other out, thus leaving the end of the barrel in the same place the entire time?

Anyone else every think about this stuff? Is this one of the great mysteries of accuracy no one can explain, and why that one hole group is so elusive?

 

[gme]

put on a pot of coffee, lots to absorb.....
http://www.the-long-family.com/OBT_paper.htm
Gary Eliseo

 

[CJ6]

Go to Varmit Als web site . Lots of stuff for you there

 

[chino1969]

Otter,
Sometimes we need to simplify a concept to make it understood. You are right when you assume a barrel is doing more than going up and down but unless you posesss an engineering degree and are familiar with sinusoidal waveforms, elasticity modulus and calculus stick with the following simplified explanation. When a bullet is fired, it travels down the barrel causing the barrel to vibrate at a certain frequency or harmonic. In slow motion and freeze frame photography, you would see the barrel whip upward (pause for a nano second) then continue downward (pause for a nano second) and repeat the above for a given number of cycles until it becomes still once again. So for simplicity sake let's say the barrel is accelerating from rest to the upward position (point A). At point A the acceleration of the barrel is 0 for a nano second. After this the barrel starts to accelerate in the downward direction. At this point B, the barrel's acceleration in the down direction is zero. Ideally, you want the bullet exiting the barrel when is at either of these two extremes as the acceleration of the barrel will not influence the barrel the way it would if the barrel were in between these two points. Again, a simplified explanation that you can wrap your mind around without going crazy.

Lou Baccino

 

[Bergonzi]

Lou, that makes sense to me. I wonder if the barrel vibrating on its length produces nodes of inactivity along the way. If this is the case, then we would want the bullet to exit on one of those nodes...yes? As I understand it different bullets, powder/primer combinations, barrel lengths, etc. will produce "harmonic ripples" (that we call barrel whip) down and over the length of the barrel....?

Bergonzi
I'm just a hunter.

 

[CPorter]

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



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

Take a garden hose and hold it 24" from the end. Have a quick acting ball valve on it and crank it wide open really fast and shut it off as soon as water exits. You will see some of the same motions at a speed you can comprehend.

 

[chino1969]

Bergonzi,
Click on the Varmint Al links to see a simulation of what a barrel goes through and I think it will help. After you take a look at that check out the graph of a sine wave on Wilipedia. The sine wave is the red graph starting at 0 (your action). The red graph represents your barrel as it reaches a peak on either side of the x axis. You can see that it crosses the x axis in between peaks. When the barrel is at either of these peaks it has stopped for a nano second before the elastic nature of the steel brings it back in the other direction. The experiment with the water hose, previously mentioned, is another way to envision what is going on.


Lou Baccino

 

[mikecr]

Nobody knows because it's never been measured and defined.
It remains an abstract.
Same can be said for optimum seating depth, or one barrel's best performance over any other.

My guess is any common upward movement in a barrel would be the result of it's droop -taken out under pressure. Without this droop, there would be no reason for cyclic movements other than circular(due to twist).
And movement aside, I'm sure there are bore dimension cycles at the muzzle, causing the bullet release to be tight -or loose, no matter where it's pointed. (OBT theory).

I think it would take a very fast scanning laser mic to see it, and alot of expensive testing to define it(to the point of working prediction/design).

 

[BoydAllen]

You need to get a copy of Harold Vaughn's book Rifle Accuracy Facts. One thing that he points out is that when a rifle is fired it rotates around its center of mass. This acts to load a free floated barrel in a vertical direction so that the first move that the muzzle makes, relative to the rifle, is downward. When this deflection hits the point where the stiffness of the barrel stops it, it springs back up, hits the stop, so to speak, and back down, repeating this cycle. The reason that the rifle rotates is that its line of thrust (CL barrel) is offset from the center of mass. If they were in line, the only vertical force would be the straightening of the barrel droop, as the barrel is pressurized behind the bullet. Harold built a rail gun and measured barrel vibration with and without weight added above the CL of the bore (added to balance the weight below). With the added weight, the vibration amplitude was much less. This is probably why the most modern short range Benchrest stocks are of minimal depth, keeping the center of mass as close as possible to the line of thrust.

Another source of input to vertical barrel vibration during firing that he mentioned is the force that is transmitted to the bottom of the barrel's shoulder (at the front of the tenon) by a Remington style recoil lug. He built a bedding block that allowed the barreled action to recoil enough that the bullet had left the muzzle before the lug hit its padded stop. This had a great effect on barrel vibration.

 

[Steve Blair]

Mike: A Laser interferometer will measure minute barrel movement very accurately. I have also used the Kodak Ektapro 1000 system to analyze automatic pistol and rifle problems at 1000 frames per second. Using scales and high-speed strobes allowed visualization of events that I would not have believed if I had not seen them.

 

[mikecr]

Sounds like alot of fun Steve.
Have you captured a rifle muzzle in action, with the precision to measure or see what's going on?

Vaughn pushed more sticks in the mud, but didn't define anything. He just didn't have what it will take to do it.

 

[Steve Blair]

We were only concerned with the action and functional problems, and did not image anything else.

When you cycle the action of one of your autoloading rifles or pistols, it feels smooth and controlled. When you watch them during a firing cycle, they look like pieces will fly off. Very interesting effects occur when masses are accelerated that quickly. I would very much like to undertake a comprehensive investigation into barrel dynamics. The tools available now are so much better than we had in the 80s and 90s and are actually affordable for a moderately funded project.

I made the switch to IT years ago during the last recession and have only been a shooter like everyone else, since.

 

[SS427]

Gary - read that info...understood about half. College physics doesn't go far toward understanding this stuff and that was 20+ years ago. So if I understood it right, his theory is the barrel actually bulges outward and then contracts, bulges, contracts, bulges, contracts, until the energy is gone. This causes the bore to get larger, smaller, larger, smaller, again until the energy is gone. The key is to get the bullet to exit the muzzle at a point where the muzzle is static...neither too big or too small. Interesting idea, and about a good of explaination as I've heard. Not convinced it is true, but a good read. The entire website was pretty good actually. I'd never found that one before, so thanks for directing me there.

CJ6 - I've read pretty much everthing on Varmint Al's website. I'm a proud own of a bi-fur pod! Good stuff and fun reading. Al's videos of barrel vibration is what got me thinking a barrel shudders like a shaking dog a lot more than being a uniform wave up and down.

Lou - this is the version I usually hear, and don't doubt what you say. High speed photography doesn't lie. But what did the high speed camera mount above the gun show? I'm guessing they didn't film it from above, and if they did, you would see as much side to side motion as up and down.

BoydAllen - I could see some rotational forces acting on a barrel from the spin of the bullet. This rotational force is one I hadn't even considered. One more force acting on barrel to contend with. You say it wants to rotated downward...why downward? Why not upward or off to one side?

Thanks for all the responses. I can't say I understand it any more than I did before, but more to think about.

 

[castleton]

On VarmintAl's site, specifically look at the first graph "Typical Muzzle Projection Curve" on the page http://www.varmintal.com/aeste.htm and read the summary below. To compensate for different muzzle velocities, you want bullets coming out of the muzzle when it is rising, not when it is stationary.

The rifle rotation that Boyd is talking about is in the vertical plane, not around the axis of the bore. Think of a football receiver having just caught a punt. If he is tackled around the ankles, he falls face first. If he is tackled around the shoulders, he falls backwards because the tackling force is applied above his center of mass. Like a high tackle, the rifle experiences a force on its bolt face when the powder ignites that is above its center of mass. Therefore, the action rotates in the vertical plane and gets the barrel motion started.

Try this sometime: shoot a rifle upside down. The recoil force will cause the muzzle to drop, instead of rise, because its center of mass is now above the bore.

Cheers,
Keith

 

[Steve Blair]

Keith,

What you describe is correct if the rifle is shot free recoil. If it is tightly supported, you must also consider the center of support relative to the bore centerline. If the fulcrum is sufficiently above the bore centerline, the muzzle will deflect downward, reflecting the induced moment.

Ideally, a rifle would have the buttpad centered on the bore. Practically, the sight line would be so high as to cause other problems. Why must we compromise on everything?