Video Illustration of tuning loads to barrel harmonics

[JLT]

I try not to pick nits, but when the point was made that a bullet's exit must be coincident with a 'frequency node' for the best groups, I gotta speak up . . .

The video describes 'frequency node' as the point where y=0 (amplitude divided by 2) or, said a little differently, the point where the frequency sinusoid intersects the barrel's axis at rest. This is the point at which the vibrating medium (barrel, in this case) is moving at maximum velocity and the rate of change in velocity is at its lowest. This is the worst possible point for bullet exit as the deviation around the mean (the 'frequency node') will be the greatest and, therefore, the groups will be at their largest.

Smallest groups are obtained when bullet exit is timed at the extreme top or bottom of the sinusoid. The velocity of the vibrating medium is lowest at these points - zero, in fact, at the functional maximus and minimus.

 

[savagedasher]

View attachment 1087169 On this same subject, recently had a 100 yard test that looked like this. I could clearly see the barrel moving up and down in the groups. Which node would you go with based on group location (barrel harmonics) between 3 and 4 or between 7 and 8?

7 and 8 it has the best tune window
Tuners you can get the same results with less bullets
I also don’t see where you marked the bullet by number 1 2and 3
I personal have not seen a group getting smaller with the third shot
Myself I would rather seen another two shot then two 3 shots

 

[KMart]

Holy cow! I think I need to draw the line when the cost of a barrel is as much or more than the cost of a custom action.

So much for that saying you see so many times here..."buy once cry once"

 

[Ledd Slinger]

I try not to pick nits, but when the point was made that a bullet's exit must be coincident with a 'frequency node' for the best groups, I gotta speak up . . .

The video describes 'frequency node' as the point where y=0 (amplitude divided by 2) or, said a little differently, the point where the frequency sinusoid intersects the barrel's axis at rest. This is the point at which the vibrating medium (barrel, in this case) is moving at maximum velocity and the rate of change in velocity is at its lowest. This is the worst possible point for bullet exit as the deviation around the mean (the 'frequency node') will be the greatest and, therefore, the groups will be at their largest.

Smallest groups are obtained when bullet exit is timed at the extreme top or bottom of the sinusoid. The velocity of the vibrating medium is lowest at these points - zero, in fact, at the functional maximus and minimus.

Good point. Being an electrician for many years that makes sense to me. If you look at a Variable Frquency Drive sine wave (chopped DC) compared to a standard AC voltage sine wave, you can see that the pulses from the drive are sustained for a longer period of time, ie; more stable, at the peaks of the sine wave.



Thank you for pointing that out JLT. I cant tell what position my barrels are in when the bullet leaves the muzzle, but that definitely makes more sense to me.

 

[Bart B.]

Smallest groups are obtained when bullet exit is timed at the extreme top or bottom of the sinusoid.

I disagree. That might work if all bullets have the exact same muzzle velocity and barrel time, but they don't.

Smallest groups happen when those leaving at slower velocities need to do so at higher angles above the LOS than faster ones. Their greater drop at target range is compensated for.

If all bullets leave about either extreme, half will not be compensated for.

The ideal place for all bullets to leave is on the muzzle axis upswing just before the axis starts back down.

 

[jsthntn247]

Can you load faster than group 8 or is that max pressure? If it holds together with same accuracy and POI at higher speeds than group 8 then you are getting into a nice node.

Loads 9 and 10 opened up and started getting pressure at 10. I would say the node around 8 would be the highest possible with this combo. If speed doesn't matter and group size was nearly identical would a node higher or lower on target at 100 be preferable?

 

[chuckshooter]

Good point. Being an electrician for many years that makes sense to me. If you look at a Variable Frquency Drive sine wave (chopped DC) compared to a standard AC voltage sine wave, you can see that the pulses from the drive are sustained for a longer period of time, ie; more stable, at the peaks of the sine wave.

View attachment 1087175

Thank you for pointing that out JLT. I cant tell what position my barrels are in when the bullet leaves the muzzle, but that definitely makes more sense to me.

What we need now is the new and improved three phase rifle.

 

[okie]

What we need now is the new and improved three phase rifle.

View attachment 1087194

LMAO

 

[Ledd Slinger]

Loads 9 and 10 opened up and started getting pressure at 10. I would say the node around 8 would be the highest possible with this combo. If speed doesn't matter and group size was nearly identical would a node higher or lower on target at 100 be preferable?

"WHERE" the bullet impacts on the target doesnt matter as much as "consistency" of impacts. You want accuracy as well as consistent point of impact on the target between the various charge weights. Whether they consistently impact high or low on the target doesnt really matter so long as they are consistent.

What's your seating depth in relation to the lands? If touching, I would tune seating depth .005" jam and test again. Then tune seating depth .005" off and test again.

 

[Ledd Slinger]

What we need now is the new and improved three phase rifle.

View attachment 1087194

Hell yeah! Thats what I'm talkin' bout

 

[AckleymanII]

When I want to save weight, I just go down a notch in contour size...bug holes still abound.

 

[Ledd Slinger]

I disagree. That might work if all bullets have the exact same muzzle velocity and barrel time, but they don't.

Smallest groups happen when those leaving at slower velocities need to do so at higher angles above the LOS than faster ones. Their greater drop at target range is compensated for.

If all bullets leave about either extreme, half will not be compensated for.

The ideal place for all bullets to leave is on the muzzle axis upswing just before the axis starts back down.

How can anyone agree or disagree? Does anyone here have a $20,000 ultra high speed camera that can capture the bullet leaving the muzzle while measuring the flex of the barrel when fired? And then of course the targets would need to prove consistent accuracy benefits at that point of exit...

All most of us will ever be able to do on this subject is "assume" we know when the bullet is leaving the muzzle and make an "educated guess" for what is best.

I would "assume" the bullet leaving at the upper or lower peak of the swing would be best, but I'll never know if that is truly occurring on the groups that shoot with excellent accuracy.

 

[Bart B.]

How can anyone agree or disagree? Does anyone here have a $20,000 ultra high speed camera that can capture the bullet leaving the muzzle while measuring the flex of the barrel when fired? And then of course the targets would need to prove consistent accuracy benefits at that point of exit...

http://www.geoffrey-kolbe.com/articles/rimfire_accuracy/tuning_a_barrel.htm

Works the same as center fire barrels.

If the slowest and fastest bullet leave at the same angle centered about the muzzle angle high or low point, the group's vertical spread will be greater than if they all left on the upswing.

I don't think it takes precision equipment to figure that out.

https://www.varmintal.com/alite.htm

 

[JLT]

How can anyone agree or disagree? Does anyone here have a $20,000 ultra high speed camera that can capture the bullet leaving the muzzle while measuring the flex of the barrel when fired? And then of course the targets would need to prove consistent accuracy benefits at that point of exit...

All most of us will ever be able to do on this subject is "assume" we know when the bullet is leaving the muzzle and make an "educated guess" for what is best.

I would "assume" the bullet leaving at the upper or lower peak of the swing would be best, but I'll never know if that is truly occurring on the groups that shoot with excellent accuracy.

Well said.

Given a distribution of muzzle velocities, the sum of the first derivatives of the harmonic function will be smallest if the median velocity of the distribution places the bullet exit at the upper or lower peak. That's just a fact. That's math. With regard to trajectory compensation for lower velocities in the distribution, one must note that the physical characteristics of the barrel will have much to do with whether or not the compensation is helpful. The amplitude of the oscillations increases as barrel length increases and/or barrel mass decreases - as amplitude increases, so do the acceleration and velocity of the system between the functional maximus and minimus.

 

[SGK]

Interesting discussion (and product) here https://www.shootingsoftware.com/barrel.htm

 

[gunsandgunsmithing]

Well said.

Given a distribution of muzzle velocities, the sum of the first derivatives of the harmonic function will be smallest if the median velocity of the distribution places the bullet exit at the upper or lower peak. That's just a fact. That's math. With regard to trajectory compensation for lower velocities in the distribution, one must note that the physical characteristics of the barrel will have much to do with whether or not the compensation is helpful. The amplitude of the oscillations increases as barrel length increases and/or barrel mass decreases - as amplitude increases, so do the acceleration and velocity of the system between the functional maximus and minimus.

That is true, over time. It's also important to remember that we aren't dealing with a single sinusoidal wave here. The overriding element is the deformation of the barrel under pressure. There are many different frequencies occurring at the same time. A tuner seems to work because it adjusts the "average" of those frequencies, low and high. The highs have less influence but are of importance..

 

[Bart B.]

What we need now is the new and improved three phase rifle.

Most barrels have 7 or so harmonic frequencies up to near several times their fundamental resonant frequency at several dozen cycles per second.

Rifle barrels might be called an 8 phase thing.

 

[Bart B.]

A tuner seems to work because it adjusts the "average" of those frequencies, low and high. The highs have less influence but are of importance..

Tuners change all frequencies. The one most effecting bullet departure angle is the one the last few inches of the barrel vibrates at for a couple milliseconds after the primer fires.

This shows all of them: https://www.varmintal.com/amode.htm

 

[Iggyort]

What we need now is the new and improved three phase rifle.

View attachment 1087194

If we reverse phasing will the bullet go back into the barrel??

Great thread!!!

Thank you OP

 

[Ledd Slinger]

That is true, over time. It's also important to remember that we aren't dealing with a single sinusoidal wave here. The overriding element is the deformation of the barrel under pressure. There are many different frequencies occurring at the same time. A tuner seems to work because it adjusts the "average" of those frequencies, low and high. The highs have less influence but are of importance..

I agree that this is also something to consider because surely the harmonic vibrations are not only occurring on the 'vertical' axis (z) with a spinning projectile traveling down a circular bore. In fact, there has to be forces creating harmonics on an immeasurable amount of axis. Perhaps the forces most easily 'seen' with a high speed camera are on the vertical axis, but that doesnt mean that is all which is occurring.