Thoughts on Positive Compensation

[Popsbengo]

Further to our earlier discussions on positive compensation (PC), rather than hijack that post I've started a new discussion.

 

I've a question that I can't get my head around.  I can’t see why a barrel clamped conventionally in a stock (ie the action bolted and bedded) would only vibrate vertically?  Just to be clear PC is not about muzzle flip due to rotation caused by off-axis forces, it's vibrational movements in the barrel.

 

I understand the principle of positive compensation in that launch angle and muzzle velocity are ‘tuned’ to match the vertical dispersal due to MV, with launch angle thereby achieving a tight mean point of impact at the tuned distance.   But what about barrel vibration in the horizontal plane?  Why do we not see horizontal stringing of the point of impact?

 

I read G. Kolbe's excellent paper  "The Vibrations of a Barrel Tuned for Positive Compensation" and appreciate the principles of PC.  But what about vibration of the barrel in the horizontal plane?  Why do we not seem to see launch angle variation horizontally ?  Obviously MV is somewhat irrelevant to the horizontal but if the barrel's vibrating then a horizontal and vertical component is present and therefore an uncompensated (by MV variation) horizontal spread should be seen.

 

Taking Kolbe's measured test set-up he has a barrel varying its launch angle by 0.8MOA (1010fps at 9.3MOA and 1060fps at 8.5MOA) for a 50yd .22 shot.  If we had a horizontal vibration of similar magnitudes then we should see 0.4" horizontal at 50yds but that is not apparent so either barrels don't vibrate horizontally or I'm not seeing something.

 

Thoughts? 

 

[1066]

I really think we are only scratching the surface with our understanding of barrel vibrations - lots of theories and anecdotal evidence but very little repeatable data.
My own idea is that the muzzle of an untuned, undamped barrel moves in a vertical figure of eight sinusoidal type movement. I believe the mostly vertical component is just due to gravity. I've never tried to measure it but there must be considerable amount of droop with a heavy, long barrel. Take a typical old school heavy barreled .22lr match rifle. The fore-end is quite stiff, likewise is an inch steel bar but it takes only finger pressure to move a floating barrel in the channel. I think if you were to measure the clearance gap, then turn the rifle upside down and measure again there would be a measurable difference. 

Here's a short video demonstrating "null" points in a metal rod..

 

[terryh]

All sorts of things to think on here, as to what really makes a difference to accuracy (consistency 🤔) might be moot I.e. picking the fly that which promotes growth and vigour out of the pepper in gains.

But one comment re null points, some BPCR shooters use a rest near the end of the barrel counter to common practice. I believe they put talc along the top of the barrel, tap it, see where the talc piles up ( I.e. a null pint) then put a bit of tape at this point as the ref. to rest on?

[1066]

12 minutes ago, terryh said:

But one comment re null points, some BPCR shooters use a rest near the end of the barrel counter to common practice. I believe they put talc along the top of the barrel, tap it, see where the talc piles up ( I.e. a null pint) then put a bit of tape at this point as the ref. to rest on?

In the vein - has anyone any experience of approaching the problem from the other end and trying to deaden vibration altogether i.e. Limbsaver de-resonator style ?
I'm thinking about putting heatshrink tubing on the full length of the barrel. Although a "dead" barrel won't have the benefit of PC tuned to a fixed distance it might be interesting to try.

[terryh]

I’ve tried sets of O seals spaced in groups along a barrel, same concept as Limbsaver, but did not do enough to come to any conclusion 🤷🏼‍♂️

[No i deer]

I've gotba limbsaver thing..

I might have a play with it..

When I tried it the ammo inused was crap..

Beyond help

[ds1]

TACOM HQ offer an alternative to the dick-‘ring with structured barrels in an attempt to reduce harmonic effect.

https://tacomhq.com/structured-barrels/

Mark and Sam had one to play with.

 

[Popsbengo]

All interesting but what about horizontal vibration effects ?   Any observed ?

[Big Al]

6 hours ago, No i deer said:

I've gotba limbsaver thing..

I might have a play with it..

When I tried it the ammo inused was crap..

Beyond help

Dont taste your time, its too easily moved so offers no consistency, there is also no fine adjustment whatsoever, its pure guess work.

I would liken it to being as good an idea as filling your cases with powder by eye rather than using a scale.

[No i deer]

8 hours ago, Big Al said:

Dont taste your time, its too easily moved so offers no consistency, there is also no fine adjustment whatsoever, its pure guess work.

I would liken it to being as good an idea as filling your cases with powder by eye rather than using a scale.

It would only be a short test for fun/curiosity.. 

I did think I found the sweet spot when I tried it on 42gr winnie subs but it must of been luck as it wouldn't repeat but I wasn't surprised.!

From memory mine was hard too move.

As the eley match shoot so much better I might get lucky...

[Big Al]

20 hours ago, Popsbengo said:

Further to our earlier discussions on positive compensation (PC), rather than hijack that post I've started a new discussion.

 

I've a question that I can't get my head around.  I can’t see why a barrel clamped conventionally in a stock (ie the action bolted and bedded) would only vibrate vertically?  Just to be clear PC is not about muzzle flip due to rotation caused by off-axis forces, it's vibrational movements in the barrel.

 

I understand the principle of positive compensation in that launch angle and muzzle velocity are ‘tuned’ to match the vertical dispersal due to MV, with launch angle thereby achieving a tight mean point of impact at the tuned distance.   But what about barrel vibration in the horizontal plane?  Why do we not see horizontal stringing of the point of impact?

 

I read G. Kolbe's excellent paper  "The Vibrations of a Barrel Tuned for Positive Compensation" and appreciate the principles of PC.  But what about vibration of the barrel in the horizontal plane?  Why do we not seem to see launch angle variation horizontally ?  Obviously MV is somewhat irrelevant to the horizontal but if the barrel's vibrating then a horizontal and vertical component is present and therefore an uncompensated (by MV variation) horizontal spread should be seen.

 

Taking Kolbe's measured test set-up he has a barrel varying its launch angle by 0.8MOA (1010fps at 9.3MOA and 1060fps at 8.5MOA) for a 50yd .22 shot.  If we had a horizontal vibration of similar magnitudes then we should see 0.4" horizontal at 50yds but that is not apparent so either barrels don't vibrate horizontally or I'm not seeing something.

 

Thoughts? 

 

Every ladder test Ive ever shot has shown some horizontal dispersion.

Of course the wind would play its part but Ive also shot .22RF in a 100yd tunnel and CF rifles on incredibly calm days and Ive still seen it. That can only be because the barrel has moved that way, not just up and down. It is my understanding that the barrel moves in a elliptical shape rather than a straight line up and down plane.

I remember one summers evening doing a PC load development at 600yds.  My wind flags were every 200yds from zero to 600yds and there wasnt a twitch on any of them, the flags were very sensitive to even a 0.5-1mph wind. The test showed about 6"-12" of horizontal and about 24" of vertical over the tested charge weights.

Once a powder charge has been established you can get rid of the horizontal with a seating depth ladder test.

[Big Al]

Duplicate post.

[No i deer]

I could shoot this a million times and it won't move on the barrel

[Triffid]

If I'm doing load development - for me starting with three-round groups at different powder charges - as well as the ES and group-size changes I'm measuring, I find changes in group MPI in both horizontal and vertical. Actually I look for loads where there is a minimal change in group MPI between different charges.

So I don't think that the barrel just vibrates in the vertical plane.

 

Triffid

[Popsbengo]

So I wonder whether those PC old service rifles performed ok vertically but sprayed the rounds left and right, but this was put down to windage variation?  😉

My observation is that PC may be useful for ammunition that can't be tuned to the gun such as rimfire or where factory ammunition is mandated.  Furthermore it may just tweak group size down on a CF rifle with otherwise optimised ammunition.   I noticed that the heavy gun benchrest record didn't employ a tuner however I feel it's possible that the load used was optimised to the distance (1000yds) so thereby not benefitting from a barrel tuner (given environmental conditions being optimal).  Air temperature variation will alter POI so any PC optimised load must take that into account to be worth considering,  I guess that's where a tuner could be useful as running adjustments can be made as the day warms/cools.

Another observation is that "perfect loads" should all end up in the same place at any distance provided barrel vibrations are at a null when the bullet exits the muzzle.  The closer to perfect the better and PC is not a panacea to sub-optimal round manufacture and development.

I will continue to develop loads at 100 and 200yds and accept that on a given day at 1000yds I may have a slightly less than optimum load.  The one exception I'd like to try is for my .338 but getting onto a range and having time to shoot variable loads at say 600yds is not easy.  Shooting my 200yd developed loads out to +1600yds seems to give good results so I'll just have to accept limitations.

[Big Al]

7 hours ago, No i deer said:

I could shoot this a million times and it won't move on the barrel

You have no control over adjusting its position in any kind of measurable way or going back to a previously good spot.

A proper tuner needs to be moved in very small increments of the same time each time, impossible with that butt plug.

[1066]

Don't they work on totally different principles - A tuner is used to manually vary the PC to the desired distance. The "Limbsaver" is a de-resonator, i.e. designed to deaden the barrel so it doesn't ring like a tuning fork.

I would think the position of a de-resonator would not be critical, as long as it's not positioned at the natural primary null point or about 6 inches from the end of the average barrel. Of course, the damper system wouldn't give the benefit of PC at a tuned range - on the other hand it wouldn't be so range sensitive. De-resonators or mid barrel dampers are often to be seen on long barreled battle tanks I believe. 

[No i deer]

2 hours ago, Big Al said:

You have no control over adjusting its position in any kind of measurable way or going back to a previously good spot.

A proper tuner needs to be moved in very small increments of the same time each time, impossible with that butt plug.

🎃🤣

[Big Al]

20 hours ago, 1066 said:

Don't they work on totally different principles - A tuner is used to manually vary the PC to the desired distance. The "Limbsaver" is a de-resonator, i.e. designed to deaden the barrel so it doesn't ring like a tuning fork.

I would think the position of a de-resonator would not be critical, as long as it's not positioned at the natural primary null point or about 6 inches from the end of the average barrel. Of course, the damper system wouldn't give the benefit of PC at a tuned range - on the other hand it wouldn't be so range sensitive. De-resonators or mid barrel dampers are often to be seen on long barreled battle tanks I believe. 

Yes I guess your right but on that basis you only have two points of accuracy, either with the de-resonator or without it. I would imagine finding accuracy on that basis is quite pot luck.

At least with a proper threaded tuner you can work your way through a fixed series of adjustments until you find the accuracy you require and that will then be repeatable as you can got back to exactly the place the accuracy was found time and time again,  or make precise and trackable changes to keep up with different temps and atmospheric conditions.

[Ronin]

Some interesting reading from a very old web page created by Varmint Al - who is a very knowledgable guy with (when he did the testing) access to very high end government equipment 

Thrre is quite a bit to look at with various pages dedicated to rifle accuracy, effect of stress and harmonics on barrels and actions 

 

Well worth taking time to read his pages 

 

Created in the 1990’s by the way 

http://www.varmintal.com/amode.htm
 

http://www.varmintal.com/atune.htm
 

http://www.varmintal.com/alite.htm
 

Some interesting work done on the effect of polished chambers too - and the effect of stages of polishing on case head separation

(without derailing  this thread )

 

I was reminded of his web pages after listening to a pod cast recently released on MDS with Aaron Hipp

https://www.podbean.com/ea/pb-cfyry-13551cd
 

Thought all the above would provide further information towards this thread