How Barrel Harmonics Can Affect POI

[rsterne]

It's even easier than that.... Simply turn it a turn at a time and watch the group size.... When you find a spot that has the smallest group, try a turn either side of that, moving it about 30 deg. at a time.... When you find the best spot, try even smaller adjustments to see if you can refine it further.... Don't worry about the POI, just look for the smallest groups.... When done, resight it....

If you change the pellet or velocity, double check it.... I'm guessing it will be within a turn either way, unless you make a huge change in ammo.... but on the other hand I haven't tried it yet.... Chris Alls, who won the 2015 EBR with my .308 cal 200 gr. BBT "Whiteout" told me that adjustments of just a few thou might double the group size, or cut it in half.... He was astounded how small the final adjustments were....

Bob

[plinker81366]

thanks bob, i was thinking of look for the highest poi first then do the group shots for fine tuning?

[rsterne]

Should work.... In theory you would want to move the weight further from the muzzle to slow the vibration and move the pellet exit onto the upswing....

Bob

[Ooops]

As usual another Robert Stern instituted migraine but interesting reading none the less.

ROTFLMAO I thought it was just me. If I'm luck I understand about a third of it but I'm always fascinated and want to know more

[aPpYe]

Now I am confused, are these vibrations, at least those worth mentioning, always vertical?  I thought they are in all directions, randomly.

With my 24" 2400kt/Disco(?) barrel (ninja regulated bottle gun) I cut shims from a beer can for the barrel band to put a little upward tension at the end of the barrel...  The thought is that since the tube pressure stays constant, it's dimensions would stay constant, and I would place the band all the way at the end to dampen vibrations. 

At 50 yards, the best group I have achieved with this gun so far is .35" CTC, groups under .5" are reasonably common, and I am disappointed with anything approaching .75" ...

I never did try the "trombone" trick, looking for optimal band placement.  Did I just luck out with my band placement, or did the tension dampen the harmonics as I had intended?  I have this shimmed band all the way at the end of the barrel:



I hope you can see this image.  I am using google photos.  It is a little clunky, but I hate that photobucket has changed their policies...

[immortl]

Applying powder burner concepts - is anyone here familiar with the work done with rimfires and tuning to a specific harmonic (barrel length)?  On a rimfire board I frequent, there was a good explanation and a spreadsheet that would calculate the various lengths to hit the nearest harmonic, greatly reducing the amount of trial and error for finding the optimal tuner setting.  I believe it was called the Purdy Prescription and was written by a guy named Purdy.  I will see if I can dig it up.  Based on my understanding of this thread, I think we are talking about the same concepts and it would be applicable. 

Added on edit:  I found the original doc and spreadsheet and have attached the doc.  Doesn't look like we can attach spreadsheets.  The spreadsheet basically does the calculations outlined in the doc for you.  I haven't looked it up in a while to see if there has been any refinement.  As it is, it worked really well for me with a 10/22 clone.

Thanks,
Joe

[grand-galop]

We have talking about barrel stiffness and weight at the end but WHAT ABOUT A STRETCHER???  I cant remember where I have read on it but the benefits were good.. Having a good anchors  point at the base and a point to stretch the barrel end with adjustable screws for rigidity seems a good solution for adding stability..

[rsterne]

Barrels tend to oscillate mostly in one plane.... The only way to make sure your barrel oscillates in the vertical plane is to test it and index it.... Putting a weight on the end of a free floating barrel will bend it downwards somewhat due to gravity, and accentuate the vertical movement.... Even if the barrel vibrates sideways, you should still be able to find an adjustment that improves the group size....

Yes, tensioning the barrel helps tame harmonics because it increases the frequency and decreases the amplitude.... Having the outer tube / shroud mounted rigidly to the receiver.... or at least having it fit on a snug shoulder, will help as well, because then the barrel acts like a larger OD where it meets the receiver....

Bob

[aPpYe]

Hm.  So it seems that the only way to know for sure which way your barrel oscillates would be to test at an indoor range...  Outdoors introduces wind that could be nice an still at your bench yet moving faster or even a different direction at your target.  At least that is the way it works for me, living at the top of a hill!  No wind at all seems to only occur at 3am every other third wednesday after a lunar eclipse for me.

[triggertreat]

Yes, on my .25 Synrod, I do screw my shroud down very tight against the receiver.  I use an O-ring between them to offset any part that is not perfectly square on the end of the shroud.  I also use a Hill barrel band that holds the front of the shroud fairly steady, but does allow the smallest of movement too.  It is only tighten down after the shroud is in place.  I have the baffle spring compressed tightly as well.  All of this seems to work better than the loose fit OEM design.  I can clean the barrel with a pull through without POI shift and groups seem to hold steady day to day with these changes.  I would definitely like to add a CF sleeve to make the barrel even more rigid.

[grand-galop]

Yes, on my .25 Synrod, I do screw my shroud down very tight against the receiver.  I use an O-ring between them to offset any part that is not perfectly square on the end of the shroud.  I also use a Hill barrel band that holds the front of the shroud fairly steady, but does allow the smallest of movement too.  It is only tighten down after the shroud is in place.  I have the baffle spring compressed tightly as well.  All of this seems to work better than the loose fit OEM design.  I can clean the barrel with a pull through without POI shift and groups seem to hold steady day to day with these changes.  I would definitely like to add a CF sleeve to make the barrel even more rigid.

I think I have read of the tensioner on a tread about the Edgun R3....  The significance of tensioning on the barrel is what Bob mention earlier.. The harmonics are higher with less AMPLITUDE... Resulting in a lesser change of POI..

[rrdstarr]

It's even easier than that.... Simply turn it a turn at a time and watch the group size.... When you find a spot that has the smallest group, try a turn either side of that, moving it about 30 deg. at a time.... When you find the best spot, try even smaller adjustments to see if you can refine it further.... Don't worry about the POI, just look for the smallest groups.... When done, resight it....

If you change the pellet or velocity, double check it.... I'm guessing it will be within a turn either way, unless you make a huge change in ammo.... but on the other hand I haven't tried it yet.... Chris Alls, who won the 2015 EBR with my .308 cal 200 gr. BBT "Whiteout" told me that adjustments of just a few thou might double the group size, or cut it in half.... He was astounded how small the final adjustments were....

Bob

Bob, I know we are both in Canada and suppressors are a No-no!  But I have powder burners in the states with NFA titled suppressors.  Question is if you were to change the position on the baffles with different spacers in the expansion chamber side, do you think it would affect the POI?  One of my Sleeved Rem 700's in .308 Win has dropped off in accuracy.  The throat is still good and the only thing I have changed is the Suppressor guts with identical parts.

[rsterne]

Yes, it could change the POI, and the group size as well, IMO.... Even just moving the weight of one baffle could change the harmonics.... Get yourself a slide-on "Limbsaver" and try that....

Bob

[Horatio]

Barrels tend to oscillate mostly in one plane.... The only way to make sure your barrel oscillates in the vertical plane is to test it and index it.... Putting a weight on the end of a free floating barrel will bend it downwards somewhat due to gravity, and accentuate the vertical movement.... Even if the barrel vibrates sideways, you should still be able to find an adjustment that improves the group size....

Yes, tensioning the barrel helps tame harmonics because it increases the frequency and decreases the amplitude.... Having the outer tube / shroud mounted rigidly to the receiver.... or at least having it fit on a snug shoulder, will help as well, because then the barrel acts like a larger OD where it meets the receiver....

Bob

Barrel tensioner, neat idea. Basically the same thing happens as tightening up a guitar string I imagine.

[rsterne]

Exactly.... and you can tune so that the muzzle is at the top of the upswing and pausing briefly as it reverses direction.... This tightens the vertical POI downrange....

Bob

[Nvreloader]

Bob

Would this not along with Chris Longs bbl nodes for tuning and getting the best accuracy possible,
and clocking the bbl etc?

Tia,
Don

[rsterne]

Don, all of those methods are part of the puzzle....

Bob

[mpbby]

Any ideas about what could happen with a (magnum) springer accuracy when you bend the barrel due ‘barrel droop’?

[rsterne]

No idea.... bending the barrel upwards to eliminate droop should not change the frequency of vibration....

Bob

[Horatio]

I’m assuming wrapping the barrel in carbon fiber or some type of fiber reinforced plastic acts as a vibration dampener?

Also, I wonder if some people reporting better groups after getting an heirloom .22lr resleeved is partly due to harmonics dampening of the plastic between the 2 tubes?