vibration node Q

[Mikecr]

A little research results in several "theories" w/regard to barrel vibration nodes. Could someone who actually knows, please straighten this poor southern boy out?

What I see:
I think everyone agrees that a short/thick barrel has less muzzle moment overall than a long/thin barrel.
BUT
I read that barrel vibration originates from a bowing barrel.
(thin barrel, more bow -larger waves, -freq/node width?)
I read that barrel vibration originates from rifling.
(Fast rifling/hi velocity -higher freq, wave amplitude?)
I read that barrel vibration originates from pressure peak.
(like an engine "ping", hi Pk, -hi amplitude, -freq/node width?)
Then, more abstract, firing pin, action and any connected metal anomalies.

What I lack:
The actual cause of the major vibration waves.
Are they resonant?
The frequency of these waves, and following nodes.
Thick barrel=hi-freq/narrow node, or lo-freq/wide node?
How parameter changes affect node frequency.
Rifling, powder speed, barrel time, peak pressure amplitude, barrel length, barrel stiffness?

I need help here.
Just stepping back and looking at it; It would seem that if I strike a short or thick barrel at the action with a hard plastic mallet, I should expect a higher frequency/lower amplitude wave, as measured with hall effect sensors adjacent to the muzzle, compared to the same test with a long or thin barrel.
To me this would suggest that stiff barrels would deliver lower overall dispersion throughout a range of loads than a whippy barrel.
But, its higher frequency(the double edge) would cause it to be difficult to keep within it's relatively narrow nodes.
What do you think? Do stiffer barrels (at the magic load) need lower ES to shoot consistant? Is a whippy barrel (at it's magic load) more tolerant to higher ES?

Thanks

 

[Brent]

<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>To me this would suggest that stiff barrels would deliver lower overall dispersion throughout a range of loads than a whippy barrel.
But, its higher frequency(the double edge) would cause it to be difficult to keep within it's relatively narrow nodes.
What do you think? Do stiffer barrels (at the magic load) need lower ES to shoot consistant? Is a whippy barrel (at it's magic load) more tolerant to higher ES? <HR></BLOCKQUOTE>

Mike, I've came up with the same info and same conclusion from all the material I've read too.
This part I quoted pretty much sums up what I too think at this point.

I think it's harder to define the node on the stiffer barrels, but they may just be less critical as dispersion is so much less, hence the wide variety of loads that shoot really well, verses the finiky lighter whippy ones.

Interesting to hear more peoples thoughts on this too, yours echo mine exactly. I wish I could be of more help but, I haven't gained much more insight on this one for a while now.

 

[Guest]

There is always the exception to rules as well. I have had some extremely light profile barrels shoot very, very well for 3 shots before they warm up and also very consistent for CBS.
Very interesting thread.

 

[BountyHunter]

Doubt you will find that a higher ES from any barrel is conducive to LR accuracy. If it was every untuned gun would be winning.

The key is tuning your barrel and load together, no matter what the barrel length.

I use the ladder method and have found that it works extremely well as long as the gun is accurate enough to begin with to consistently shoot in a known spot. If it isnt .5 MOA do not use this method.

BH

 

[Michael Eichele]

BH hit the nail on the head when he said to tune the load and barrel together.

Sure short fat barrels are nice cause they are stiff, but who cares if a barrel whips a little. The key is make it whip the same. The reason I use a short heavy barrel is because I live and hunt in a cold enviornment. When the barrel is cold and bullet #1 is fired it whips very violently and not harmonious with the normal whip sending the bullet to who knows where. At 600 yards, its a clean miss. The short fat barrels with a medium charge help fight that.

 

[Brent]

You guys are right, ES will open the groups up at LR, but that's nothing to do with the barrel being whippy, or stiff and its accuracy per say.

Tuning to a node, whether it be narrow or wide, which is more tolerant of ES, and the reasons seems to me what Mike is driving at here??

The whippy barrel may dwell longer and therefore be more tolerable of ES within a node, and this consistant exit will affect accuracy at LR, but if the ES "is" tight, shifting the MV to the node on a stiffer barrel that might not be as tolerant should be easy enough.

If you need a whippy barrel to tolerate a 50 fps node or more, 50 fps ES isn't going to make up for the fact that at 1000 yards ES does kill you.

Working up a load on the right side of the node may cancel out some of the ES caused dispersion, but it won't remove it all. Working on the wrong side of the node, if you go to this degree to find which side you're on, "would" make ES and dispersion in effect, stack up.

The stiff barrel may not tolerate MV shifts as well if the node is narrower, and temp insensitive powder and tight MV control in charge weight could be a must to deal with it.

How much does temperature have on the vibration itself?

How far off can your MV be before it starts to string shots?

What's the high level, and low level... node width in MV?

When you go too hot, which way does it string shots, down low, or up higher?

Ideally you want a load that starts to string shots lower as you peak over the intended MV range, and stay away from the low side of the node. Unless by going lower on MV, shots begin to actually rise, then this would be the side to work on, and avoid the other side.

If I've got a load that does great at 70 degrees F and only works to 40 deg, then goes stringin shots, I'd have to take ammo at a different charge weight to compensate for MV change if I was hunting at 20 deg, that or alter seating depth if I could compensate for barrel time enough that way.

Charge weight creates the pressure and MV that creates the vibration, so if that changed enough, seating depth may not necessarilly make up for it. Easier to load for a target MV anyway, then everything should remain the same...

This one load I have for the 30-338 Lapua, it's near max load, and just before it got to that level and stabilized, it jumped about .75 MOA on the target!

If I let the MV fall backwards, I'll not only be dropping .75 MOA from POI shift, I'll be stacking the lower MV on top of that.
I can't go any higher to get to the other side of the node, pressure's too high, so a downward POI shift as I increase MV just isn't possible with this load at this OAL, I very well may be able to alter that by increasing the OAL .020-.030" into the lands, just don't know yet.

Here's a pic of the load. 210 JLK 90-97gr Retumbo 30-338 Lapua. 95gr on up grouped high. Circle is 2", fired at 300 yds.

 

[Mikecr]

This is great info. Thanks all
I'm researching for projects in the works.
These are based on large cap wildcats burning slow powders. So ES will likely be a battle. I'll have to use fairly long barrels(28"), and weight/balance is a factor.

Am considering breaking with tradition alittle. Nothing to loose really.
Barrel block, tuner, damping, whatever. But I have to first understand the beast.
You should feel pretty good if you know the answers. I'm getting the impression that these things just aren't known by many today. For some reason.
I would have thought that the moment the shooting world realised barrel vibrations have a large effect on consistency, great efforts would have tamed and manipulated them, for gain.

 

[Jeff In TX]

Mikecr,

This is a good subject. I think Jim Ristow understands this particular subject better than most folks. I think Brent will agree. I'm going to copy your orginal post to Jim and see what he thinks. We I get an answer, I'll post it.

 

[Brent]

I agree.

Mike and Jim might enjoy conversing too. 888-634-8028

 

[Jeff In TX]

Mikecr,

Here's Jim Ristows response

1) The barrel moves in what I can only describe as a 3 dimensional wave. The amplitude is of course affected by profile, thickness and length.


2) It is caused by the mass of the bullet being accelerated into a twist. The rate of acceleration and twist determines the frequency.


3) Imagine a smooth bore with all rifle mass in line so there is no muzzle rise. There would be no harmonics just slight "ringing". Two entirely different issues.


4) Shorter and thicker barrels are more rigid with less muzzle movement but it is still there. In practical application the nodes (where it will shoot) are simply wider. A "Whippy" barrel is less tolerant of ES and nodes are narrower.


5) Where the barrel shoots best the muzzle is at the extremity of swing, not at the static location....this is the nugget of info that should bring it all together. You can prove this yourself by loading a long thin barrel to several nodes and shooting at the same 100 yard aim point (utra lights are perfect for this). Often lower velocity nodes will shoot up and to the side. I have seen 9 or 10 inches between groups (most only 4 to 5) which proves at least 4 or 5 minutes of muzzle swing on thinner barrels.

Hope this helped

 

[MAX]

Mikecr, you may be a poor country boy but you ain't no dummy.

Harold Vaughn's book, "Rifle Accuracy Facts", covers a number of vibration issues and modes, and may be of some help. If not specifically, perhaps in general.

The discussion above is interesting in regards to the weapons, but don't forget about bullet imbalance. It adds to the chaos. Robert McCoy had a few things to say about that.

 

[Mikecr]

I'm emailing Jim to see if he can expand on this for me. If his methods are sound, then it could be a real break for me.
Else it's back to the grind!