Barrel Harmonics????

T

tiny68

Member
I am trying to tune my 6BR. I am new to this caliber. Rifle is a Savage Target Action in a SSS LVT stock with a 28" Krieger 1:10 twist 0.272 neck 6BR. I shooting 95 BIB set 0.010" into the lands. I ran the load ladders with three different powders: Varget, RE-15, and RE-17. I shot the ladders at 300 yds. I chronied each shot and got velocity at each 0.3 gr increment. I originally plotted the velocity versus charge to look for patterns in velocity and stable regions and then looked for regions of minimum vertical on the target. I decided to calculate the bullet drop using the initial muzzle velocity and Randy's listed B.C. (0.45) with the PCB ballistics software. I took this drop due to gravity (all as negative values) and added it to the vertical deviation from the point of aim for each target. I know I have read something like this below. I was trying to calculated a corrected vertical deviation. In my simple mind this should yield something similar to the barrel vibration (sine type wave) as the end of the barrel whips up and down at different powder charges. The attachment shows my results with all three powders when plotting this corrected vertical versus the muzzle velocity.

What the heck am I seeing? Never tried this before. Certainly not a sine wave. I am wondering if this is the multiple modes of vibration. Something happens at 3050-3060 fps with every powder. Is this a node? Or is this data trash since each point only represent a single shot and it has no statistically value. However, I see a pattern.... but I don't know what to make of it.

Any of you barrel harmonic pros want to straighten me out here.... I'm a little lost.

Here is a different link with my complete load data if it helps. http://www.websitetoolbox.com/tool/post/6mmbr/vpost?id=3205403

Corrected Vertical Displacement versus Velocity
http://i159.photobucket.com/albums/t151/tiny68_photos/CorrectedVertical6BR.jpg

Thanks for any clues, tiny
 
tiny68,

Now you've got me confused too.... :)

How are those plots "not a sine wave?"


Now, I'll agree that using only one shot per data point is incomplete but IMO the plot is classic.....just lay it "flat" to see it as a sine wave. I thought you ladder guys referred to "plateaus" anyway? If those aren't classic "plateaus"........

What Am I Missing?? I read from your limited test results that you should play around at 3050-3075fps.

al
 
I guess in my mind as the barrel vibrates (cycles) above and below it should be making a more traditional looking sine wave. I have read some of VarmintAl's stuff showing the multiple modes can exist. I didn't know if the constant positive (up) slope is one of the lower frequency modes and the other apparent cycle a different mode in my velocity window. If so, where do I want to tune too? On the rise of the smaller peak? At the maximum? On the fall? I may be trying to make too much engineering out of this, but that is my nature.

Thanks again for any help, tiny
 
tiny68 said:
I guess in my mind as the barrel vibrates (cycles) above and below it should be making a more traditional looking sine wave. I have read some of VarmintAl's stuff showing the multiple modes can exist. I didn't know if the constant positive (up) slope is one of the lower frequency modes and the other apparent cycle a different mode in my velocity window. If so, where do I want to tune too? On the rise of the smaller peak? At the maximum? On the fall? I may be trying to make too much engineering out of this, but that is my nature.

Thanks again for any help, tiny
Click to expand...



Tiny, although some argue that the bullets should exit on the up slope, slightly before a positive peak or is it slightly after a negative or maybe before and after a,,,,,,Oh what the hell; who cares? :eek: :)

Here, we are talking about the typical, 22 or 6mm BR rifle used in 100/200 group shooting.

The rifle can be tuned by increasing or decreasing the powder charge in .3 grain increments until vertical disappears or, now that we have the option of using tuners on our barrels, by adjusting the tuner. :cool:

I have conducted many tests in the tunnel on the sine wave you mention and can state with certainty that you want the bullets exiting at EITHER a positive peak or negative valley. But who cares; what difference does it make? :rolleyes: The rifle will tell us when it's in tune by showing no vertical on the sighter target.

Now that we have the option of adjusting a tuner at the line, we can tune our barrel as easily as focusing a scope; the powder charge is never changed. :eek: :D

The better understanding we have of such things as barrel vibration the better, but there is no need to over-complicate things.

With the Beggs tuner, one goes to the line with his pet load. Tuning is done only with the tuner. You can never be more than a half turn out of tune regardless of atmospheric conditions or the starting position of the tuner.

"But how do I tell when the rifle is in tune?" you ask.

Simple,, fire a three shot group on the sighter. There is one chance in four the rifle will be perfectly in tune with the reference mark set to 12:00 o'clock. If the group shows two bullet holes of vertical, turn the tuner a half turn in EITHER direction. One bullet hole of vertical requires a quarter turn of the tuner. "But which way?" you ask, "Quarter turn to the left or right?" Turn the tuner a quarter turn in either direction. If the group opens up to two bullet holes of vertical, you went the wrong way, in which case, you simply make a half turn in EITHER direction and the rifle is in tune. Simple; huh? :)

Later,

Gene Beggs
 
Barrel Harmonics

Harold Vaughn describes the procedure in his book "Rifle Accuracy Facts" On page 85, he shows the "sine wave" chart and states that the impact points have to be corrected for the differences in gravity drop of each bullet. Based on my reading of your procedure, you subtracted the drop for one bullet from all of them. To do it Harold's way, I think you would have to calculate the expected drop for each bullet and subtract that from its point of impact. As a practical matter, I think you could do what Alinwa suggests and see everything you want to know.
 
I've worked up loads and/or helped develop loads for six different 6br (or 6br improved) and they have all shot their best at 3030 to 3060 fps with the 95 grain bullet. There definitely is a node at this velocity!

The last barrel I worked on was just last week. I shot three shot groups of .5 grain increment charges at 100 yards over a chrono while watching for good group size and low standard deviations. Found a good node that shot .3 and SD of 4 fps. The load above this spot and below also had good numbers and groups which further indicates a good load and good choice of components. I loaded up about 20 of this good load, fired a couple at 100 yards to get them zeroed and verify my impact point spot on. Then I dialed all the stuff into my PDA with ballistic software and dialed up the appropriate amount (in IPHY) for a 900 yard shot. I then commenced to fire two groups. The first was five shots and measured 4.25". The second group was 7 shots and measured 4.75". It's that easy. The whole process (not including load time) as about 45 minutes. Brand spanking new barrel of unknown accuracy to under 1/2 moa accuracy at 900 yards that quick. Forget the ladder and sine waves and all that quickload "prediction" garbage.

BTW, the kicker in all this is that not one of my 6br's have similar barrel lengths. They range from 26" to 30" and THEY ALL STILL SHOOT THEIR BEST AT 3030 TO 3060 FPS. So much for sine waves.:eek:;)
 
Tiny68

Tiny
Gene Beggs got everything right except the distance.In 600/1,000 yard benchrest you can't tune at the bench like you can in point blank benchrest.
His tuner theory is good but at our distances we need to set it and forget it just like in those info-mercials.
Looking at your charts I would go with the Varget at your top load.You can then add a muzzle device weighing 6,13 or 22 ounces 2.5 inches in front of the muzzle.
The new Berger BT Match bullets are shooting very well this year.If you can get one your twist rate likes I'd give it a try.
Lynn
 
And for lighter bullets am I right in thinkng that 3170-3200 fps, 3320-3340 fps and around 3420fps are the next nodes
 
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