Velocity "nodes," is there a

308sako

308sako

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Best accuracy/velocity node, which the other "nodes" do not equal?

If I have an accurate load and assumed node at say 2820 ft/secs, can the next node up or down be superior to that, or should they be equal. The cartridge by the way is .308/155gr , not 6 PPC or derivative of same. Another short way of phrasing this would be is there a single best accuracy node in a given barrel?

Part two of this question would be if the "nodes" are expected to be approximately 120 ft/secs apart i the 6 PPC, are they spaced similiarly in other cartridges, or does each cartridge have it's own spread or reach between "nodes?" For instance would the .308 be spaced at say 165 ft/secs?

Thank you for your considered responses.
 
Barrel Vibratory Nodes

Generally speaking the velocities at which the vibratory node of the barrel is in a null or minimum movement condition are 120-130 fps apart. Since no two rifles are exactly the same, there will be relatively minor variances from rifle to rifle. Varmint Al over on the short range benchrest section did one of the best analysis I've seen in awhile on the actual mechanics of vibratory nodes.

There isn't any better or worse node in relation to velocity, it's all a matter of being able to tune the load for the node. Matching the velocity to the null and achieving a very low ES/SD means the bullets will be exiting the muzzle at the best spot on the null.

My .308 (155 gr Lapua Scenar), 6.5 X 284 (139 gr Lapua Scenar) and .338 RUM (250 gr Lapua Scenar, yeah it's a 1,000 yard rifle but note the velocity) are all on the 2950 fps node with minor tuning of velocity due to the fact they are different platforms. But only about 16 fps difference between the two. Good velocity for a null node.
 
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Two groups shot this morning including a sight adjustment inbetween. Range was 300 yards, 308/155Lapua's. I will assume this to be the upper node, but the chronograph battery was dead, so once again I will trek to the range and do it all over again.
 
Rust, something else to bring up. I take it that 1 node will be with the barrel at the top, then the next node will be with the barrel stopped at the bottom. I heard of the British using "regulation" I think was the term, where they tune it so it is slightly to one side of the center of the node, so that at short range, the fast shots will be slightly low, while the slow ones will hit slightly high. Then when the gun is shot at longer ranges, it compensates for slow and fast shots. What is the go with this? Is it a realistic theory? Could this be one of the reasons why some guns which work realy well at long range, are not as super at short range??? Could being on the bottom node or top node improve this?
 
rooshooter said:
Rust, something else to bring up. I take it that 1 node will be with the barrel at the top, then the next node will be with the barrel stopped at the bottom. I heard of the British using "regulation" I think was the term, where they tune it so it is slightly to one side of the center of the node, so that at short range, the fast shots will be slightly low, while the slow ones will hit slightly high. Then when the gun is shot at longer ranges, it compensates for slow and fast shots. What is the go with this? Is it a realistic theory? Could this be one of the reasons why some guns which work realy well at long range, are not as super at short range??? Could being on the bottom node or top node improve this?
Click to expand...

Mr. Rooshooter, you have some interesting theory here, please would you like to tell us where the full information comes from?

Con
 
Concross. I,m not sure where I read it, I think it was in a full bore website or Australian fullbore target magazine.I now think the word the British used for this practice might have been compensation, not regulation. I,m no expert at this, so I am still interested in hearing any other theories to 308Sako question.
 
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You do realize the barrel vibrates in three dimensions simultaneously, right? Seems like that would kick any 'top' vs. 'bottom' idea to the curb, but if someone has actually tested something like this (and got consistent, reproducible results) I'd be interested in seeing the data...
 
"You do realize the barrel vibrates in three dimensions simultaneously, right? Seems like that would kick any 'top' vs. 'bottom' idea to the curb, but if someone has actually tested something like this (and got consistent, reproducible results) I'd be interested in seeing the data... "

While I do not have the articles that explain it from some years back, it has been observed that "most" (always some goofball exception to everything) barrels/rifles tend to vibrate in a fat bottomed figure eight pattern, not just a purely vertical motion, they can be a very thin fatter bottomed figure eight though, which is better. This generally means that better accuracy is observed when muzzle exit occurs at the upper end of the vibratory pattern.

As far as working on velocity to time muzzle exit at the best portion of muzzle motion, as I mentioned earlier Varmint Al posted one of the better analysis of this that I have ever seen over on the short range benchrest section. I agree with what he said.

It would all seem to support my experience over more years than I care to admit to, where some nodes seem to pattern the group (I like to try to set up targets far enough out that rather than bughole, I have enough spread to the group to see a pattern) a little better (rounder, more uniform rather than strung which tends to occur at 45 degrees or so) and I suspect this is at the upper portion of the barrel motion.

But although nice to know, in practice it doesn't matter to me since my standard practices of load developement and rifle tuning produce repeatable results from rifle to rifle, and have for many years and many rifles from hunting rifles to match rifles. Of course the degree of accuracy varies depending on the platform, I wouldn't expect a lightweight magnum sporter to equal a heavy match rifle.

And the same things apply no matter what, a good barrel with a properly cut chamber mounted true on a quality receiver properly bedded into a stable stock with a quality scope properly mounted. The variables should be as much as possible limited to the load and tune of the load.

I will admit that I have always been pleased when things come together resulting in a competitive rifle. But there has always been the fly in the ointment rig that defied all attempts at accuracy. I no longer spend money trying to make a silk purse out of some sows ears, if it ddoesn't show some promise out the gate it's out the door.
 
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