Half nodes?

[Ned Ludd]

No.
The optimum release time is when the barrel is just short of its apex on the upward vibration cycle. Slower bullets will exit at the dwell area at the top of the wave.

This has always been my understanding as well. At either end of the arc of travel, the movement of the bore should be at the slowest of any point in the cycle because the bore/muzzle will reach the end of the arc, hesitate momentarily, then continue back along the arc in a different direction. Bullets released in the region of the high end of the travel arc and, to a lesser extant the lower end, will be the most stable with respect to velocity variance because the bore will be moving at its lowest average velocity at the apex of the arc of travel. For that reason, the bore will exhibit the least amount of movement over time at that point and bullets will be minimally sensitive to velocity variance.

In contrast, bullets exiting the bore in the middle of its arc of travel will be released when the bore is moving at its fastest rate of any time during the cycle. Therefore, they will be subject to greater bore movement, and hence, changing POI, for even very small changes in velocity. In terms of bore movement, the most stable load should be one where the point of exit is at the ends of the barrel arc of travel. Below is an image taken from a post by Dan Newberry (OCW Testing Method) that illustrates this concept nicely:

Edited to add: the barrel harmonics described above by Dan Newberry are a different concept from Chris Long's OBT Theory, which is mainly about barrel length and precision nodes that are based on the position of longitudinal shock waves, which continually move back and forth lengthwise through the barrel, relative to the muzzle. The last I heard, Dan Newberry and Chris Long had found that there were commonalities and mutually supportive elements between the two theories, although they had both approached the notion of accuracy nodes from a different perspective. Although I now routinely use Quickload and target OBT nodes whenever possible during load development, I have also carried out plenty of ladder and OCW tests. My own observations tend to fit well with their notion that we are likely dealing are elements of both OBT and traditional barrel harmonics during the reloading process. Certainly there is ample evidence that both processes are occurring after the trigger is pulled.


Normmatzen - as far as the effect of different powders on barrel times, they can have a very profound effect. This is the primary reason that H4895 will generally give you about 20-30 fps greater velocity than does Varget at an OBT node. Optimized loads with both powders can be developed to yield essentially the same barrel time, but the velocity of the one loaded with H4895 will be faster, in part, due to different acceleration. For the same reason, when loaded to the exact same muzzle velocity, these two powders will give quite different barrel times.

 

[Scott Harris]

Got my orange jumpsuit on for when everyone throws tomatoes at me, but here's my .02 on "nodes." I recently shot a match and velocity in the morning was 2670 (SD 6) right where it should be. Later in the day (sun out, much hotter), average velocity was 2720 (SD 6). Rifle shot great both strings... I'm not a big believer in finding sweet spots. I am a big believer in putting as heavy of a contour as you can manage on the barrel so harmonics are not as important. My "load development" consists of finding a "node" where the velocity changes are minimized as the amount of powder is changed. Beyond that, I think it's more important how well you prep your ammo versus finding some magical sweet spot. Of course, I'm probably wrong and just rationalizing my laziness and general disdain for load development

 

[Joe R]

.... I like to just think about what the muzzle of the barrel is doing when it completes an up down vibration cycle. One would think the objective would be to catch the muzzle in the perfectly flat position, not pointed up or down. But there are really two flat positions; one with the barrel moving up and one with the barrel moving down.

IMO this is TOTALLY wrong.

It would be correct if the barrel was a solid bar like a tuning fork and the source of the vibration come from the only place it can come... the exterior. However, we all know that the barrel is actually a tube and the source of the vibration is from the interior... the chamber.

I believe the barrel does not whip up and down, or side to side, like a tuning fork. I believe the explosion sound waves cause tiny little waves that force the barrel to ever so slightly expand and contract. The low part of the wave forces the metal towards the center (constricting) and the high part of the wave cause the hole to enlarge, alas ever so slightly. As I understand it, the idea of OBT is to time the exit of the bullet when the low part of the wave is at the crown and it is tightest, thus giving us more accuracy. If the bullet exits when the crown is at its largest, when the wave is at the highest, you get less accuracy.

I think of the bullet like a surfer riding the wave. The wave starts out wide, then as the surfer picks up speed and enters the tube, it gets narrower and eventually (if he's really good) the surfer comes out of the tube very fast when the wave is at its high point again. But then I could just have a vivid imagination and be totally wrong.

If you buy into this idea, there could be many other times when the bullet could exit the crown when it is tight, therefore giving validity to the observation that some have found "half" nodes at other velocities. For me OBT, QL and my labradar work together to guide me to the right spot. YMMV.

Kindest regards,

Joe

 

[aj300mag]

Couple of things to remember,

Barrels vibrate at the frequency of the speed of sound traveling through metal.
All barrels sag (droop or what ever) from their own weight. Dan Lilja has the formula on his web site to calculate the amount of sag.
Barrel contour doesn't affect the frequency of the vibration, it changes the amplitude.

IMO this is TOTALLY wrong.

It would be correct if the barrel was a solid bar like a tuning fork and the source of the vibration come from the only place it can come... the exterior. However, we all know that the barrel is actually a tube and the source of the vibration is from the interior... the chamber.

I believe the barrel does not whip up and down, or side to side, like a tuning fork. I believe the explosion sound waves cause tiny little waves that force the barrel to ever so slightly expand and contract. The low part of the wave forces the metal towards the center (constricting) and the high part of the wave cause the hole to enlarge, alas ever so slightly. As I understand it, the idea of OBT is to time the exit of the bullet when the low part of the wave is at the crown and it is tightest, thus giving us more accuracy. If the bullet exits when the crown is at its largest, when the wave is at the highest, you get less accuracy.

I think of the bullet like a surfer riding the wave. The wave starts out wide, then as the surfer picks up speed and enters the tube, it gets narrower and eventually (if he's really good) the surfer comes out of the tube very fast when the wave is at its high point again. But then I could just have a vivid imagination and be totally wrong.

If you buy into this idea, there could be many other times when the bullet could exit the crown when it is tight, therefore giving validity to the observation that some have found "half" nodes at other velocities. For me OBT, QL and my labradar work together to guide me to the right spot. YMMV.

Kindest regards,

Joe

Varmit Al explains how a barrel whips...
http://www.varmintal.com/apres.htm#WithoutTuner

 

[Joe R]

Couple of things to remember,

Barrels vibrate at the frequency of the speed of sound traveling through metal.
All barrels sag (droop or what ever) from their own weight. Dan Lilja has the formula on his web site to calculate the amount of sag.
Barrel contour doesn't affect the frequency of the vibration, it changes the amplitude.


Varmit Al explains how a barrel whips...
http://www.varmintal.com/apres.htm#WithoutTuner

I'm going to have to study this link, there is lots to digest. But right off I can say that Varmint Al's is talking about a 6ppc (light weight) not a 30 cal 200 grain bullet. Big difference in mass.

Obviously there is some torque depending on the caliber and weight of the bullet and there may be some very slight coaxial movement of the barrel, but I'm pretty certain that is not what Chris Long's model was trying to capture. His model is not caliber dependent, I believe Chris was addressing the vibration aspect not the mechanical torque created by the rifling and bullet interaction.

 

[jsthntn247]

Varmint Al's descriptions make sense to me. If the barrel does indeed whip up and down. A node that was near ( but not completely at) the top of the whip cycle would produce the least amount of vertical because faster bullets would exit lower (just before the pause at top of the cycle) and slower bullets would exit higher (near the top at the pause) which would make up for differences in velocity. In my current target rifle I have a good node at 2780 and another one at 2850. Both shoot one hole at 100 yards and the node is .3 wide for both. However, the higher node has 1/3 the vertical at distance even though they have nearly identical ES and SD's. I have assumed that what I was seeing is what Al was explaining but have no way to verify.

 

[rardoin]

Google high speed video of barrel whip...or variations of that search for visuals of the muzzle end movement at bullet exit. There are many complex movements, axial and likely radial that come into play.

 

[Joe R]

I have assumed that what I was seeing is what Al was explaining but have no way to verify.

Me neither.

But getting back to OBT and nodes.

Just last month I posted here http://forum.accurateshooter.com/threads/my-new-260-ai-build.3909184/ about my new 260AI build that shot well at almost all powder charges, but really came into it's own this past weekend when I finally increased the charge and was right on top of the node that QL led me to. OBT works, but you gotta squeeze out all the variables out of your reloads.

 

[RonAKA]

In contrast, bullets exiting the bore in the middle of its arc of travel will be released when the bore is moving at its fastest rate of any time during the cycle. Therefore, they will be subject to greater bore movement, and hence, changing POI, for even very small changes in velocity. In terms of bore movement, the most stable load should be one where the point of exit is at the ends of the barrel arc of travel.

Actually the whole purpose of tuning a barrel to a bullet velocity is to compensate for variations in muzzle velocity. A barrel moving upward in the vibration cycle is what we are looking for. We are not looking for a stopped barrel. There are some in the rimfire world that think a barrel tuner when properly set up "stops" the barrel, and once tuned the gun can shoot any ammo velocity at any distance and the barrel will still be tuned. Not true. Barrel tuning in a rimfire by changing the tuner weight and location to change the vibration frequency of the barrel is just the reverse of having a centerfire barrel with a fixed vibration frequency and changing the velocity of the bullet.

A fast bullet will exit sooner in the vibration cycle, while a slow bullet exits later. If you use an average target velocity that gets to the muzzle when the barrel is on the upward vibration cycle a fast bullet exits with the barrel pointing lower. A slower bullet exits with the barrel pointing higher. If you choose the right part of the upward cycle the normal velocity variation from bullet to bullet at the same load will be compensated for, and the vertical variation reduced.

As far as where the exact right spot may be could be (just my theory) at the 1/3 up position (velocity), and then another similar one at 2/3 up. Right at 50% up where vibration vertical velocity of the barrel is at a maximum, may be too much. This is probably where you see a dip in POI increase with velocity increase. The ideal spots may be just before that or just after.

So getting back to the OP's question, yes there may be two "sweet spots" fairly close together separated by an area where there is some over correction. But remember, you have to go all the way around the clock with increasing velocity to get to those spots again. So if we categorize the vibration cycle crudely into a clock starting at 12:00 you will have a bad area which peaks at really bad at 3:00 and goes to 6:00. Somewhere between 6:00 and 9:00 there will be a sweet spot, then an over correction zone at 9:00, followed by another sweet spot between 9:00 and 12:00. In some cases there may not be a dip or it is very minor at 9:00, and there is quite a wide sweet spot.

At least that is the way I see it. As far as the OBT, OCW, I remain skeptical. Ladder works if you have an accurate gun and do the test properly. Hard to do with a scatter gun though.

 

[RonAKA]

Barrel contour doesn't affect the frequency of the vibration, it changes the amplitude.

Sure it does. A tuner on the end of the barrel is like a reverse taper and it has a very significant effect on frequency of vibration. Check Varmint Al's site. I recall he even modeled a barrel with a reverse taper. Think of a guitar string. Low frequency from a fat string. High frequency from a skinny string...

 

[Ned Ludd]

My point was simply that when considering the effect of barrel harmonics on directionality of the muzzle (and therefore POI/precision), the ideal point for the bullet to leave the bore is when the muzzle is moving at the slowest speed, which should be the point where the rate of movement/directionality of the muzzle is minimized and therefore provides the most consistent and stable POI. That point will be at/near the ends of the arc of travel, not in the middle, and not partway up in the cycle.

OBT is a whole other ball of wax. I use it and it has worked very well for me and many others, but I cannot say with any certainty that Chris Long's explanation/mechanism is either the correct or complete explanation of what is going on. What I do know is that I have had success using it as a loading approach, so I will continue to use it whenever possible.

 

[RonAKA]

My point was simply that when considering the effect of barrel harmonics on directionality of the muzzle (and therefore POI/precision), the ideal point for the bullet to leave the bore is when the muzzle is moving at the slowest speed, which should be the point where the rate of movement/directionality of the muzzle is minimized and therefore provides the most consistent and stable POI. That point will be at/near the ends of the arc of travel, not in the middle, and not partway up in the cycle.

The problem I have with that thinking is that it does not explain what I see when I do a ladder test. Some things to consider:

1. If a sweet spot or "node" occurs when the barrel is stopped, then a faster bullet will have a higher POI than a slower bullet.
2. However the ladder graph below shows increasing velocity with no increase or even slight dip in elevation of POI.

When you consider that even the best load has deviation in velocity, it is of significant advantage to have those deviations result in no change in the POI. In this specific example velocity deviations in the range of 3400 to 3440 fps have zero or almost zero affect on group size in the vertical. This cannot happen when the barrel is "stopped". It can only happen when barrel movement compensates for velocity deviation. In other words a "sweet spot" does not occur when the barrel is stopped.

 

[Scott Harris]

The problem I have with that thinking is that it does not explain what I see when I do a ladder test. Some things to consider:

1. If a sweet spot or "node" occurs when the barrel is stopped, then a faster bullet will have a higher POI than a slower bullet.
2. However the ladder graph below shows increasing velocity with no increase or even slight dip in elevation of POI.

When you consider that even the best load has deviation in velocity, it is of significant advantage to have those deviations result in no change in the POI. In this specific example velocity deviations in the range of 3400 to 3440 fps have zero or almost zero affect on group size in the vertical. This cannot happen when the barrel is "stopped". It can only happen when barrel movement compensates for velocity deviation. In other words a "sweet spot" does not occur when the barrel is stopped.

"When you consider that even the best load has deviation in velocity, it is of significant advantage to have those deviations result in no change in the POI. "
Agreed. Looking at your graph, its pretty clear the 3400-3420 velocity range is a nice forgiving place to be. If you want to shoot well, that would be the end of the conversation. Law of diminishing returns, time to go work on the wind reading!

 

[Ned Ludd]

As Scott stated, you've clearly found your window and this discussion is starting to powderize my brain. I'll leave it at that.

 

[Alex Wheeler]

If you have ever played with a tuner and seen how little you have to move it to change groups then I think you would find it hard you could ever predict exactly where a node will be...

 

[RonAKA]

If you have ever played with a tuner and seen how little you have to move it to change groups then I think you would find it hard you could ever predict exactly where a node will be...

I think the problem is "computers". Anyone can program a computer to calculate anything, especially when there is no known right answer to check the "computer". In fact you can not only calculate an "answer" you can do it to 8 decimal points or more. How can you ever question that accuracy? Computer programmers figured that out a long time ago. "Garbage in, garbage out".

 

[Ned Ludd]

I don't find it unusual at all that it might be possible to predict about where a node might be using a computer. Note I said "about". A computer can be an extremely useful tool for this process, allowing a person to save time, effort, components, and $$$. No one is saying the use of Quickload takes all human observation and intuition out of the process, just that it can help you get there with much less effort. You still have to roll the loads, shoot them, and interpret the results. But the computer can cut way down on the range of variables you have to test.

So how could this possibly work, using one 'o them there newfangled computer thingies for loading' up their shootin' irons? Well, just take a look at the original premise of this thread, "Half-nodes". There are many, many shooters using 200 Hybrids, and now the new 200.20X bullet in F-TR. As discussed previously in this thread, it is extremely difficult to hit OBT Node 4 at safe operating pressures in a .308 with a 200 gr bullet, even with a 32" barrel. This would generally require velocities in the range of about 2720-2750 fps, and pressure well in excess of ~65K psi. So the folks using them have been shooting them in the range of about 2650 to 2675 fps out of 30"-32" barrels. And there've been an awful lot of matches won using them.

Note that that window is only about a 25 fps velocity spread. It may actually be even narrower than that. This is where people all over have been finding an accuracy node with the 200 gr bullet. The velocity range is little (if any) wider than I would expect for a typical optimal charge weight window. If people all over the world are using barrels in the 30"-32" length range, with a wide range of different contours, are all hitting almost the exact same spot, it should be pretty obvious that there is something inherent about this particular accuracy node that, in fact, is very predictable. So I'd say this example is is pretty strong evidence why it's totally possible to predict pretty closely where an accuracy node might be using a computer. Would this be beneficial in every type of precision shooting known to man? Maybe, maybe not. All I can say is that it works very well for F-Class. It works because barrel length and velocity are variables most anyone has the tools to measure with a fair degree of accuracy. Most people do not own the tools necessary to do vibrational and moment of inertia analysis that would necessary to predict the behavior of a tuner with the same degree of accuracy. However, it would likely also be possible to make tuner setting predictions if you had access to the right equipment.

RonAKA - regarding your graph above, I think the real the question is whether the y-axis of that graph actually reflects only the position the muzzle was pointing at the time the bullet exited the bore, or if there might be several other factors influencing POI at the same time. You might take a look at Harold R. Vaughn's "Rifle Accuracy Facts" pgs. 84-88, particularly Figure 4-39, which does a very nice job of correlating the velocity of muzzle movement with vertical dispersion.

 

[RonAKA]

RonAKA - regarding your graph above, I think the real the question is whether the y-axis of that graph actually reflects only the position the muzzle was pointing at the time the bullet exited the bore, or if there might be several other factors influencing POI at the same time. You might take a look at Harold R. Vaughn's "Rifle Accuracy Facts" pgs. 84-88, particularly Figure 4-39, which does a very nice job of correlating the velocity of muzzle movement with vertical dispersion.

I've read Vaughn's book, but I only borrowed it, and can no longer remember what it says on those pages. Vaughn has some interesting theories. I will always recall the one about him blaming inaccuracy of Remington barreled actions on the fit between the barrel and the action being loose, and moving during the firing cycle. In fact I'm sure I will think about that theory in not kind terms when I go through my upcoming job of removing my original factory Remington barrel and finding it is very well seized in place!

My graph only shows the relationship between muzzle velocity and vertical POI, nothing else. There are three cases of it shown and the result is pretty consistent. Vertical POI rises, flattens out, and then rises again with increasing velocity. What reasons could explain increasing velocity without the expected increase in elevation?

On computers, I won't say they are always wrong. I will just say that you have to be really careful with the inputs and how you interpret the outputs. Before I retired, one of my areas of expertise was steam turbine blading vibration and failure analysis. It is a complex field, and we used finite element analysis services to help us understand the root causes of cracking. It is a very powerful tool essentially not possible without a computer, but the output is extremely difficult to interpret with very high confidence. One of the things you have to watch for is significance. The FEA modelers can easily blow up and show animations of many multiple modes of vibration, just like Varmint Al does with his Gun Analysis. However many, if not virtually all of the modes are so small in amplitude that they are negligible. I have not read all of Varmint Al's stuff in detail, but I did not see anywhere he put the movement of one mode in the same scale of enhancement as the other modes. I suspect it would show most of the complex modes are simple negligible. The first trick is identifying the mode of interest. I suspect that in the case in the gun it is likely up down barrel movement and angle. Yes, the barrel probably does expand and contract in diameter and it would feel good to think that your bullet is exiting when it is in full choke mode, not improved cylinder. My suspicion is that this mode is one of the insignificant ones. But anything is possible I suppose.

 

[Alex Wheeler]

I think the problem is "computers". Anyone can program a computer to calculate anything, especially when there is no known right answer to check the "computer". In fact you can not only calculate an "answer" you can do it to 8 decimal points or more. How can you ever question that accuracy? Computer programmers figured that out a long time ago. "Garbage in, garbage out".

My point about the tuner is that you can turn it and make the barrel think its .003" longer and you just moved the node. No way any one is measuring barrel length to that type of accuracy which is what would be required to make accurate predictions in my opinion. My advise to anyone looking for the best groups is to tune the rifle for the smallest groups at the distance your shooting. The node which will produce the smallest groups at long range will be just before the barrel reaches its highest point in its movement. That will promote positive compensation which is needed to shoot tiny groups at long range. If you want to tune with group and es you will be relying on the cone of dispersion which is fine on a hunting rifle which needs to shoot at many distances but will never yield the best group. None of that matters though. Just bring your gear to the range, load and tune at the distance you want to shoot and you'll win. JMO

 

[Ned Ludd]

My point about the tuner is that you can turn it and make the barrel think its .003" longer and you just moved the node. No way any one is measuring barrel length to that type of accuracy which is what would be required to make accurate predictions in my opinion. My advise to anyone looking for the best groups is to tune the rifle for the smallest groups at the distance your shooting. The node which will produce the smallest groups at long range will be just before the barrel reaches its highest point in its movement. That will promote positive compensation which is needed to shoot tiny groups at long range. If you want to tune with group and es you will be relying on the cone of dispersion which is fine on a hunting rifle which needs to shoot at many distances but will never yield the best group. None of that matters though. Just bring your gear to the range, load and tune at the distance you want to shoot and you'll win. JMO

It probably wouldn't be necessary to take hyper-accurate measurements regarding apparent changes in barrel length with different tuner settings. All that would be needed would be to analyze the barrel harmonics at different tuner settings. The effects of tuners at the correct setting on barrel harmonics have been fairly well-characterized and should be reproducible simply by monitoring the barrel harmonics. With the correct equipment, it might even be possible to do this by simply tapping the barrel with a mallet (or something similar) to set up the harmonics. This is not my idea, but was recently suggested to me by Steve Blair. However, a number of recent posts here regarding the use of tuners and my own reading on the topic suggests it is a reasonable hypothesis. At the very least, with the correct equipment, it would be pretty easy to test whether optimal tuner settings "predicted" by this approach matched, or were even close, to optimal tuner settings determined empirically through firing and group analysis. To be honest, I'm not sure exactly what is currently considered to be the optimal equipment necessary to do barrel harmonic analysis. However, I'm pretty sure that whatever it is, most people don't own one, which is likely one of the reasons people don't do it this way.