How far apart are "nodes"

[FearNoWind]

... In fact, nobody knows what a 'node' actually is. We don't know what 'tune' is

Sorry, Mike, I have to disagree with you there. When the rifle is fired a shock wave runs along the barrel causing some deformation of its shape. Consequently the center of the bore is generally misaligned with the target at several points before the bullet leaves the barrel. The "node" we're looking for is the state of mechanical equilibrium that occurs as the shock wave is at its lowest amplitude (node within a sine wave). No two barrels are identical so calculating where the "node" might be for a specific size/type of bullet traveling down the barrel at increasingly higher velocity over the barrel's unique length, rifling resistance and other factors means the node for a given load in your rifle won't match the time/space relationship for determining the node for my rifle.
"Tune" is when you've got all the pieces together in such a way as to have the bullet leave the muzzle at precisely the time the barrel is in equilibrium. Some shooters like to use those barrel tuner attachments that the slide up and down the length of the barrel until their favorite load works well.

 

[Mikecr]

That's a couple good notions, but not the only ones.
Don't forget there is more going on than than a single accoustic wave causing bore dimensional changes(not misalignment). There is barrel expansion with bullet travel, elongation, untwisting, whip, interference waves, and recoil. Atleast.
Then you have pressure tune, powder tune, and timing tune. Atleast.
Then you add temperature and air density changes, barrel stresses, contours, fluting, relative barrel hanging forces, bedding, theading & threading type, action, recoil stop point/sys, scope attachment, gun hold, bag hardness, etc.
We cannot see any of this. We're just wading through summnations of it.

The closest anybody's come to isolating these various factors, is Harold Vaughn as published in his book Rifle Accuracy Facts. In this, he doesn't conclude anything as simple or cut & dry.
It isn't simple.
We have Varmint Al's animations, and Chris Long models, rambling Bill Calfee node theories(all three different),, but you got to remember these are not real. They're theories.

I'm suggesting that we really don't know, because we're unable to isolate and affect each alone. For example, no matter what you think seating adjustments are doing, you can bet it fails tests somewhere. There is no predicting of it. It's a significant unknown, always has been. Same with primers & striking. Both of these, seating and ignition, produce far greater deviations than seen with OCW testing.
Is there an animation out there for these?
How bout for kitty litter -vs- sand? Or 10deg slope shooting -vs- tune?

When we actually know something, we define it. That's science.
We can't even see atoms yet, but we've defined splitting of them to astonishing accuracy.
It would take a relative manhattan project to define and predict gun tune just the same. It's that abstract.

 

[FearNoWind]

... Don't forget there is more going on than than a single accoustic wave causing bore dimensional changes (not misalignment). There is barrel expansion with bullet travel, elongation, untwisting, whip, interference waves, and recoil.

You're right, of course. I sometimes use the analogy of "bore misalignment" with the target for the neophyte who may not have enough experience to understand the physics affecting barrel dynamics. They want to know why the bullet doesn't hit the target consistently when the node is out of sync. I figure all they need to understand is what barrel whip does at the muzzle.
Reading your comment I too find that my terminology may be a bit too simple. Thanks for the insight. I'll try to find a better way to express that phenomenon for my students.

 

[Toptuna]

great thread

 

[el matador]

This has been a great discussion. I buy the "theory" of an oscillating shock wave, and that there are optimal times for the bullet to exit with respect to such phenomena. Using barrel time seems like it has merit, and I would like to explore that. I also believe there are many other variables at play which create additional vibrations or shock waves, one of the most significant being seating depth. And on top of that, the OCW method holds some water too! I'm starting to think that a great load is a combination of good barrel time, stable powder charge and finding a sweet spot amidst all the other vibrations.

The reason I posted this topic to begin with was that I found a relatively accurate load for a 300 WM. It is a pretty mild load however. With virgin brass and 76.0 grains H1000 I sent a trio of 208 Amax's into a .320" group, chrono'd at about 2750 fps. I believe with fire formed brass I'll get a little better velocity, and I can step up to 78 grains or more...but how far do I need to go before I hit another node? I think 2950 fps is about the max I can push those bullets.

Looking at the chart from the shock wave theory: Each node is 8-13% greater than the one preceding it, so roughly 8-13% more velocity would be a likely spot for another node. 8% puts me at 2970 fps, so it might be just barely possible with this powder and bullet. It's not real scientific but it's the kind of rough approximation I was looking for. I wish I had quick load right now so I could make some virtual reloads!

 

[benchracer]

So what factors do you use to extrapolate node distance and predict nodes? Since you have experience and confidence in this methodology, could you provide is with a real world example, and explain? I am curious how all the variables that play into load development are are included or excluded from your method. If you can explain it, and it can be replicated, I'll be the first to eat humble pie and post my results. If you can't explain, or won't, you have no business trying to tell someone about real world results you cannot support. At least that's my take on it.

The best place to start is by entering good data in Quickload. That is where most of the variables are captured. It is also important to do a min-max workup to reconcile predicted velocities and max with real world data in your rifle.

When you generate a load table in QL, it also spits out barrel time data, which can then be compared to the chart I referenced earlier. The chart gives the OBT in milliseconds for a given barrel length. According to information in the paper I referenced, best accuracy typically is within + or - .020 ms of the OBT. I have found that this typically yields nodes that are about 60 fps wide.

There are limitations to this method. For example, QL cannot tell you which primer will work the best for your combination, nor can it tell you what your optimum seating depth will be. I typically start my load development by performing the Berger seating depth test using a starting load, which gives me a good place to start. Though seating depth typically has a significant effect on accuracy, it has only a very small effect on OBT. Using the right seating depth in your rifle will shrink your groups, but it doesn't measurably change the velocity range for a given node.

Another limitation of this method is that it cannot tell you if your rifle will shoot a given component combination well. It can only tell you what the velocity range will be for a given node. Just because you are in a node is no guarantee that the combination you are using will produce tiny groups.

Now for real world results. Here is the thread where I was introduced to the OBT concept and used that data to walk another shooter through the process. His best load was produced in the predicted velocity range:

http://www.longrangehunting.com/forums/f19/375-rum-guys-137112/index4.html


Here is a second thread where I repeated the same process, with the same results. This time, best accuracy appeared in the upper half of the predicted node. Note that the results were obtained without tuning seating depth:

http://www.longrangehunting.com/forums/f19/finally-time-test-375-rum-320-ce-140162/index2.html

I have also been going through my notes and running my data through QL to see where my accuracy loads fall relative to OBT predicted nodes. Out of six different accuracy loads, I have one that does not appear to fall in an OBT predicted node and one that I cannot evaluate because the powder (Superformance) is not supported in QL. The issue there is that I developed all of these loads before I owned a chronograph, so I still need to shoot them over a chrono to see what the actual velocities are before I can definitively confirm them. I am adding that to my project list.

It is also worth noting that all of my personal accuracy loads were developed using OCW. The OBT paper mentions a strong correlation between OCW and OBT. It looks to me like OBT data is confirming my OCW data. The advantage that I see with OBT is the ability to reach the same place as OCW with fewer rounds expended and/or fewer trips to the range. OBT also makes it possible to get a better idea of the overall performance potential offered by a certain combination before burning any powder.

In closing, I want to make it clear that my intentions in making this argument do not involve making anyone eat humble pie. Everything that I do relative to shooting/hand loading is something that I approach as a learning opportunity. When I learn something, I try to share it with interested people. That is my motivation for entering this debate. I did not do so to show anyone up. Rather, to show what is possible. I am just learning to use some of these tools myself. It is quite possible that I am following a chimera. If that is the case, I will be the one eating humble pie and I will gladly do so. In either case, others will benefit from the discussion, and that serves us all well.

 

[benchracer]

Ok Benchracer, predict a tune with OBT:
223REM
24" barrel
50gr FB bullet
H322 powder
Then predict the seating depth that takes me from 3/4moa to 1/4moa(without changing MV)

Just twirl it through your software & read me the printout. Then I'll pass along the actual answer for MY gun. Let's see how well you do.

First, you seem to have an incomplete understanding of what OBT does or how it works. I strongly recommend that you read the info I posted, along with Dan Newberry's explanation of OCW. Even if you do not agree with the conclusions in either source, you will still benefit from understanding the concepts and how they apply relative to this discussion.



As I mentioned in a previous post, OBT does not predict an accurate seating depth. Though seating depth can have a dramatic effect on overall group size, it has a very small effect on barrel time and a consequently small effect on the velocity range of a given node.

I ran the data you provided through QL, using an OAL of 2.260 and a Sierra 50 grain SP, which is a FB bullet. Knowing your actual base-to-tip length and the actual bullet used could provide more refined results, but the data should get us in the ballpark.

The OBT chart predicts nodes at .900 ms, 1.030 ms, and 1.120 ms. Information in the paper I posted states that a given node typically falls within + or - .020 ms from the OBT value.

Below are the values I used in the prediction, along with the results. There are three distinct nodes in the predicted data. Compare for yourself and let us know, which, if any, node your accuracy load falls into:

 

[benchracer]

How you can test to see if you truly have a stable load I.E. The POI does not shift with temperature and SLIGHT variants in powder charges (from miss throws) or from different powder lot numbers. An OCW test is not necessarily trying to find the most accurate load, you are trying to find the most repeatable/stable load and then you adjust seating depth to tighten groups after you find your stable load. So just because you have an accurate load, that does not necessarily mean that is a "node" by the OCW method; although, most nodes are significantly more accurate than the scatter groups will be. His website spells it all out and what it's purpose is.

This is a good summation of what I have in mind when discussing a node. It is possible to achieve an accurate load outside of a node, but such a load is likely to behave in a temperamental fashion.

 

[benchracer]

The reason I posted this topic to begin with was that I found a relatively accurate load for a 300 WM. It is a pretty mild load however. With virgin brass and 76.0 grains H1000 I sent a trio of 208 Amax's into a .320" group, chrono'd at about 2750 fps. I believe with fire formed brass I'll get a little better velocity, and I can step up to 78 grains or more...but how far do I need to go before I hit another node? I think 2950 fps is about the max I can push those bullets.

This appears to be an excellent opportunity to test/demonstrate the methods being discussed.

 

[Mikecr]

The best place to start is by entering good data in Quickload. That is where most of the variables are captured. It is also important to do a min-max workup to reconcile predicted velocities and max with real world data in your rifle.

When you generate a load table in QL, it also spits out barrel time data, which can then be compared to the chart I referenced earlier. The chart gives the OBT in milliseconds for a given barrel length. According to information in the paper I referenced, best accuracy typically is within + or - .020 ms of the OBT. I have found that this typically yields nodes that are about 60 fps wide.

There are limitations to this method. For example, QL cannot tell you which primer will work the best for your combination, nor can it tell you what your optimum seating depth will be. I typically start my load development by performing the Berger seating depth test using a starting load, which gives me a good place to start. Though seating depth typically has a significant effect on accuracy, it has only a very small effect on OBT. Using the right seating depth in your rifle will shrink your groups, but it doesn't measurably change the velocity range for a given node.

Another limitation of this method is that it cannot tell you if your rifle will shoot a given component combination well. It can only tell you what the velocity range will be for a given node. Just because you are in a node is no guarantee that the combination you are using will produce tiny groups.

I couldn't agree more with you through this. I basically do the same, beginning with ~Berger seating testing during fireforming, and then with fireformed cases I move into ladder & incremental load testing, then a final tweaking of seating to shape grouping, and on to cold bore testing.

My test to you earlier wasn't fair. You didn't have opportunity to calibrate to my conditions.
For instance, I do not FL size and my cases are higher in capacity having taken to chamber dimensions. So my velocities/pressures are lower. I'll concede that if you had known this you would have nailed it.
My H20 capacity is a tad under 31gr, and my accuracy load is 27.2 H322 for ~3585fps. QL shows this at 903ms. It's pretty clear I cannot reach higher to confirm other OBTs from referenced table. But the 900ms point does correspond.
Good job.

I've discounted OBT and OCW because in the past these never produced the most accurate loads for me. But this was a long time ago and back then I was developing like most people today -powder first, then seating. Since OBT/OCW discount initial seating, it would be random to find the most accurate load with them, by themselves, even while finding powder nodes. But I'll concede that with best seating established first, OBT/OCW may reduce efforts overall, and may help with decisions to go for another node or not.

I'm still skeptical about predictions from scratch. But the next time I calibrate QL to a new barrel, I'll look for OBT correlations(without reaching), and consider the numbers again.
Maybe save time & barrel life.
I appreciate your efforts & input benchracer.

 

[Unofficial Gun Addict (UGA)]

Benchracer, I appreciate your taking considerable time to respond with your methodology behind the OBT process. Deer season is approaching rapidly, but afterwards, I'll have a lot of time to get out to the range and experiment. As promised, I will attempt to thoroughly test this out because I'm always looking for ways to improve technique, in shooting and reloading... and I'll definitely post my results back on this thread. I'll also compare OBT workups to loads I have already refined, see how they compare, and test to see if using the data from the OBT table and the timing data that quikload puts out improves or hastens load development. If need be... my wife cooks a mean humble pie and I always have my fork within arms length.n. Thanks again.

Boss

 

[benchracer]

I couldn't agree more with you through this. I basically do the same, beginning with ~Berger seating testing during fireforming, and then with fireformed cases I move into ladder & incremental load testing, then a final tweaking of seating to shape grouping, and on to cold bore testing.

My test to you earlier wasn't fair. You didn't have opportunity to calibrate to my conditions.
For instance, I do not FL size and my cases are higher in capacity having taken to chamber dimensions. So my velocities/pressures are lower. I'll concede that if you had known this you would have nailed it.
My H20 capacity is a tad under 31gr, and my accuracy load is 27.2 H322 for ~3585fps. QL shows this at 903ms. It's pretty clear I cannot reach higher to confirm other OBTs from referenced table. But the 900ms point does correspond.
Good job.

I've discounted OBT and OCW because in the past these never produced the most accurate loads for me. But this was a long time ago and back then I was developing like most people today -powder first, then seating. Since OBT/OCW discount initial seating, it would be random to find the most accurate load with them, by themselves, even while finding powder nodes. But I'll concede that with best seating established first, OBT/OCW may reduce efforts overall, and may help with decisions to go for another node or not.

I'm still skeptical about predictions from scratch. But the next time I calibrate QL to a new barrel, I'll look for OBT correlations(without reaching), and consider the numbers again.
Maybe save time & barrel life.
I appreciate your efforts & input benchracer.

Very interesting results! Your conclusions regarding the utility of OBT/OCW are similar to my own. I am glad that I could make a contribution to the discussion and I hope my efforts will prove to be useful.

 

[benchracer]

Benchracer, I appreciate your taking considerable time to respond with your methodology behind the OBT process. Deer season is approaching rapidly, but afterwards, I'll have a lot of time to get out to the range and experiment. As promised, I will attempt to thoroughly test this out because I'm always looking for ways to improve technique, in shooting and reloading... and I'll definitely post my results back on this thread. I'll also compare OBT workups to loads I have already refined, see how they compare, and test to see if using the data from the OBT table and the timing data that quikload puts out improves or hastens load development. If need be... my wife cooks a mean humble pie and I always have my fork within arms length.n. Thanks again.

Boss

I will be very interested to see your results when you have them. If OBT proves to be useful to you, I would consider that to be a healthy dollop of ice cream to go with the pie and the sweet taste of success!

You're very welcome! It was my privilege.