Half nodes?

[memilanuk]

So... been using QuickLoad / OCW / OBT off and on, in one form or another, for a number of years. For a lot of cartridge / bullet / powder combinations, it's worked just ducky. More recently, though, I've been kind of scratching my head trying to figure why a particular bullet / powder / barrel length combination just seems to be... off.

Basically, any which way I run the numbers for a .308 Win, 200 Hybrid and a 30" barrel, the nodes show as being around ~2530, and ~2715 fps. The one is slower than I'd care for, and the other is not practical (unless one considers brass to be disposable after one shot, even with Palma brass). The region where the vast majority of shooters find 200 Hybrids to run really, really well - from a little over 2600 to maybe 2650 fps - is about as far from an OBT node as its possible to get. I've tried and tried, and can't find any combinations that shift the node up or down to that range.

I have also noticed, that the tune window at the actual 'node' seems to be frustratingly narrow, at least compared to other OBT nodes I've experienced in the past. Not very forgiving of variation in any way. Very accurate, but I really have to work to stay on top of it.

Recently at a match while BS'ing in the pits with a fellow shooter, this topic came up in discussion. The term 'half nodes' was brought up - that there are smaller, narrower 'sweet spots' about every 100fps apart, vs. the approx. 200 fps between OBT nodes. I did some cursory searching for information on this, but without any real success. I'm curious... anybody else out there have similar experiences and care to comment?

 

[jvw2008]

If you look at Chris's calculated nodes and calculate the mSec difference between 1-2, 2-3, 3-4, etc. you will find a repeating sequence in these calculations. I have struggled with the same observations you mentioned. The 6.5 Creedmoor has a definite node at somewhere around 2830 fps, but that muzzle velocity doesn't even come close to node 4,5,or 6. But if you halve (half-node) the difference between 4-5 or 5-6 (sorry don't have QL in front of me right now) the optimum barrel time sits right on top of a MV of 2830. As an example : add node 5 and 6 and then divide by 2 to get barrel time for the half node.

Disclaimer : this formula or approach has NO correlation with Chris Longs publications that I have found so far. Therefore I am simply offering it as an observation.

Jerry

 

[jlow]

Very interesting post as I am pretty much just got to the same boat…

This is a 30” Bartlein shooting the same 200 grain Hybrid. When I looked at QL, pretty much the same as what you found i.e. the high node i.e. node 4 that gives a projected MV of 2753 fps is right at the cusp of 60,000 psi i.e. max pressure. I’ve also noticed that most are in fact shooting it at 2600 to 2650 fps.

Right now, all I have done is to shoot single powder weight looking for pressure. I’ve gone as high as 44.8 grains of Varget that got me to around 2680 fps but have not shot anything hotter. My idea was to explore this area and see what the group size would be.

One thing I noticed though is at least with this barrel, the POI from the last 4 powder weight I tested i.e. 43.9, 44.2, 44.5 and 44.8 grains (so 0.9 grain range), all were in the same place and a quick check with OnTarget gave me a group size of 0.5 MOA. This could be a fluke but I don’t think so because this barrel has been doing the same thing with 185 Juggs, so at least I am hopeful…

 

[memilanuk]

I'd let my QL license lapse for a while. When I renewed, I initially ran the numbers for my 185 Juggernaut / Varget load @ 2750fps from a 30" Krieger. Turned out it was right smack on top of a node - not a huge surprise, as it was a *very* good load - very accurate, super easy tune, tolerant of just about everything - lot-to-lot variations were largely irrelevant. Yay.

Then I ran the numbers for my 200 Hybrid + H4895 load with another 30" Krieger, running right @ 2650fps. Unlike the 185 load, this one is fairly fussy - really have to stay on top of lot-to-lot changes in *everything*, including minor variations in ogive dimensions. Still very accurate, but a pain in the posterior to keep it that way. I tried (and succeeded, if you call it that) to hit the upper node @ 2715-2720 fps... but lost way too many cases way too quickly. No deal.

One thing I did notice when tweaking the numbers in QL was that that same 2700+ node, while what I considered safe with a 30" barrel... was theoretically reachable with a 32" tube. So my barrel this year was a 32" Bartlein, running 44.2-44.3gn Varget @ 2705fps. When I do my part, it seems to shoot pretty well

Next barrel though, I'm going for the 'node' in the lower half of 2600. Just curious as to the how/why of the apparent disconnect with the OBT theory...

 

[Ned Ludd]

It's not a disconnect with OBT Theory. OBT nodes simply represent a subset of total "accuracy" nodes. If you do a typical ladder test over a sufficiently wide charge weight window, you will find a certain number of potential accuracy nodes. Only a very few of them will correspond to OBT nodes. I can't tell you exactly what the different between them is, but it is almost certainly due to harmonics, as opposed to the exact timing of when the bullet exits the bore.

The 2650-2675 fps node many people have been hitting with their 200 Hybrid loads is not an OBT node. Bt clearly it works because a lot of very good shooters have been winning high-level matches using it. Is a non-OBT node necessarily narrow or less forgiving than an OBT node? I suspect that may largely depend on the rifle itself. Barrel contour doesn't play any role in OBT theory, only barrel length is important. However, we know that barrel contour can have a very big influence on harmonics and how a load tunes. If this wasn't the case, barrel tuners wouldn't have the beneficial effect they obviously do. So the question whether a non-OBT node has a less forgiving tune window than an OBT node may have more to do with considerations other than just barrel length.

Finally, I have run the numbers in QL on numerous occasions for 200 gr bullets. For 30" barrels, your generally talking about running pressures in the 66K+ psi range with Lapua Palma brass and a freebore in the .170"-ish range in order to reach OBT Node 4 with most single-based powders. That increase in velocity over the 2650-2675 node people have been running will buy you maybe 0.2 MOA wind resistance at 1000 yd. Is it really worth it? If you are using a 32" barrel, with a much longer freebore, and brass with larger case volume than Lapua Palma, you just might be able to reach the OBT node without having to push the pressure so high. Whether that is worth such a relatively small gain is obviously up to the user.

Edited to add: one other thing to consider is whether non-OBT nodes such as the one for 200s that seem to fall halfway in-between the OBT nodes can also be predicted with any certainty using QL. This clearly works with OBT nodes because you can use barrel time as a readout and can target a defined OBT node. Whether the non-OBT nodes in between can be also targeted using barrel time remains to be seen. I have been trying to figure that out using other people's reported specifics for optimized loads with 200s, but I haven't reached any conclusion as yet. If the response is linear, or at least close to linear, it might be possible. However, if those in-between nodes are linked more strongly to barrel harmonics (as opposed to barrel length), they may differ widely enough between different rifles so as to make any kind of prediction using QL very difficult. In the meantime, it's pretty easy to simply test in the velocity range we know others have successfully been getting the 200s to work and empirically determine where our own rifle wants to shoot.

 

[memilanuk]

Edited to add: one other thing to consider is whether non-OBT nodes such as the one for 200s that seem to fall halfway in-between the OBT nodes can also be predicted with any certainty using QL. This clearly works with OBT nodes because you can use barrel time as a readout and can target a defined OBT node. Whether the non-OBT nodes in between can be also targeted using barrel time remains to be seen. I have been trying to figure that out using other people's reported specifics for optimized loads with 200s, but I haven't reached any conclusion as yet. If the response is linear, or at least close to linear, it might be possible. However, if those in-between nodes are linked more strongly to barrel harmonics (as opposed to barrel length), they may differ widely enough between different rifles so as to make any kind of prediction using QL very difficult. In the meantime, it's pretty easy to simply test in the velocity range we know others have successfully been getting the 200s to work and empirically determine where our own rifle wants to shoot.

That was kind of what I was getting at with the term 'half nodes'... the calculated OBT nodes are typically ~200 fps apart - in my instance lets say 2530 and 2720. Split the difference gives you 2625. There are a *lot* of people running 200s @ 2620-2640 with outstanding results. I'm wondering if that's repeatable in any other cartridges or circumstances; the earlier post regarding the 6.5 Creedmoor suggests it may be. I'd been told that these so-called 'half nodes' were some well-kept secret... just trying to stimulate some open conversation on the topic.

 

[MattPeetz]

Monte

Do you know, does the 284 fall on an OBT node at 2830-2850 where a lot of shooters are at? Or 2710? With the 180's

I have a good node around the 2710 mark in my daughters 284 with 180's with 30" barrel.

 

[memilanuk]

Matt,

I don't know enough about a .284 Win to enter all the particulars into QL.

Monte

 

[Ned Ludd]

That was kind of what I was getting at with the term 'half nodes'... the calculated OBT nodes are typically ~200 fps apart - in my instance lets say 2530 and 2720. Split the difference gives you 2625. There are a *lot* of people running 200s @ 2620-2640 with outstanding results. I'm wondering if that's repeatable in any other cartridges or circumstances; the earlier post regarding the 6.5 Creedmoor suggests it may be. I'd been told that these so-called 'half nodes' were some well-kept secret... just trying to stimulate some open conversation on the topic.

I'm not sure they're any "well kept secret", because people have been shooting those nodes for many, many years before the advent of Quickload. My main question remains: because they do not correspond to OBT nodes, which are solely based on barrel length, can they be predicted like OBT nodes can? If these "half-nodes" are more dependent on barrel harmonics rather than barrel length, they may be far more dependent on barrel contour, etc. Thus, they may vary much more widely from rifle to rifle than do OBT nodes, which in my hands seem to be very consistent for a given barrel length, regardless of caliber or contour. I don't know the answer to this question either, but hopefully someone with the knowledge will post it here. Thanks for starting the topic, Monte.

 

[jvw2008]

Matt

I also run a 284 Shehane. 180 gr hybrids over H3841SC, 29.625" Bartlein. I cited my experience with the 6.5 CM, but I'm believing what I posted because I've documented the same findings with the Shehane and a 223. As I said, the time intervals between the nodes from Longs equation show a very repeatable sequence - one interval is short and one is longer. Dividing the longer interval in half is where I'm finding a fairly stable node. My findings suggest the equation nodes are a little more stable than the half node I described - at least in my rifles.

With my Shehane, the node below 2830 fps is at 2720. This MV sits right on top of an equation node (#5 I think) and I use it to fire form. I use 54.4 grs of 4831 to fire form cases that have much more variance in case capacity than what I would normally tolerate, but even with the capacity variance the node is stable - 0.4 MOA on average. I'm not willing to compete at 2720 so I've pushed up to a barrel time that is exactly in the middle between nodes 4-5 to get a 2830 MV. Once I have my case capacities sorted and consistent, this half node has shown itself to be very stable. Do I believe in half nodes - yep.

 

[MattPeetz]

Matt

I also run a 284 Shehane. 180 gr hybrids over H3841SC, 29.625" Bartlein. I cited my experience with the 6.5 CM, but I'm believing what I posted because I've documented the same findings with the Shehane and a 223. As I said, the time intervals between the nodes from Longs equation show a very repeatable sequence - one interval is short and one is longer. Dividing the longer interval in half is where I'm finding a fairly stable node. My findings suggest the equation nodes are a little more stable than the half node I described - at least in my rifles.

With my Shehane, the node below 2830 fps is at 2720. This MV sits right on top of an equation node (#5 I think) and I use it to fire form. I use 54.4 grs of 4831 to fire form cases that have much more variance in case capacity than what I would normally tolerate, but even with the capacity variance the node is stable - 0.4 MOA on average. I'm not willing to compete at 2720 so I've pushed up to a barrel time that is exactly in the middle between nodes 4-5 to get a 2830 MV. Once I have my case capacities sorted and consistent, this half node has shown itself to be very stable. Do I believe in half nodes - yep.

That's quite interesting. I am only running the 2710 for her because the barrel can't reach the 2830 node. It is 30" long but its just slower than a turd. I don't want to leave it at that but it will allow her to get used to handling the 284. I fully plan to put a new barrel on it to get up to the 2830 node. I haven't fully finished the load work up on it but that node shows a .6gr spread that will shoot .4" at 200 yards with less than a 1/4" of vertical. I still have seating depth and tuner settings to do.

I was just wondering if that 2710 would be a half node and it appears to be with what you are finding.

 

[Tommie]

I need an education. What does "OBT" node mean?

 

[Ned Ludd]

Optimized barrel time. It is a theory put forth by Chris Long that correlates barrel length and the time the bullet takes to traverse the barrel (barrel time) to accuracy nodes (link below).

http://the-long-family.com/optimal barrel time.htm

 

[Tommie]

Optimized barrel time. It is a theory put forth by Chris Long that correlates barrel length and the time the bullet takes to traverse the barrel (barrel time) to accuracy nodes (link below).

http://the-long-family.com/optimal barrel time.htm

Ah yes. Thanks!

 

[Mozella]

........ snip.........The term 'half nodes' was brought up - that there are smaller, narrower 'sweet spots' about every 100fps apart, vs. the approx. 200 fps between OBT nodes. ......... snip........

You'll find even more and better matches between your testing and the OBT theory if you work on a "sixteenth nodes basis". I'm kidding of course.............kind of.

Lots of folks have tried to come up with a scheme to predict what kind of load would produce good precision without going through all that pesky testing. Others have schemes to predict ideal tuner settings based on a small number of factors, some of which are downright silly.

When I first heard about OBT, I took a look. Since I had a background in physics I was skeptical because of how simple the concept was and, no surprise to me, the predictions of where the "nodes" were didn't match any of my rifles. In my opinion, you just can't ignore really important factors like barrel contour and hope to have a formula which works. But since so many people rely on OBT, I figured it was worth examining. None of my "good stuff" correlated with what OBT predicted.

After I bought Quickload I wondered if I could take a known good recipe and apply it to a different powder. If OBT has any validity, even though it might not predict a good load from scratch, surely once I found a good load for a particular rifle/barrel combination, matching that barrel time would also produce good accuracy, right? Using the Quickload program, I tried several alternate powders and charge weights carefully adjusted to produce a barrel time equal to that of my "good stuff", keeping everything else the same. No dice. The measured chronograph data matched the QL prediction, so I assume the barrel time (something I can't measure directly) wasn't far off. But the accuracy wasn't there; not even close. Trying to match MV didn't work either.

In other words, either entering recipes into OBT known to produce winning results in several rifles, or working backwards from a known good recipe and trying to match it using a different powder, I found that the OBT theory didn't have any correlation at all to what has proven to be accurate after a lot of careful testing.

I measure all my 5-shot test groups using a computer based program on scanned targets and I study all that data, along with chronograph measurements, etc. etc. I have developed good loads for all my rifles, some of which I successfully use in competition. And I have thousands of archival records entered into various Excel spreadsheets. None of them correlate in any way with the OBT theory, which is no surprise to me. Finding the amount of powder, along with the best seating depth, neck tension, primer type, bullet shape, bullet weight, head space, and all the major and secondary factors which might effect precision isn't easy. It takes careful testing.

I'd love for someone to develop a crystal ball for load development, but until then I'll ignore the OBT theory and continue to test at the range. Of course, if anyone finds that OBT works for them, don't let me stop you.

 

[jvw2008]

A lot of folks start with QL and then give up because nothing seems to come out as predicted. Odd that the software works great for others. For those who just can't seem to get good results, the problem frequently arises from incorrect data being supplied to the software - crap in crap out. Correlation of the OBT depends entirely on the correct calculation of barrel time out of QL. If you don't have a correct answer from QL then the OBT theory appears to fall apart. When something doesn't work for me but does work for a lot of other folks I start looking for what I'm doing wrong rather than immediately assume the theories and calculations are just BS.

JV

 

[Ned Ludd]

OBT works fantastically well for me. If it didn't, I wouldn't be trying to come up with a way to apply the technique to half nodes.

 

[RonAKA]

I find "nodes" to be a bit of a misleading term, and 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. This is also the point of maximum velocity of muzzle movement. Then there is the very bottom of the cycle and top of the cycle. The barrel muzzle is actually stopped in these positions. The problem is that is a very narrow range. Now, if we go back to the two horizontal positions, and consider the downward moving horizontal. If a bullet exits early (more velocity), it exits with the barrel pointing up. If it exits late (slower bullet) the barrel is pointing down. This exaggerates the effect of slow and fast bullets, and represents the worst possible velocity and position to target. Perhaps could be called the "sour spot". But the upward horizontal position is the opposite. Fast bullets exit early when the barrel is pointing down, and slow ones are late with the barrel up. This is the self correcting zone of velocities. Right at the horizontal and moving up may even cause a dip in POI with increasing velocities. There, or right before it, or right after it, would be in my opinion the preferred sweet spot. I don't think you get another legit sweet spot until you get to net horizontal upswing part of the cycle.

Using a computer program to tell you where that is, sounds nice, but I remain a little skeptical. It has to compensate for barrel profile as well as length, to accurately predict vibration frequency. I like the old fashioned ladder test. Here is an example. I find it tends to be bullet weight independent and mainly depends of the specifics of the barrel and just velocity. 3420 looks like a "sweet spot" to me...

 

[normmatzen]

Interesting.
No one has mentioned the effect on different powders on QL calculated barrel times.

I recently fitted a new barrel on my 284 WIN heavy gun/F Class gun. I had this one cut for 28" instead of the 30" of the last one.
To determine a load range to test this new barrel, I tried H4350, H4831sc and RL-17. Even though all powders had similar velocities, all had different barrel times. Drastic enough to make me ignore all but RL-17. I loaded 6 groups of 5 each with a different loads of RL-17. I shot at 200 yd and monitored MV with a chrono and compared groups. I picked a load based on lowest ES and then with my tuner (screwed all the way back for the ES test) I set the best adjustment with further group testing.
I was always disappointed with the old 30" barrel, but the new one shoots so well I am going to relegate it to matches only and practice with the old barrel set back 2 "!

 

[aj300mag]

One would think the objective would be to catch the muzzle in the perfectly flat position, not pointed up or down.

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.