Compensation Tuning Again?.......... Sorry!

[williada]

Tim thanks for pointing out the graph error. I have adjusted it and as you have indicated it doesn't change the outcome. Its just shifted the peak node back a bit and is probably where I suggested you load at 47.3 grains. As for the reasoning with the previous graph, for the benefit of others, it still applies. Have fun. David. Edit: If you upper node String 8 thereabouts does not work out for group size come back to String 5. However with a trough node, for a bit of insurance and opposite to the peak node place your tune slightly to the right of the trough node because the slow shots will be pitched higher for compensation purposes.

[williada]

Fine tuning comments - I know a lot of people jam bullets particularly the VLD's but consider altering the neck tension to remove the horizontal? Gene Beggs once reported on jammed bullets saying in his underground test range they would always give 2 horizontal shots in a long string. He remedied this by taking his free flight out in .005" increments from just kissing the lands and playing with neck tension. Of course small increments in powder charge was used to alter the vertical. The object was to get the best group possible before attaching a tuner.

[Tim L]

Thank you so much for your input David.
I believe the Pennants match is all long range so I can concentrate on the upper node.
I'll run up loads as you suggest and do the rest of the testing at 600. Probably only 5 round groups as I don't want the barrel giving up on the day!

I'll be prepping a 308 barrel shortly, I'll lay my targets out in a string for that, no scope adjustments, and run a 10 round ladder. If you're willing to comment, I'll post them up again for another run through the process. It's been very useful, I would have been working on the 48gn group rather than the 47 (and a bit) peak.

[williada]

Happy to comment Tim. Could I say that testing at 6 hundred is mid range and there are more influences such as wind and mirage and overturning moments that could distort the results. Your load at string 5 would be suited to this. Five hundred would be a better proposition if you have the longs in mind and String 8 thereabouts needs thorough exploring here.

Seeing you have started this process with jammed bullets, follow it through as it would be interesting to see the results.

I now add some other comments for those people who do not jam projectiles. Remember everything is a trade off and our goal is consistency.


If you take Gene Begg’s line, that the jammed projectile can lead to horizontal problems; it means the benefits of the jam such as induced barrel lift, concentrically lined up projectile, the breaking of any bond between two dissimilar metals (case and projectile through electrolysis) and consistent ignition must be outweighed by the lateral shots. The down side of the jam is the problem with carbon build up and the wear problems on the locking lugs and the situation that can be created by the fact that the case shoulder may not be putting the correct tension through the action and barrel threads and affecting harmonics and a tendency to produce those sensitive barrels because everything is keyed up any small variance in tensions will have a bigger effect on dispersion.

I think the fact Tim’s barrel showed that with very little variance in extreme spread the group dispersion and height can vary considerably and contrary to what most people think, that group size was not dependent on small extreme spread, but rather the group proximity to the node. Of course his barrel can handle a positive compensation tune near the node and through the effect of gravity can shrink those groups further at the target. So in his case, the jam is probably inducing barrel lift and his nodal groups are tight and more than likely he does not have to worry about the lateral. But off the node that lateral comes in quickly with increasing charge volumes.

It also means that in a sensitive barrel, if a tuner is applied that adjustments need to be finer than ¼ turns on a variable tuner. This is because the turning of the tuner has to be related to additions and subtractions of powder charge as it effects barrel lift as a bench rest shooter would adjust for changes in heat and humidity during the day to stay on the node. This is why I work through small powder charges for different conditions in my load development first. While determining a fundamental load, I note how far small increments in powder charge push me off the node. Then I introduce the tuner to duplicate the pattern and correlate charge changes with tuner movements. If you want a shortcut, do your load development with the variable tuner on but don’t move it. Then only move it little bits if you feel you are off the node. But you will not know what powder charge the tuner corresponds to. My habits die hard, and I want to know the equivalent powder for the move as conditions change. Yes it is a longer process to duplicate the load pattern with the tuner on, but satisfying.

So back to load development without the tuner to begin with.

You have to ask yourself why are those lateral shots occurring and can you achieve a similar benefit like the jam does by playing with free flight. I believe a boat tail bullet in a free flight setup presents a greater surface area and can be skewed off line before the gas seals. Once offline it will travel through the barrel misaligned because the projectile is malleable.

But I think that altering the jump will determine a position where the gases have least chance of skewing the projectile hence Gene starting the fine tuning from just kissing the lands and moving back in .005” amounts. However, I don’t like just kissing the lands as this induces too many variables with ogive contact unless bullets are sorted. I start with .005” off the lands and do my first run of tests there to identify points to explore with minor adjustments first. With the old Sierra I found .010" a good place to start.

The other benefit with a jump, is that it mitigates against any primer spike in pressure. It’s like the effect of a stone dropped into a bigger pond – the ripples are less. However, the jump does effect bullet timing with muzzle lift if you go too far. This is a different ball game and using jump for muzzle timing not lateral, you can explore it from .005" to .080". On the other hand increasing or decreasing the charge will have more influence in .2 or .3 of a grain is far easier, then making only tiny movements in jump for lateral removal. If you are worried about extreme spread in terms of groups size and your groups even on the node are not as tight, you need to do a jump test from say, .005", .010", .015", .020", .030, .040, .050", .060", .070" and .080" from the lands as an example and lay 3 or 5 shot groups out in the horizontal to graph the results. This is very time consuming.

The other thing that is said to induce horizontal or lateral, is the neck tension. I believe the right neck tension goes a long way in determining a full powder burn closer to the breach and reduces extreme spread. With slower burning powders, they keep burning longer and can waggle the muzzle. Whether the group dispersion is on the node or not, if it is getting bigger with more charge, back off the charge. I have tended to increase neck tension for more velocity which I use to take out vertical, but reduce tension if I am getting lateral.

Of course you should fire form the cases with the bullets jammed, so that when you reload and if you have minimised run out of the projectile it will actually mean something. Otherwise your reloaded round will sit in the chamber at 6 o’clock and be misaligned anyway. I like to neck size the fire formed case and bump the shoulder .005”, and unless you have custom full length dies suited to your chamber alignment problems will always be present.

With regard to tuners, only a fundamental weight determined by adding weight will broaden that node, but a light variable tuner will be sufficient to follow that node and not necessarily broaden or dampen vibrations just like you were adding or subtracting powder charge for bullet exit timing. The benefits of a heavy weight with a variable attachment are better still but that may induce weight problems and bag handling problems. Variable tuners enable you to change barrel lift characteristics on the spot. Bench rest shooters can change load as the go and use multiple sighters, we do not have that luxury. If your rifle is a hummer after initial load development, use it without a tuner because mastering a tuner needs to be methodically determined as well. David.

[ecomeat]

Williada (David....and anyone else who will comment !)
When your refer to "nodes" in relation to tuning a rifle/load, for the sort of accuracy that we need to be competitive in F Class, I am hoping you could clarify a couple of things. I am also hoping that this post isn't seen as hijacking a very interesting thread. I have been shooting in the bush my whole life, but only discovered F Class a bit over three years ago, and have much to learn.
Question : do you basically agree with Chris Long's Optimal Barrel Time (OBT) theory ?
There seems to be a very tight relationship between his OBT paper, The RSI Pressure Traces, Dan Newberry's OCW , and QUICKLOAD software.
Presumably "accuracy nodes" and harmonic nodes (that produce accuracy) are basically the same thing, on lay mans terms.....they are the final load parameters...the sum of all things...that produce a very accurate, reliable load.
Please don't get me wrong...I fully appreciate that the is a hell of a lot more to perfecting a load, than just "speed in fps". I figure I will still be learning about the intricacies of all the other factors when I die of old age, so I am hoping that some of the really technical minds on this forum keep sharing their knowledge !
I bought the QUICKLOAD software, and it arrived a few weeks ago......so needless to say, there have been a few hours spent learning about it, playing with it and tweaking it.
However, something that it highlights has basically got me completely baffled !
Entering the data for the 180 gr Berger 7mm projectiles , there basically isn't an OBT anywhere near the extremely common "speed node" of 2820-2840 fps. They are at 2720-2740 .......a widely accepted "lower speed node", and again at 2940-2960 fps ....again (it seems) a widely accepted speed node.
But nothing any closer than those two "speeds"to that with 28-34" barrels. So the immensely popular muzzle velocity of around 2820-2840 fps is simply nowhere near an OBT, using 7mm Berger 180gr projectiles.
My twisted logic tells me that the has to be "something in it", when the OBTs tie in perfectly with the lower and higher nodes. So I just can't fathom how it can possibly be the case that 2820-2840 fps is literally in the middle of nowhere, as far as OBTs go.
If I had to take a guess, I reckon maybe 60% or even more, of all the 7mm loads/data that I have ever seen on accuracy Forums online, are somewhere very close to that 2820 fps. Yet that is literally nowhere near an OBT node.
Could you please comment ?
Tony Berry

[williada]

Ecomeat, you are not hijacking the thread and aspects of OCW, OBT and Pressure Trace form part of the commentary I have used in a number of threads although I have not spelled it out in so many words. Yes I do think they are relevant and but they are not the whole picture as I will endeavour to explain. Like anyone, we build on someone else’s work and sometimes we find new things to add to the sum of knowledge to give it a new name. So here goes.

The notion of “Pet Loads” was first brought to my attention by Ken Waters, the author of the book that is a quick reference bible. This has been used for years by many, and I view a pet load as a shortcut to load development in the same way Long et al are using the term in acoustic shock wave theory with the use of technology such as Pressure Trace and Ballistic software such as Quickload to come up with a starting point for load development or pet loads. It is a starting point because all barrels are not the same because of exterior profile, internal dimensions, length and metal used. The starting point is valid because it is a statistical approach but remember that it is all about averages, standard deviation, range etc. and your position in the analysis can be different.

Acoustic shock wave theory basically says many vibrations are excited from the breach to the muzzle and move back and forth even before the bullet exits. What is critical to this theory is that these vibrations alter the bore size. So if the bullet exits when the bore size is distorted at the muzzle, groups open up. It may be oval as I have mentioned this before, in a throw away line. In this case gas leakage will yaw the bullet like a bad crown. Also, that a micro bend in the barrel due to vibration could point the exit of the bullet in another direction. This part fits well with node theory and sine waves and micro trends but does not necessarily explain the macro trend of a barrel for compensation purposes in terms of total barrel lift.

However, the OBT theory does explain why those tiny, tiny changes in powder or free flight change group size and position. It explains why some very minute changes on a tuner can have a dramatic result. It explains why low extreme spread is not the holy grail as Tim’s groups demonstrate. Why, because returning vibrations interfere with outgoing vibrations and the bore size is varying and the bullet is caught in the middle. It is very hard to quantify these interruptions because each barrel is different. This is why powder changes or free flight changes are not necessarily seen to have a regular group size pattern. I think the fact that peaks and troughs seem rhythmic with or without extreme spread changes is that the barrel acts like a gut and can flex to allow the general flow of the projectile forward to be relatively unimpeded but because of the pressures involved it only takes a small flick or small blockage or a different path to affect dispersion. Even a low extreme spread is not sufficient to counter the vibrational impacts as seen in Tim's groups. Those that attended the Bendigo presentation for the Australian Palma Team early 2000’s will remember the slide show I gave showing the barrel distortion and particularly the one induced by rifling torque that Varment Al use to have on his site. OBT recognises 7 vibration impacts and I read somewhere years ago there were at least nine. It could be visualized that the OBT tune has the barrel in its least flexed state as the bullet exits the muzzle. OCW theory applies OBT.

The important thing about OCW is the most forgiving group is selected in amongst groups side by side about the same elevation. Advice I gave to Tim. So yes, I use the acoustic principles, but I don't consider them to be a node because the centre group selected may not be the tightest group. It needs to be fine tuned in small increments. This OCW selection is said to cope better with environmental changes during a shoot where sometimes nodal tuners slip off the node. It does demonstrate that short range testing ain’t the bigger picture at a further target distance if the atmosphere changes if you are a nodal tuner. You have to test this with quite a few rounds to confirm it as the OCW as it only uses a few shots with each varying round charge. Just like other early diagnostic tests. It can happen that OCW is the node and more than likely it shows up as a neutral barrel profile in terms of compensation where slow shots present at 6 o'clock at long range and more often due to crook primers. So in this situation getting your extreme spread down does matter.

I used a Pressure Trace in load development back in the 2000’s, along with a Border pressure meter Corby lent me. It was beneficial in that it showed me the shape of the pressure curve and when I was getting a complete burn. I also linked this trailing end of the curve to fouling towards the muzzle we often get. It also demonstrated to me that a case full of slow burning powder was the way to go with slow burning powder for an efficient and complete burn. But I found faster burning powders were less sensitive and did not need to be a full case to get an efficient burn in .308. I do think this warrants more exploration with fclass where smaller cases can be used in a better match of components.

I do not have a copy of Quick load. But perhaps I can generalize. Firstly, believe only what is happening on the paper, as your gear and your mass will determine how it groups. Harold Vaughn from a vibration perspective examined the symmetrical aspects of the action and the effects that had on groups. To give one example, he drilled a corresponding hole to the blowoff hole and noted the differences. We know our scope size and position all effects the reflection pattern in the vibration and the macro barrel lift. These patterns integrate with OBT.

The thing nodal tuning, OBT, OCW etc. do not take into account is the macro barrel lift characteristics at longer ranges that determine compensation through the effect of gravity. Computer programs often give a simulated spread in the vertical with different velocities as I have demonstrated earlier. But they are virtually meaningless as I demonstrated earlier too, with tiny groups, with huge extreme spread that were compensation tuned to the positive. Our game is long range shooting, not 100 yards and subtle variances are magnified but can be turned to advantage.

The other thing, I would point out, is that the drag characteristics of a bullet perform best at a given speed in a given atmosphere. So the role of external ballistics may have more answers than internal ballistics as to why people are using different velocities than Quickload may suggest. Others might know.

There is still a lot to learn. David.
Edit: Just a bit of tidying.

[williada]

I have had another look at Tim's groups and re-read early comments he made and been in contact with him since. If he was travelling about 2850 fps that would place him about the trough node in String 2. In this section of the macro barrel compensation profile it is pretty neutral even though it is a micro trough. I understand Tim lost 3 shots in the NQ Queens at 900m that were low, having completed two 60's in the second stage. As such, I am grateful to Tim for sharing this information and Ecomeat for allowing me to comment on OBT and OCW tuning. The problem I have is giving information overload and writing too much. But hopefully I can bring people along by topping up their knowledge bit by bit. So Tim's results fit the discussion beautifully.

The danger with a neutral barrel, as I have highlighted before is at the longs, with slow shots. It is often linked to primer selection which in the dramatic can be fizzers. In Tim's case he needed more charge to get into the overall positive range his barrel profile exhibited, not that he needed more charge to get to 900m. To handle a neutral barrel at the longs it is best to have a higher group to handle any slow shots rather than try and centre it. So recognising what your barrel profile is, is the key factor in the management of shots. Managing extreme spread does count with a neutral barrel at the longs and not with a positive one, in a general sense.

OBT theory demonstrates that only small changes in powder or free flight can have a dramatic effect in the fine tuning about a node. These changes are not linear. As others have experienced, the most you can expect from environmental changes is perhaps .6 of a grain in a calibre like .308 but most changes are in the order of .2 or .3 of a grain about the node. So its important to integrate OCW thinking about a relative flat spot in elevation of test groups about minor peaks and troughs. In fact ladder testing finds those flat spots, I happen to like the visual effect of the horizontal plot to examine both macro and micro trends. However the danger with OCW or ladder testing is you will select a neutral profile which is fantastic until you get to the championship distance.


Where some of the lessons from OCW come in, is where you try to select a node and a peak for the longs i.e. is where the groups either side of the micro node are pretty close in elevation terms. In Tim's case groups from string 8 to 10 are the best place to be overall. Groups through strings 11 to 12 are opening up and this is typical of the slow burning powders burning too far up the barrel with greater charges and disturbing the muzzle vibrations. Slow burning powders can respond to full capacity for a complete burn. I understand Tim had more capacity but I don't think in his case his sensitive barrel would have brought the dispersion in that much.

I have also said before, a trough node can give outstanding results if conditions don't change because it takes more energy to lift the barrel. Its a bit like drop-offs catch you in the wind. They happen quickly due to lack of wind energy to hold them up against gravity and it takes more energy to lift the flag or dwell time to respond and shooters can see this build up more easily, as the energy overcomes the gravity. So the peak groups are not as tight because gravity strings them more. This is what you see in short range testing and you often go for the tightest nodal group. But don't be fooled, at the longs gravity pulls those slow shots into the group with a slightly positive barrel because the have been tossed up by the rising barrel. OCW also is not fooled as the authors select the middle group on an elevation string which is not necessarily the tightest group, but is the most accommodating group for environmental changes.

Like OCW principles often the middle wave of the macro trend can show better results as Tim's groups clearly demonstrate. We can often see an energy overlap by plotting the velocity against the string as Tim has in his spread sheet graph. It is denoted by that flat spot in the line. That is something ladder testers also look for. That is where I would test further and expect a peak to be with fine tuning, because I have slight positive compensation in mind.

Because Tim's barrel is sensitive, I would pick, just before the peak micro node for positive compensation which would allow us to hit the node as the day warms up and generates more velocity and not have low shots in the cooler morning because the positive compensation would lift the slow shots. This area may not be as tight as the trough at string 9 which rises to minor positive compensation in string 10. Also a good spot. But after string 10, you are quickly into strings 11 and 12 with vertical and horizontal. This means you have only one string of safety at the trough node at String 9. I repeat that with a trough node you have to do the opposite to a peak node and move your group slightly to the right so any slow shots will move into positive territory. Otherwise you will mimic a neutral barrel, assuming the trough node is the low baseline and will not go lower, but at long range slow shots do. Its all very testing for the brain.

Finally, in Tim's case, the least variable area in elevation terms is between strings just before string 8 and to string 10. This is his widest window in elevation terms.

I would close by saying, an OCW (Optimal Charge Weight test) result would be a great all-rounder as it is not subject to minor environmental changes but subject to long range drop out with slow shots because the barrel mimics a neutral compensation tune. A fundamental weight tuner attached to the barrel mimics the OCW tune. A nodal tune without weight on the muzzle in the form of a barrel tuner or a ladder test will be great if you can stay on the node but is subject to failure with environmental changes unless you can use a variable tuner attached to the muzzle or have other rounds with different charge weights prepared for the condition changes. A slightly positive tune can be marginal, but manageable at 300 but be great over all ranges and if selected before a node on a peak or after a node on a trough will cope with environmental changes and prevent those vertical groups destroying scores at all ranges and particularly at the longs. Of course a variable tuner with a fundamental weight is beneficial to all types of tune if the mounds vary the bore angle to the target. David.

Edit: Judging by some of the angular spreads throughout the macro trend in Tim's groups, I don't think the curvature of the barrel is set in the vertical plane.