Neutral Compensating 6 x 47 Broughton barrel

[ecomeat]

As David (Williada) noted in his last response today to Denis Aarons, we have been liasing about a barrel of mine, with the concept being that the information will be shared widely amongst F Class shooters by discussing the concepts and wonderful detailed responses from David, to the benefit of all in our sport.
Some of you probably remember my posts a while back about a red hot Broughton 6mm barrel that i killed by inadvertantly (read "STUPIDLY" ) introducing some Redding Application Media.....the ceramic balls with graphite...and doing this to the bore with about 80 shots only


Before i killed it, that barrel shot this 3 shot group at 300 yds with shots 35-37


After killing it, I concentrated on my 284 barrels for a while, but finally got another Broughton barrel off Phil Jones, chambered in 6 x 47 Lapua. It shot quite well when i used it, until i finally ran out of 105gr Berger VLDs, and with 1000 105gr Hybrids on the shelf it was time to get a load working with them.
This was about 6 weeks ago, at the same time as i got the QUICKLOAD software.
QL showed me an Optimal Barrel Time (OBT) node using N150 powder (i had an unopened 1 kg tub) that was going to be 100% of powder burnt on bullet exit, so i decided to go with that.
That could well have been my first really bad decision, coz in hindsight my very first F Open rifle 3 years ago only ever shot AR2209 and i did enough with it to trouble some 7mms at OPMs. Hindsight is wonderful !!
Anyway....I just couldnt get the damn thing to shoot a decent group at all with the N150 powder. Being Hybrids, I tried jumping everything from 005", 010", 015", 020", 025", 030", 040", 060" ....all at about 3100 fps and couldnt get it to shoot. Tried jamming 008-010" and it started to look OK.
I then went back to AR2209, and shot some groups, basically trying to find a node around a Quickload OBT node, and then sent the results through to David. I didnt jam this straight online becmause i was forced to deal with some pretty annoying wind changes while shooting, which to me would mean that the "groups" themselves were totally compromised. I was just trying to keep the groups inside their targets, dealing with a really choppy wind, changing quickly from 10 oclock to 3 oclock, and up to 6 mph. I was simply hoping that David could see some sort of trend by assessing the vertical ONLY


It was shot at 300 yards, genuinely “Round Robin” with a minimum of 45 secs between shots, with velocities written in the TOP LEFT of each target area, and the AVERAGE of the drop from the point-of-aim, in decimal INCHES recorded in the bottom right of each target square.
I shot them in the order : 38.5gr, 39.0gr, 39.5gr, 40.0gr, 40.4gr, 40.8gr, 41.1gr, 41.42gr, 41.66gr, and finally 41.22 gr.
The “round robin” firing order was maintained in the same order, for all 3 sequences, resulting in 10 x 3 shot groups.
I should explain that the 41.22 gr was added last, after I had already prepared the target for use, because a triple check of some bullet data averages for Quickload dropped the faster OBT from 41.42gr down to 41.22gr
Hopefully David can/will come in now, and post his handiwork for us to continue to learn

[Cameron Mc]

Some of you probably remember my posts a while back about a red hot Broughton 6mm barrel that i killed by inadvertantly (read "STUPIDLY" ) introducing some Redding Application Media.....the ceramic balls with graphite...and doing this to the bore with about 80 shots only

Tony, we are blessed having you on board to teach us the do's and don'ts. First hand experience is invaluable.
Keep us updated with anything that will be relevant to keeping our gear in good shape.

Sorry
Cam
ps You could write a book one day

[DaveMc]

Tony,
A couple of things come to mind when doing these. Firstly, it is very handy and much more powerful if you can tell us which shot corresponds with which velocity. This is simply done by plotting impacts as you go. Then we can enter individual shot heights against individual velocities. This gives far more statistical power than averages of 3 shot groups and truer patterns will emerge. Average heights of 3 shot groups when the sd of either velocity or vertical height is large have very little 'power".
If you give us the individual velocities vs individual height figures I can give you a much better compensation picture all the way from 300-1000. I was going to do it with a few of Davids earlier pictures but the power of the mathematics is lost without the individual entries.

Secondly, (and you may or may not have done this I cannot tell from the information) with a fishtailing wind, if you were holding or winding then the results are totally invalid. Aerodynamic jump (the 10:00 to 4:00 slope) can be much more dramatic at closer ranges and if you hold to form groups rather than one center aim then you could be holding left and being driven low and then holding right and driven high - or other wind/atmosphere related vertical issues. It is very important to hold the same aimpoint on these tests. -Ideally done in pristine sighting and wind conditions but that cannot always be done but If not it is essential you hold the same aimpoint and keep a steady shot timing done in round robin style as you have done. Then the slope of the line can be taken into account.

[williada]

Guys I have a bit on in the next few days so I will come back to Tony's comments in more detail later and in the meantime I am interested in receiving his latest plots to sort out best he can, a pill that might not maximise his barrel's potential.

Dave, all those things you mentioned about velocities and identifying each shot with that velocity and distance to minimise coning effects were passed onto Tony which he quickly adapted to in subsequent plots he sent. Timing is everything, and you can't always get the conditions you want when you have timelines for testing. So the best thing you can do is make approximations. We still got something from those plots even though they were magnified by the conditions.

What I would point out is that it is impossible to recover group size if the angle of departure varies for each shot. The only exception is through positive compensation where gravity pulls that slow shot down into faster shots when tuning the barrel to pitch shots with an angle of departure roughly in the vertical.

With positive compensation, the best group will be distance specific. The spread of the shots for a distance can still be very accurate up to that distance with a minor positive compensation. However, determining the range of that spread depends on what part of the rising trend line you select before the peak node on an upward trend line or after the trough node where the upward trends starts again.

All the vagaries of manipulating the angle of your spread and its general group size come into play as well as picking a spot in the barrel that throws groups with the least environmental sensitivity. Positive compensation will buffer environmental changes as will OCW tune and you can combine both.

However positive compensation is not absolute, in that it can change in more severe environmental circumstances and if the launch angle changes due to mound angle/target angle or pilot error and more so the fulcrum point on the butt. Only a variable tuner can assist you quickly on the spot unless you have different rounds prepared. However you need to learn how to use a tuner properly.

To maximise its effects you still have to select the best primer, and eliminate any ignition problems associated with springs etc. You have to select the best, bullet and powder combination for the barrel by looking at group size in the nodal tune and you have to reduce extreme spreads of velocity when determining that point off the node. On the node, extreme spread variance does not matter that much. The later relates to OCW tune.

If you have a properly tuned rifle and understand its grouping pattern, then it helps you learn and judge conditions and you gain confidence in that you don't even think about pilot error because you are automatically pulling a perfect shot because you are constantly rehearsing it and not doubting yourself.

David.

[ecomeat]

David looked at my 300 yard groups, and did his best to convert my results into something of use :

His reworking of my results into a type of graph, with a red colored Sine Wave showing him an overall trend was accompanied by his comments, which i will wait for David to add himself. You can see the area that he wants me to work with highlited by the neon green square.
in the interim, i had worked out what the average "distance below the Point of Aim" of each plot, and lacking the skills to do it in Excel, had simply drawn a trend graph by hand :

The flat line starting at 41.22 gr was an OBT "node" from Quickload, which as you will see later gave me one of Williada's "Teasers" that he talks about.

By now, I fully understood the need to have :
1) Five (5) shot groups for maximum data. 3 is better than nothing, but 5 is best.
2) Plotted all shots
3) Velocity for all shots
4) Equal increments in powder charge. My failure to do this meant Williada had to approximate the two "out of whack" powder charges, below where they should have been fitted, and as you can see they basically fit his red Sine wave perfectly anyway.
5) Holding a consistent point of aim

[williada]

The information below comprises some of the notes regarding areas to test and also a further in depth explanation as to how I went about it. Due to my own time constraints I cannot edit it sufficiently. Apologies.

I have tried to lay major spreads out in approximately .4 grain which is probably the right ratio for the incremental charge, broad picture for the 6x47. Also, I note your minor spreads in .2 grain, and if you moved the diagrams of 41.1 and 41.42 up they would roughly follow the red sine wave. I have re-arranged your plots as follows: the blue line represents the highest and lowest shot outliers to form a channel. Then I check the trends in this channel for barrel lift considerations and micro peaks and troughs to fine tune about. I use groups which form in the upper part of the channel in preference, although groups that form troughs on the sine wave about the middle of the channel can be excellent.

In the case of this barrel, I am using the red sine wave as it appears to be fairly predictable in the sine wave pattern and is the outlier that would do you with your barrel. Normally, I would look at the group as a whole. Do check that your barrel is drawn up firmly.

What I do, is mark on a plot sheet the speed of each projectile, and transfer that again to the actual card I shoot, for future reference. It’s a bit fiddly, but I like to know what speed each individual shot is doing for a more informed interpretation. What I look for is, when that slow shot turns over, is left or right high or low. I also try to find an energy overlay where the broader incremental powder velocities merge. This is a bigger window to operate in and saves fiddling with a tuner or charge.
Your barrel, going by my spread sheet graph of your average velocities, shows this energy overlay in a flatter section which is between 40 and 40.5 grains although it is still pretty steep. I would expect a peak somewhere here that would give you your best energy overlap or be the most tolerant of charge variance and therefore environmental tolerance. However, you need to consider which groups, side by side, (these satisfy OBT and OCW principles) are the most tolerant in elevation terms. Also look for a slow shot being tossed higher than the fast shot. Your final tune should then sit just left of the positive peak to capture both positive compensation and the nodal tune as the velocities increase as the day warms.

This barrel seems very twitchy with this projectile, given that your groups show that as little as .2 of a grain will tip you off a node one way, then the other, in the second half of your barrel’s lift profile. I would expect that it would be extremely difficult to hold a group in changing air densities as I only adjust powder charge or the equivalent on the tuner in .2 or .3 of a grain to accommodate air density changes. While 41.22 demonstrates a node as a triangular group, is about a relatively neutral barrel lift section, and it lies about the upper middle of the channel; the fact that it is a trough, it should be a bit tighter. Also, its mates either side, trend in opposing directions on the sine wave. It’s probably a teaser and I would think there are better loads to use.

The barrel also shows a propensity to gravitate to the lateral even though you indicated a variable wind direction. It may mean, that the wide shot in 40.4 had a little added to it, it may not. I would advise that fine tuning, (in an area I will suggest shortly), that that your neck tension be explored which can often remedy this pattern or you may need to change projectile powder or both. If you think you are altering for wind in a test, it’s most important to test at a shorter distance of about 130-140 yards and not alter. If the groups are showing a particularly rhythmic pattern, then we can discount the wind factor even if groups are enlarged with the groups you have supplied. I use 300 yards or 500 yards to make fine adjustments in elevation terms having established my fundamental load up short to form round or triangular groups.

The area I would explore is indicated by the yellow rectangle which ties in pretty well with the flattest section in my spread sheet graph. I would like to see you retest with loads 40.2, 40.4, 40.6 and 40.8 using your same jam settings. If you are getting lateral dispersion, run the same test again with different neck tensions as you have indicated free flight adjustments have not helped. If a lateral problem still persists, your barrel suggests it likes fine changes. Your previous free flight tests may be too large an incremental change. So you may need to test about your current jam settings in .2 thou increments but only for a total range of 6 thou. i.e. two groups ( -.2 thou and -.4 thou from your current setting), one at you current setting, and 2 groups after your current setting (with .2 thou differences).

May I suggest you do five shot tests with fine tuning round Robins. This gives more data to work with. We can take it from there if you are still not satisfied, with armoury suggestions if you cannot find another projectile or powder or both.

More explanatory notes I sent to Tony,

Yes, the normal sine wave is a regular pattern of cyclic motion or oscillation travelling in one direction. This can be mapped point by point (if you know them) as you have done in a similar process with your data or it can be mapped by applying a formula which was originally based on a pattern of data but using a line of best fit. There is nothing wrong with using a pencil.

Your data set was complicated, so I had to adapt procedures based on experience. It was an interference pattern of different shock waves interacting. So in that sense it was not representative of the regular cyclic pattern traveling in one direction. Acoustic shock waves distort the traditional shape of the sine wave in a barrel i.e. the returning waves interfere with the outgoing waves. In addition, the bullet changes the tension in these waves by altering bore size, so changes their shape and harmonics. There are also three moments of inertia in the barrel when the bullet is fired i.e. in the chamber, along the barrel and by escaping gasses at the muzzle which can change the shape of vibrations amongst other things.

These waves clash to give harmonic frequencies or a standing wave. Your whole group harmonic frequency was composed of a fundamental frequency and higher harmonics. This was indicated generally by a couple of close shots and one tossed right out by the higher harmonics, which I had to tease out in the form of the red line. Probably a more accurate plot of your barrel fundamental frequency would have been in .2 grain increments. But would there be any barrel left after testing?
For a minimum statistical sample to be relevant, you need at least 30 data points. This would mean you would need to multiply each of your three shot groups by ten. So test results with say three shots with each sample are statistically weak but can point you in a direction to retest to verify results. Sometimes they do not verify results with more shots because the first sample proves to be statistically weak, so you explore another point. Therefore I have bastardized the definition of sine wave a bit by separating the cream from the milk and we homogenized it because statistically we did have not enough data and the data supplied varied in increment charge value.

The one shot regularly tossed right out, is the heart breaker when going for that perfect score, so I tried to identify its pattern as the red line because it was representative of a separate wave pattern. Call it a heartbreaker plot.

The first indication there was a pattern in such a short range of data to study, is where the rough shot touched down on the bottom of the channel twice. Stockbrokers use this technique in technical analysis and call it a double bottom. I then connected the rough shots between the touch downs with the rough shots on the inside of them and drew lines of best fit. Where these intersected I found the first peak of the rough shots. Although this pattern had insufficient data to show the peak it was an educated guess based on your .2 grain plots as opposed to the .4 to.5 grain plots that confirmed my thinking. Thereafter the peaks and troughs for the balance of the red line were easy to find following the rough shots. Again your .2 grain plots fell beautifully on this red line to the right hand side to confirm this.

My first impression of these groups was the barrel joint could be moving, so stringing shots. Now you mention you change up barrels which may have insufficient tension on the threads and so may give that finicky experience. The issue is not weakening threads etc, but holding that joint still as the barrel heats. The joint only has to have minute movement at times and spring back means its hard to detect, so in that sense the barrel does not come loose. If you change barrels regularly it would be a good idea to blue the action face and see if the barrel shoulder has an even ring around it. This is a separate problem but can be similar in effect. Mate, I could write another book on switch barrels. Mostly negative because it leads to altering the position of barrel curvature where you think you lock it up and changing the harmonics for extreme accuracy.

[ecomeat]

Davids comments have since proven to be spookily accurate, about his recommended area to explore, and about the "worthiness" of my Quickload OBT node at 41.22 gr . As Dave Mac noted, because i was holding off and twisting knobs coping with a bloody awful wind, i kept thinking that there had to be every chance that my data was virtually irrelevant.......but not so !
David directed me towards 40.2gr - 40.8 gr to explore, but i still liked the flat line on my graph, from 41.22gr--41.66gr
We were shooting at 400 yds at Beaudesert the next weekend, and i wanted to satisfy myself that my Quickload OBT load (41.22 gr) was not the best one to pursue...so that is what i loaded. I planned to do the 5 shot groups that David requested a few days later.
My first string encountered some pretty wild winds, typical for Beaudesert when its hot and dry in October, and only just held X Ring elevation.
My second string was 65-7/66 and finished with 7 X's. I have been struggling to get any decent "form" back since breaking my leg in early June, and only getting back shooting a month or so before, so i was pretty pleased with myself, and this OBT node load

MPI
Diameter X Y
7.2mm -1.6mm -0.9mm
The point i dropped was just a shitty execution of a shot, so i had to own up to that after initially telling David that i missed a drop off (I finished with 1.75moa of wind on)
After sharing the plot above with David, he pointed out that it falls into the predicted area of "trough nodes"........potentially one of those "Heartbreakers" that he has posted about.

The next Friday, Craig McGowan (Mr 7mm.com) invited me to come and have a play at Belmont at 8 oclock in the morning, so i gladly accepted the kind offer, as David was waiting patiently for my 5 shot groups.
Mirage was surprisingly bad, but i had driven for over an hour to get there, so with Craig plotting for me we achieved the following at 100 Metres. I fired 5 shots @ 40.0 gr to warm and foul the barrel, which werent plotted, and then shot Round Robin , waiting a minimum 45 secs between shots to finish with 4 x 5 shot groups


David will share his comments on these groups in a couple of days, when he completes some urgent farm work that needs his attention

At Beaudesert on Sunday we were shooting at 500 yds, so I shot the first string in the middle of "David's Zone", with a load of 40.4 gr of AR2209, and shot the second string of 11 shots with "my" OBT zone load of 41.3 gr AR2209.
It was very warm, plenty of mirage and washing machine winds, coming from all directions and really rapid changes. Ignore the scores, and just look at the Vertical that resulted
Firstly the 40.4gr load, in the middle of Davids recommended zone and ten shots


Then the OBT zone load, 41.3 gr, which David had predicted could be extremely "environmentally sensitive"
.
I think he was correct !
It seems that i might have a great 300-400 mtr load.....provided the weather is mild enough...and still plenty of work to do if i want to shoot this sweet little thing any further out than that !!
David will comment again after he finishes his farm work, and in the meantime, I will follow his directions closely and record a number of 5 shot groups,at 0.1gr increments in AR2209, as well as trialling 001' more neck tension, and 001" less.
I just have to get some 7mm loads ready for North Arm first.

[plumbs7]

Wow! I know nothing !!thanks David for the info! I'll keep trying to get my head around it!

And also thanks Tony . Great to see u shooting again ! Regards Graham.

[williada]

Comments relating to wide shot to Tony

However, the shot you say was lost to a drop off, I feel was more related to atmospheric density changes which may have been sufficient to push you back to group patterns to the left i.e. to that trough node which has a bit of vertical or a tad further back which also has lateral. This is on the cards given that you said mirage was bad and conditions were steamy and you had to wind your scope back in magnification. Because your adjustments are so fine, under .2 of a grain, there is the possibility that powder variance accounted for the shot unless you use those scales Barry Davies uses that can weigh down to the last kernel.

The other evidence is that with a drop off, having left wind bias, the shot if anything would go higher. It only goes lower in a stronger left wind. Also the induced vertical is marginal at 400, but the shot still went relatively low and left. If the wind dropped off, the mirage would have slowed, and also would have a tendency to raise your shot particularly in steamy conditions. So Tony, on balance a beautiful group may have been stuffed by the environmental conditions. This spot is worth visiting again in stable conditions because it falls in the vertical range of other loads I suggested you try in that yellow rectangle on your test plots, which may not be as tight but will likely score better in worse conditions and be more suited to long range.

Comments relating to Belmont shoot of Tony

Firstly, I must congratulate you on following procedures. The reason is that we are trying to see how your rifle performs in a worse case scenario. Both the time interval and the round robin process facilitate this. We can see the effects of barrel heat and fouling. Also the short distance takes out the wind factor and the elevation is only tickled up at 300 or 500.
You must continue with 5 shot groups. Your earlier 3 shot groups were an indicator, and so far the diagnosis has been correct. As you can see, the three shot groups can mislead you as to the size and direction of your group.
With the compensation tune we are aiming to handle a worse case spread with tight group size and load 40.2 is demonstrating this and the real peak is at 40.4 where I thought it was. However that lateral is present in your gear and your next step is to play with neck tension to reduce lateral.

Seeing you use those fantastic scales, I suggest you introduce 40.3. This may introduce some minor positive compensation and give you some leeway between the groups with potential. Your rifle is pretty neutral but those minor slow shots can still be rising at short range and can be evaluated at 500. Impressive tight spread on the reloads. So run with 40.2, 40.3 and 40.4 with five shot groups.

You would also notice the different environmental conditions have on load velocities between venues. It is vital to keep records so you know what the powder changes will be needed as the environment changes. If you are brave enough in the future you can add a light tuner to mirror those changes in both temperature and humidity just like for powder.
Also apart from comparing plots, you could develop a master scatter diagram where you can overlay on top of each other, each practice group at a set distance. This will reveal over time (with many shots), what your barrel is doing, perhaps when it will do it (with more adaptions) and your technique faults.

Additional comment
Now that you think you have got it alright, we need to calculate the vertical trajectories of each shot to see if it an across the course load i.e. is it still manageable at three hundred and hold beautiful elevation out to 1000 yards with minor positive compensation. Then suck it and see. Your goal is to keep that angular elevation low still with minimum extreme spread for that minor positive compensation because you are working with very heavy barrels in F/Open where the micro pulses are smaller. Your goal is not to be range specific with the compensation but appreciate its subtle advantages across the course.

[plumbs7]

Hi David , with vertical and experimenting with neck tension , would u go tighter first or less tighter neck tension? (Really enjoying this thread , I couldn't get my head around the other thread on positive compensation and just gave up!) regards Graham.

[williada]

Graham, generally vertical requires tighter neck tension and the opposite to lateral. The tighter neck tension can give a complete and faster burn a bit closer to the breech and increase velocity as the pressure curve is altered. Like increased velocity generally reduces vertical with increased powder charge. That assumes of course there are no other factors in the mix causing the vertical in which case changing the neck tension may or may not be seen to do anything. Poor primers, bedding issues, bolt alignment, poor barrel joint or concentricity or barrel wall thickness issues are examples which come to mind. Its easier to control the vertical if the barrel is indexed properly. Cheers, David.

[williada]

Graham,

I omitted to say what I would do first because I don't have a lot of time at the moment. In the context of Tony's group and mostly when we narrow the search for the most productive positive peak pretty close to a node we are trying to reduce the lateral because we have most of the vertical performing well.

In Tony's case we have still have three loads to test, with very minor charge variance which I well extrapolate the trajectories over all ranges when I get time, to see which is the best across the course load. This is also a good time to load with changing neck tensions, one with greater than the normal neck tension, and one with lighter neck tension in the batches to test.

It will also confirm any physical faults with the gear that may be influencing the lateral I have in my mind. Of course the size of the velocity spread and group dispersion will confirm QA (quality Assurance) or the integrity of the components used. Let there be no mistake, the goal is still to reduce extreme spread, because we are picking a group that is just off the node and the barrel has a macro neutral trend. But we have a bit of insurance on the node where the extreme spread is not as critical.

Hope this helps. David.