Interpreting What You See From Testing

[williada]

Interpreting what you see from testing.

The 10 shot groups are not tight because they came from the machine rest. What is important is they are relative, because all groups are examined in the same setup, with the same shot interval of 45 seconds, with the barrel cleaned the same way before each test with 4 warm ups preceding each test.

Before any compensation test, you have to sort out the best primer powder combination because you don’t want ignition variances skewing your results. In other words, you have to run your tests with your best reloads; and your best armouring setup that is within specifications.

Below is a primer test. The powder was 2206. From left to right was Remington primer, CCI Benchrest and Federal Gold Medal. This not to say, that the same primers in a different barrel would not have performed differently.

The prerequisites for this testing are the correct firing pin protrusion (0.045”- 0.055”); as is your spring force (about 28 lb); and if any bolt that uses an Omark bolt or derivation must have that spring lubricated because the spring rubs on the internal surface of the bolt. In my old equipment I always polished the internal surface of the bolt to make it slick. Variable ignition which strings shots vertically is often confused with technique or being off the node due to conditions when it is not the case, although some with tuners can reduce the height of the string but it is nowhere near the potential the rifle can achieve. If you are dry firing, use a snap cap to protect the pin.

The group on the right, looks fairly round and almost triangular. Fantastic, it looks like a large nodal tune pattern.



But further testing will reveal how you can be sucked in.

The group below looks like a nice nodal tune and the primers are sorted.




However, the high velocity shots are forming at the top, the mid velocity shots are lateral and the low velocity shots are low. This is a negatively compensating barrel. It will still be good up to the distances indicated in another graph which is following and you can interpret. I would not use this barrel for the longs. (Note my assistant marked the velocities on the target after each shot at 100 yards).

In contrast we can see a barrel that is positively tuned.


Note the extreme spread, at 65 fps. You have to accept, I know the slow one went high. I used 10shots here at 100 yards, and tested it at 1000 yards anyway which supported the earlier test. Could I also say, that if you have a variable tuner, a bit of that lateral can be tuned out. At worst it will be a bit more positive. Go to 300 yards and test the group shape by winding that tuner in 1/3 turns effectively depending on its thread pitch (between 3-4 thou). In contrast, with a neutral barrel, some slow shots can be high or low, but well within acceptance about the centre of the group. If one is really low in velocity, it can fall at 6 o’clock.

A point with the variable tuner. If you are between nodes, you can move forwards or back and you will find a node. (I always prefer that that upper node anyway in powder testing which is not discussed). My view is based on leverage. You have more force if you have a long lever. So I tend to move the variable tuner out towards the muzzle if I get vertical, and in if I get lateral. In the case of the positive group above, I would cover that small lateral by moving the tuner in (about a third of a turn back from the muzzle) in the first instance. Edit: The opposite is the case in a negative barrel. If the elevation spread is dropping you out lower, then wind the tuner in. If getting lateral on a negative barrel wind it out. The characteristic of your barrel is a greater indicator of where you expect to lose shots. Out he top on a positive, out the bottom on a negative, and sometimes at long range out the bottom with a neutral with a low velocity shot.

Of course the old adage with a vertical spread was to add more powder. Of course you can adjust the free flight to compliment these issues. But I would go as far to say you must know which barrel lift characteristic displays to make the most of your reloading techniques. If your reloading techniques need refining, do it one variable at a time.

You test at 500 yards, for the general characteristic of compensation rather than trying to change the shape of the group or pattern which I do at 300 yards then at the distance. It’s usually sorted at 300 yards. The distance usually adjusted for mound angle which affects bore angle and compensation; and atmospheric conditions. Again testing at the required distance will only amount to very minor changes if it is set at the shorts.



The graph above, represents a computer mock-up of the angle of departure and two velocities which will indicate a positively tuned rifle for 1000 yards. If you look at the graph, you will see why it is important to test at 500 yards. Alan, this is why I said to you many moons ago to use this distance but I had not added the other reasons, and it is still valid for that nodal tune. The velocity variances are subtle with the slower bullet being tossed high. They are the red line. The black line is the faster bullet. I traced that on the diagram, because my old computer program would not overlay the trajectories. I printed them onto clear sheets, traced fast trajectory in fine black so you could see the overlay.

You can see why I advocate a short range rifle, a mid range rifle and a 1000 yard rifle from this mock-up. I also advocate a different twist rate for each of the sections of the trajectory because I know that staying on a nodal tune is velocity specific due to stability factors I explained on a previous post. You can see why 600 yards will cause problems in a Queens after a cracker start at the shorts.

I believe no one should fear competition, you should embrace it on a level playing field and that’s why I have shared this information to the best of my ability. David.

[aaronraad]

Great post David!

Keep going and we'll have to encourage Alan to set you up with your own sub-forum!

You can see why I advocate a short range rifle, a mid range rifle and a 1000 yard rifle from this mock-up. I also advocate a different twist rate for each of the sections of the trajectory because I know that staying on a nodal tune is velocity specific due to stability factors I explained on a previous post. You can see why 600 yards will cause problems in a Queens after a cracker start at the shorts.

Would there be any significant advantage then if a projectiles stability factor wasn't velocity specific, certainly over the supersonic range? How much MOA is it adding to the group size and how much is it displacing the group centre at it's extreme over the nominal 1000y?

[bartman007]

David, I subscribe to the idea that you need more than one setup to shoot from 300 to 1000 yards.

Tuning either by powder/seating depth combination or by using a tuner fixed to the barrel will give you good groups at various ranges. Do you think the following selection could allow for a single rifle to be shot accurately over all ranges from 300 to 1000 yards:

1. 7mm 284 chamber with 1:9 twist running 168 grain projectiles from 300 to 600 yards (running between 2900 and 3000 fps)
2. 7mm 284 chamber with 1:9 twist running 180 grain projectiles from 700 to 1000 yards (running between 2800 and 2925 fps)

Alternatively:

1. 6.5mm [260 / 6.5x55 / 6.5x47] chamber with 1:8 twist running 130 grain projectiles from 300 to 600 yards (running between 2900 and 3000 fps)
2. 6.5mm [260 / 6.5x55 / 6.5x47] chamber with 1:8 twist running 140 grain projectiles from 700 to 1000 yards (running between 2800 and 2925 fps)

Or you may see splitting the distance up into 3 distinct ranges, based on group size achieved with various projectile/charge combinations.

Is there merit in running the lighter projectiles at higher speeds to shoot the longs? After all, we see that the 6mm BR variants are super accurate out to 1000 yards! They just take more work to keep in the center.

As the above is clearly F Open class, in FTR class there are some shooters now using 155's for the short to mid ranges, and then turning to the 185's to run the longs with great results.

[macguru]

Great post, you are making us think about stuff I got myself pretty confused with a barrel tuner and went through alot of ammo ! I wish i had read this first...

Another (F open) approach is to get a cartridge / load that has a very small velocity spread due to efficient design and match the powder speed to ensure efficient burning. (eg a 6mm dasher with 2208) Then with a little tuning by powder load the rifle will shoot well at 100-600+ yards, more limited by wind that vertical spread. As range increases and the wind conditions worsen, you switch to your second heavier caliber (6.5mm/7mm/30) for the longs. It may not have the same super tight groups at 300, but will drop fewer shots at the longs due to crosswind even though the 6mm has a potentially higher x count in calm conditions (right out to 1000 on a good day)

[williada]

"Would there be any significant advantage then if a projectiles stability factor wasn't velocity specific, certainly over the supersonic range? How much MOA is it adding to the group size and how much is it displacing the group centre at it's extreme over the nominal 1000y?"

Aaron let me answer this through a conceptual approach, because if you get the idea or know what you are looking for, then firstly, the odd accidental finding will stick out like dog’s b...s.; secondly, most of the theory has been proved by experiment over the long distances in the past by ballisticians and simulation at a higher level than I have access to; and thirdly we are really dealing with simplified forces and their interaction for most of the models and so we are dealing with narrow ranges of application rather than absolute applications.

Therefore the overall thrust, if you keep it simple, is we can reduce the range of operation that makes it more relevant to the supercentre. By this I mean, we have the option of changing velocities for specific sections of a trajectory and using the most suited twist rate for that section of the trajectory under a given set of environmental conditions rather than on size fits all. One size fits all pretty well, if you take into account a tune for a small amount of compensation as DaveMc has found. But I am trying to get deeper into the mud. Just like G7 models can more ably predict trajectory for VLD’s etc over traditional models we can refine things more as our knowledge grows, but it depends on your view as to what you think is significant.
If you think relatively speaking, then a fullbore shooter, that has a primary goal of hitting 2 moa bull, then a half minute rifle would give a shooter 1 ½ minutes to play with in conditions. Something being velocity specific would not matter that much, to refine a group size. Given, I say the general fullbore technique has a one minute capable setup. They only have ½ a minute each side of the centre to play with. I would venture to say only the elite shooter can read wind in this sense to ½ a point anyway.

If you are talking about an F/standard setup you and your gear could expect a primary goal of hitting 1 minute, the centre bull and through extreme spread analysis and probability then you will get your fair share of super vees. Velocity spreads are becoming more relevant. You are becoming group size dependent for score and knowing the pattern your rifle produces to maximise score.

Now if you shoot F/Open, the reality is you want at least a ¼ minute rifle in grouping capability. You can achieve this because the rifle mass is greater and less likely to enlarge the group due to recoil and because you can use projectiles with high ballistic co-efficients which primarily allow you to buck the wind. But if you want these rifles to shoot in the teens (micro group) then those nuances of yaw and velocity will come into play. Knowing the effect of Magnus force now becomes relevant to maintaining elevation particularly in fish tails to maximise score.

As an aside, Rod D who is current VRA secretary, approached the Monash University Maths Department armed with statistics from the Queens to come up with a model for handicapping all shooters. At the same time, I came up with a model based on a rifle’s grouping ability. The outcome when we compared results was that the handicaps were almost identical. Now we have so much more data in over the few years, then a better model can be produced which should have a handicap factor that includes BC. No doubt Alan’s system is excellent and he led the way with this thinking. None of the models were perfect, only approximations and ease of use on the day has to be considered.

Like all approximations, BC is often quoted and used by shooters to discriminate as to choice of projectile. In reality, the BC changes with velocity. Sierra used to produce BC values for each 100 yard distance and on more advanced computer programs you could slot these values in to more ably calculate wind deflection and trajectory.

We know that we only want to pump velocity up to beat the transonic range and remain supersonic so the bullet is not subject to the chronic instability of crashing back through the sound barrier. But at long range we need a faster twist rate to stabilise it. The problem becomes if velocity is not fast enough a very fast twist rate has to be applied and so the bullet becomes over stabilised and so its tractability is affected. This means the bullet cannot follow its bending path because it lands like a jumbo jet, nose up ass down. This presents a drag problem particularly in headwinds and enlarges your group. The best way I can explain this, if you hold your hand out a moving car in the wind flow with your fingers pointing up and your hand palm down your hand will be blown off line.

Over-stabilized bullet

Source: How Bullets Fly.

So a high BC is not the answer unless the velocity range is maximised too.
At short range there is no need to launch a projectile at such a speed so as to beat the transonic range at 1000 yards. Often grouping ability is far better with a lesser charge and it is maximised by a spin rate suited to the distance. People can see the difference on the target with say ½ values e.g 7 1/2 versus 8 or 11 ½ versus 12.

The point here is, that the bullet is statically stable or dynamically stable with the matching twist rate for velocity in given conditions which enable the bullet to travel along its trajectory and minimise its drag. Like your hand outside the car window pointing into the wind flow it will not blow off line at an angle if its pointing in the direction of least resistance.
We accept that trajectory is determined by gravity, so the over tuning moment is probably a weak spot as the bullet makes its decent, because magus forces alter a bit. This is not a problem for short ranges, but for middle distance it becomes more so. If we want to move this overturning moment further forward in the trajectory we can go faster or use a bullet of less mass. All of this, involves trade-offs with drag factors such as the mach cone or pressure wave of the ogive, the lateral drag of the bullet body due to nicks etc and the parasitic drag which is minimised by the proper boat tail shape. The velocity affects the drag characteristics including the boundary layer or cushion around the bullet. So in a general way, the shape of the projectile affects its drag in its bending path but its the gyroscopic forces induced by twist gives its stability so it does not deviate from the path. The length of a bullet has a huge effect on the Magnus force (an aero dynamic force) with relation to the centre of pressure and its centre of gravity. So the suitable twist rate assuming the balance is correct will maximise stability. The general shape likes a set speed for drag and if you look at many different calibres this best speed is close to 3000 fps as I found with the old Sierra. This will vary due to atmospheric density.

Assuming all things are designed well, there are still nuances we have to deal with as the bullet slows. The transient yaw gets damped by the appropriate twist rate to give us equilibrium yaw and the bullet is said to go to sleep. When it slows further, the Magnus forces move forward and the normal precession of the bullet starts to enlarge the bullet yaw (yaw of repose) in the downward path of the trajectory. So the velocity enters the equation again and ultimately the bullet becomes unstable and your group opens. All the compensation concepts work on is the velocity/gravity factor in the equation and therefore is one small factor amongst many things in grouping ability on the day. Magnus forces also apply to spin drift, so at extreme range this will vary with atmospheric conditions like a swing bowler relies on it in cricket. This is a separate concept from bullet stability. We are in real trouble if someone comes up with reverse swing induced by the atmosphere on the spinning bullet. Am I really joking? Maybe if your spin rate is so slow, you need a bit to bite the atmosphere with in a super thin atmosphere which therefore demands a really fast twist rate.

In certain conditions air density which is determined by barometric pressure, heat and humidity can really confuse you. It is generally thought that heat is a big factor. What is counter intuitive is that in moist conditions the air is less dense and this can technically be better for rifles with a slower twist rate from a stability perspective i.e. suddenly a 1-8.5 performs better than a 1-8 which is a faster twist. In normal conditions the slower twists require a bit more velocity to stabilize them. All of these factors can be plugged into JBM Ballistics on the internet.

So in order to answer the question, it is impossible for a projectile not to be velocity specific, but within a given range of acceptable velocity for practical purposes maybe. I cannot give you details of displacement because there needs to be a specific bullet configuration, environment and barrel twist set to analyse, and I don’t have the resources at my disposal now to give absolute answers. But what I can tell you, is that it is important to keep records of your shoots in terms of air density, twist rates and velocities and observe what is happening to others. It is not uncommon at long range to barely hit the target sometimes due to these factors. You just need to be awakened to the possibilities in your group analysis.
Sorry Aaron, I don’t need another area on the site. Thanks for the compliment, but I’m losing a bit of sleep now. I am a slow typer. I often have to edit posts now for typo’s etc. When I re-read them later. David.

[aaronraad]

Thanks David, all sounds familiar.

I'll keep working towards reducing the Magnus forces. I'm planning on a +180gr 30cal prototype that will need a 1:10" twist barrel.

[williada]

Mike our posts crossed, so maybe there is something in the post that answers your question with regard lighter pills. The only downside with the lighter pills is that their BC's decay quickly and you have to note that they are often published with average BC's. However, they drop off quickly at the longs and skewed to the shorts in terms of BC's.

So in terms of McGuru's advice, the bigger pills with BC's are more acceptable because they hold a relatively higher BC with distance due to mass and are a better proposition because they will buck the wind better at the longs. Its good advice. They will remain in for score, not necessarily for group potential. The higher mass bullets crash trough aberrations in atmosphere caused by mounds interrupting laminar flow of wind as a frontal or tailwind fishtail relative to smaller calibers.

However, lookout for those guys who learn to read wind and apply Magnus forces to reading elevation changes with small calibres.

Somehow Murphy's law applies during a Queens and you get a strong fishtail bouncing your light pills more on steep mounds and reading Magnus means bugger all.

I will explain more when I catch up Mike. David.

[DaveMc]

David,
It is a great pity I have not got much time to put to this at the moment but will try and add a bit here and there (coffee break on early start now).

Within this topic "interpreting" the data we really need to try and show some real examples of how much this will effect results at various ranges. I might ask if you can put up the set of groups you posted in the other thread viewtopic.php?f=5&t=6104&start=15 figure 2 page 2. This looks like a great set of data to model up and shows a similar pattern and magnitude to what we are finding here and what you have posted now but with a better set of continuous data. I can't read the velocities but you might be able to get them/see them on the originals. This data set is also very similar to our observations plotting elevations from 100, 300, 500 and 1000. It also has a reasonable number of replicates in each set. Even without individual velocities vs shot height we can deduct enough information from averages and standard errors of each group.

With some of the good quality cheap chronographs available today it is within reach of the average shooter to do some ladder testing at short range and extrapolate the longer range results and perhaps we can put up a simple spreadsheet to help them do that.

Knowing the results beforehand leads me to believe the next stage of this conversation will then evolve to "what rifle designs would encourage more compensation at the longs".

Personally I use one rifle all the way through. Velocity spread is the key to being able to do this and that will be evident as we start to show some modeled (and real life) examples.

Cheers,
Dave

[bartman007]

Hi David / Dave / Aaron (Brains Trust),

Taking the 7mm example where I was suggesting 168 grainers for the shorts then 180 grainers for the longs, it there any merit in this? Considering that the Barrel Twist rate will remain at 1:9.

I propose that due to the 168's being lighter, I can run them quicker onto the next node (typically 100 fps higher).

Or am I better off just buying 180's and use them for the shorts and longs, and change the twist rate to 1:8.5 instead?

Interesting reading.

Keep up the good work

[aaronraad]

Hi David / Dave / Aaron (Brains Trust),

Taking the 7mm example where I was suggesting 168 grainers for the shorts then 180 grainers for the longs, it there any merit in this? Considering that the Barrel Twist rate will remain at 1:9.

I propose that due to the 168's being lighter, I can run them quicker onto the next node (typically 100 fps higher).

Or am I better off just buying 180's and use them for the shorts and longs, and change the twist rate to 1:8.5 instead?

Interesting reading.

Keep up the good work

From a frontal drag perspective, I would argue form factor over BC to minimise trajectory deviations caused by velocity fluctuations. The opportunity to arrive at the target sooner is always to the shooters overall advantage. Form factor is only one element but given F-Class vs TR & F-Std at least it is an option. The closer long range boat-tail projectiles get thrown at hypersonic velocities, the more efficient they become and a 50fps change becomes less significant. Oddly the opposite is true, but I'd much prefer to lay the targets back at 20 deg to the horizontal. Something I use to have a lot of fun with throwing the ball from 1st-slip to mid-off once the ball went through to the keeper.

Practically speaking, once you have your barrel chambered and throated, your case capacity and barrel twist are going to be optimised for a certain ideal projectile (if it exists). If you're lucky you will obtain indistinguishable results for multiple projectiles.

Personally I'd work on the longest projectile you intend to use and ensure your barrel and chamber specification get that projectile into the velocity node you want working out to your maximum range. If you can squirrel some range specific tuned loads, with or without another projectile then that might be to your advantage. Just don't burn out your barrel trying! Consider though all the extra range specific ammunition (+ the dreaded shoot-off) and equipment you require for each range. Do not dismiss the burden this will bring on your shooting ability on the day, with all the extra mental preparation and equipment organisation this will require. There aren't many Pro golfers out there caddying for themselves at the top of the tour.

As a teams event (depending on ruling) selections might be based on a shooter performance at each of the specific ranges allowing them to optimise a single piece of equipment and ammunition. In effect tuning each team shooting member for a range. Same could be said for coaches as well, given unlimited budget resources or very generous volunteers.

Just being aware of what your group size and centre changes to for each range puts your skills at a better advantage. Group size and centre at each range are influenced by a projectile's stability factor and hence my question to David. A shooter can adjust for the group centre at each range as they do elevation, but can not change the group size unless, as you indicate they change their load, or possibly their barrel compensator (harmonics).

I hear you David. These long responses require a lot of editing just to make sense, let alone any grammatical standard I'm prepared to "Submit" at a given hour. Praise be the edit function.

[Brad Y]

Again prime examples of the quality of our posters. Probably the best website available on technical tuning of rifles PERIOD. We should be capping membership so no other countries see it!

Being an average joe and only being involved for a handful of years in the sport, I have some questions and comments- please dont take them as negative or against the grain of what is trying to be achieved. Honestly alot of this stuff blows way over my head. Will happily admit that.

While I do think a smaller and very accurate caliber at short range can be beneficial, I still fear those who have one gun that plain works and that know how to use it from 300-1000. All this work seems to indicate to me that when buying barrels we should be buying 3-4 of them at a time from the same batch and of the same dimensions as closely possible, dividing them up into test barrels and match barrels. "Is the first barrel of a caliber an experiement and should it always be treated as such?"

At what point (assuming say a 20 round run in has been done and the barrel is showing minimal copper fouling) does a shooter determine the best powder/primer combo? When do you work on powder charge? At what point do you try to determine if your barrel is positively or negatively compensating? If negative compensating barrels arent "ideal" for long range work, can you change that or are you "stuck with a lemon" until being able to source a new barrel?

For now I can only stick to what I know, which is run a barrel in and follow the OCW process. Im always on the lookout for vertical spread, random shots that cant be explained and a good spread to back up the results on target. But if Im shooting high 50's/60's with good center count on target at long range then I still cannot see how its possible to have much faith in a set of numbers . Do numbers prove a target wrong or does the target prove the numbers to be insignificant? I did OCW tests to all my 6, 6.5 and 7mm barrels. The 6 and 6.5 both worked early on and I stuck with it learning little bits and pieces along the way while putting in good results with them. The 7mm wasnt so fruitful and while I learnt enough to give it another try with a slightly different plan in mind- it wont be getting anymore than that. Until then Im more than happy to read and learn and be proven new ways of doing things.

[williada]

Hope this helps Dave. Here is an enlargement of chono data so you can see it. Sorry but details of last shot plot (6) are missing but key data summary is there.
I am convinced the 7 mm diameter is probably the most effective drag wise to be the all rounder and therefore to experiment with.
You are dead right about the stock design in terms of bore centreline and centre of gravity. We have to also consider changing the location of the recoil plate to effectively form sine wave intervals further apart (less frequent) and a bit bigger which are more easily tapped into. This can be done easily with the way Barnard uses a screw head for a recoil plate. This can be located in different positions along the action, as I said in a much earlier post on recoil.
Maybe people need to explore the total barrel length from recoil plate to muzzle with Fibonacci numbers for harmonic relationships.
Anyway, I shall leave that to others to explore. I find time a problem now too. You have got people on the bike with your world class results, let’s hope they will be encouraged to push the pedals. It’s a huge area to get your head around. So the purpose of writing is to bring people along slowly with logical argument and hopefully increase objectivity. With any new development (or rediscovery), the phases of development can be seen as forming the discussion, storming the ideas, norming the results, to performing at our best.
Good luck, David.

[williada]

Aaron, an excellent response.

Brad, that has concerned me too, giving out too much information to o/seas competitors. But I see it as more important to get our bread and butter shooters up, because they lose heart and fall away and leave the movement now that people think they can buy a score. There is more to it than buying a score and anyone can do that. But to convince people it is worthy you have to have a recognisable feat such as a world championship that DaveMc, Peter and Team have achieved, because individuals hold things too close to their chest. If everyone has better gear then the talent applying for team selection will increase. At the same time, the coaches get educated and then truly manage the environmental conditions. This is what the English do so well. Never fear competition if everyone's on a level playing field.

Compensation characteristics can be changed in lots of ways as I have mentioned. The traditional way was to nip off 1/2 - 1 inch from the muzzle to get the exit timing right with barrel lift, knocking off 30 - 40 fps each time. Of course you can alter the fulcrum point on the stock where it hits your shoulder or you can rotate your barrel 180 degrees in the vertical to make that muzzle point in the opposite direction.

A case in point. I had the wife of Peter P contact me with regard to Peter's gear not handling the RG in England because they thought the powder was different. Because Peter had done some testing with me to verify results for the NRAA, and I knew his free flight, the message went back immediately to take out the free flight a further 0.015" with the universal throat reamer sent over with Phil M the team armourer. He finished with a solid badge having been running 6th. when his practice shoots were a disaster. Its a pity others didn't follow suit, but Peter was confident in the testing to be game enough to do it.

So no barrel has to be discarded because its displaying a certain characteristic at a point of time. So if some turkey says your barrel should have this or that free flight its a generalization because every barrel should be treated on its merits.

Edit: I forgot to answer your other questions Brad. The OCW is a great starting point. The tests are based on returning vibrations from the muzzle back to the breach that hold the barrel in a static state. This test finds nodal points that are pretty air density immune and are great for nodal tuning. If you think about it, its like a free recoil technique that tries to minimise barrel lift and so the elevation you get depends on your reloading technique and it is reasonably resilent to atmospheric changes. It is just an alternative way of tuning. However Dave and Peter proved that by allowing a little positive compensation you can certainly use one rifle across the course and learn all its characteristics. So a little muzzle lift is a bit different to the nodal tune. This is not to say, that the international competitors have learned that trick from their success, and the English already knew it. It is not to say if you have a short range gun and master that as well as a long range gun that top competitors are very capable of mastering both. Its just horses for courses and what's best for you. Certainly, if you have sound wind skills you are very much in the hunt with one rifle than a person that needs to rely on tight grouping to cover the lack of wind reading skill. If an OCW is a good way to cover all bases, then a 7 mm is a good all rounder in all conditions but there are times when the right conditions are present for smaller calibres to pick it off in the right conditions at the longs. Go with the law of averages.

So use your OCW load or pet load and do primer test first, having run the barrel in. Next, you can immediately do your compensation tests. The amount of rounds used in compensation testing could be 30 to ensure a significant sample. After run in, which conditions the throat because of the reamer marks, the barrel can take up to several hundred rounds to work harden in case of stainless. It is imperative to keep the barrel squeaky clean during this period because the projectiles are still lapping the bore. Keep away from hot loads and bullet jams during this period. I know the veterans used to get free ammo and many would pump a quite few rounds through quickly to stress relieve the barrel. Do not do this with stainless. I would look at serious load development after a hundred rounds. Beware the throat wears quickly to begin with then you have to follow up that throat at regular intervals.

David.

[DaveMc]

Thanks David - yes I can work something up from there. I will struggle for time over next few days but will get to it as soon as I can. I intend to model this up at various ranges and then give specific examples to explain more clearly to the readers.

As I said before this is very typical of most f class rifles we see today and when extrapolated out (will do that for the exercise) this level of compensation is barely discernible at 1000 yards.

The question (as you have put forward above) is - "what do we have to do to create a muzzle pulse significant and fast enough to compensate fully at 1000 yards?" and "is it worthwhile?"

Your movable weight work fascinates me.

I suspect we have to look harder back to the 303 days when compensation played a bigger part in the game - lower velocities (and hence longer barrel time - perhaps a larger pulse coming through at the time of muzzle exit), weaker actions, drop comb stocks, weight and cog location, longer whippy barrels etc etc. As Peter Smith said earlier many of these things we have evolved away from for accuracy reasons and creating a large vertical impulse in a long whippy barrel will impart some horizontal component as well.

Do we want to create a large vertical barrel pulse??? or are we better off where we are with small compensation and tight velocity spread to control long range vertical??

It is a fascinating subject and I think potentially could have an impact more in FTR and match rifle rather than FO etc where we may choose larger, slower projectiles in longer, lighter barrel formats.

I would love to explore a 34 inch light palma fast twist barrel from 2300 fps through to 2850 - perhaps in combination with a movable weight system. it would be truly fascinating - We now have the high BC projectiles to make this sort of combination a possible competitive outfit - whether it has a practical application or not is the question ?????

[williada]

Dave, I only see this work in its marginal utility too, but if I can save ¼ inch at 1000 yards, I just might hit the line to stay in. Interesting you mention a 34 inch barrel. The positive compensation example above came from a Krieger Palma barrel 35 inches. I ordered it through Project Penumbra with an extra inch at the re-enforce.

General discussion
The problem with all this stuff is that so much time has to go into preparation and analysing the patterns to learn what’s really happening. Hopefully we are making people think. Eventually, it only takes a few rounds if you know what you are testing for and you don’t blow away that cherry ripe throat by using in excess of 800 rounds. That is your window of opportunity with a fresh throat. So hopefully people can benefit from conclusions and cut to the chase. It took a lot of time to invent the wheel, and it takes a long time to re-invent it.

A concern I have, is that the rifle becomes an armourer’s rifle, in that you have to be always at it by nipping up throats, crowns, barrel orientation etc and adjusting loads and tuners that are appropriate to the physical changes. This is a specialist skill and does cost.

It takes a while to teach people and is really a one to one lesson. It is in fact goal based learning. So at every stage of learning development you can slip in the next goal until a full understanding is developed. I am still learning having shot for well over 40 years.

Tuners can complement the shooter skill set if they are used correctly, and IF it is applicable. A case in point. The week before David R was to travel to the US with a Palma Team, his issued and team specked barrel was not performing to his satisfaction. We spent a day, one to one, teaching him how to use a tuner. The rest is history, he won the individual long range championship and went on to win the NZ Queens. But unless the armouring or reloading adjustment keeps up with the physical changes as the barrel wears, the tuner may not be enough to maintain that purple patch.

Another case, in a previous team Frank P, who being issued a specked team barrel by the Armourer could not get a decent hit and he was justifiably concerned about the barrel. I had seen his groups and the barrel was replaced on my advice and I think he top scored in the team. The barrel reminded me on a pair I tested, where one shot under 1 minute at 1000 yards and the other could not group less than three feet out of the machine rest as verified by Peter P. This was not an armouring problem but a barrel with internal problems.

Both these cases highlight that each barrel has to be treated on its merits irrespective of specification.

If a barrel walks excessively on heat up, it’s usually due to a wall thickness problem caused by poor od profiling or linishing or not using sufficient coolant which can interfere with heat treatment. Its less likely due to a wandering deep hole bit because two sizing reamers are usually used and because they are over 5 inches long usually iron out drill bit wander. Then the barrel is lapped with a 4 inch plus lap. But it is not to say some variance can occur.

I would draw people’s attention to my previous days associated with barrel manufacturing where I learned a few things. A problem associated with buttoned barrels drawn by hydraulic ram is that the twist rate can vary. This will destroy accuracy. (The only exception is a properly configured gain twist). The top manufacturers use two tubes, one inside the other with register marks. The two have leather washers which engage the rifling and allow them to turn independently and are inserted into the barrel like a cleaning rod. If the register marks differ, then the barrel has variable rifling.

Heat treatment has to be done in appropriate stages of barrel manufacture. This is why buttoned rifles were profiled first before buttoning. I have experienced buttoned rifles opening up after re-profiling with the internal dimensions changing. Some barrels actually have a megaphone taper internally which interferes with engraving forces and gas seal i.e. the bore enlarges from the breach to the muzzle, when in fact the reverse of this internal taper is actually beneficial. But those that meet air gauge specs can be hummers. Quality assurance has to be better in this regard. There is less risk with a cut barrel.

I got a far bit of lapping practice in those days, so I will briefly discuss that. Again it is an art you have to learn about casting the lead lap, bumping it up to size inside the barrel, the finishing off the lands to sharpen them up with a copper lap due to the rounding by the lead lap. I believe the lands are more important in a good barrel than the groove. Because if you have ever looked at a new Omark moly barrel after a few shots you can see the groove has no wear at the edge near the land and the centre of the groove is shiny. In fact this is why I believe canted lands are better for gas sealing and faster bore time because the friction is encountered on the driving side of the land and the groove has an enhanced role. However groove size is very important to the throat in a smooth transition at the leed. It sets everything up for concentricity and reducing pressure spikes.

With regard lapping on a traditional barrel, done properly, it will iron out any humps and bumps which will foul and give near perfect internal dimensions. You measure along the way either the lead lap or a separate lead slug by slugging the barrel, another process and by using pin gauges on the lands as you go. You can actually feel differences to the tenths of thou, and if you are really skilled you can lap a tiny tight taper towards the muzzle. As I have said before the process of lapping will bell the muzzle, the crown has to be cut at the minimum diameter on the barrel to improve bullet yaw on exit.

It worries me that people polish barrels trying to remove carbon. Not only can they change groove dimension, but they round the lands. In fact a super smooth barrel will foul. It’s like Plasticene sticks to a smooth surface like glass but not to a rougher surface like a blackboard. This the very reason manufactures do not use fine grit while lapping and prefer a matt finish to reduce bullet friction. The right throat is one of the major factors in determining accuracy. Don’t stuff it.

Ok, the point I make is that unless your equipment is up to scratch, the any group testing can be skewed for other reasons. David.