OzFclass

I recently sent a PM to David (Willada) regarding how he looks at tuning a rifle by applying the experiences he has been constantly sharing with us. The idea was also to try and simplify things like compensation to the people that (like me) arent technically as sound in the discussion on these forums as we would like to be. No offence to anyone intended but in my case, I struggle with lots of math and science (was a biology nerd not a physics nerd and maths was only a subject that needed passing to finish school)

So I will share below the response I got from him. Its very detailed and starts with the basics of a rifle before going into the tuning method. A great point that I can see is that with sub standard equipment we can do all this work for very little gain and get frustrated. Without asking some basic questions, I feel that when something confuses us, we do not follow into it further and get to fully appreciate these years of experience and countless hours behind a rifle- and the posts that describe them. Hopefully I didnt make myself look like a goose and this has a benefit to others. I encourage anyone who has a query about compensation, nodal tuning, OCW's, primer testing and anything else to put in their questions as a reply and hopefully guys like David, DaveMc, Aaron R and Peter S will be able to oblige with their wisdom. 18 months ago I thought load testing was loading up different charges and shooting for groups to see what a rifle could do. Now we have so much more information on hand it can get confusing.


Brad, the tuning of a rifle starts with building it properly. So below, I outline some of the things I do if it was really big deal. Maths is only a shorthand way of describing something. Don't worry about that so long as you get the idea. Theories are not worth much until you can prove them. I never knock a question because someone just might have a new angle.

Anyway hope this is not over the top. Pick out those things you can do practically, there's still enough there to tighten groups. I know you don't have lathe access.

Cheers,
David

The Venue
• Select a barrel with a twist rate suited to the environmental conditions. Look up past records on bureau’s for meteorology. Check JBM Ballistics on the internet.
• Select a heavier calibre if there are lots terrain factors if laminar wind flows are affected, including high mounds trees, berms etc. A bigger mass is less affected. If not use your upper powder node if nodal tuning, if you only have one rifle to have it punching harder.
• Do your testing for node and compensation from a heavy ramp that can be adjusted for height, to mimic the bore angle of the actual mounds to the target. It’s important to use your natural position. Or test on location. Different mound positions will effect eye relief from the scope and rifle balance for bag handling, length of pull and butt plate height – all of which changes barrel whip, angle of projectile departure to change both a nodal and compensation tune.
• If flying, powder takes on moisture at 30,000 ft and takes at least a fortnight to settle. In the meantime velocities can be all over the place and you might not perform well. Most international teams ship ammo in or have it made on site by arrangement because of this problem.
The Action Prep
• If you haven’t got a custom action, have it blue printed, i.e. bolt race, lug recess and action threads in alignment.
• The bedding must allow the action to sit without flex or tension
• Bed the trigger guard and takedown screw bases in a traditional setup.
• Check takedown screws don’t bottom out in the action (because proper tension cannot be achieved and the action may bounce) and screws must pull down perpendicular to the action and not touch the woodwork through the stock, only the bottom
• Face shoulders on screws and blue for full contact on bases
• Check for full rear contact of recoil plate or lug
• Keep takedown screw force below wood density max 33 inch pounds if no pillar. If on aluminium about 55 inch lbs.
• Locktight rail and scope screws and level scope.
• Adjust trigger fit for index finger length of pull and polish engagement surfaces, lubricate and inspect springs for any misshape that allows them to rotate when tension is off and lead to variable trigger weight. Check sear engagement travel, and free travel. Locktite grub screws.


The Bolt
• Polish inside of bolt, in and out, oil for slickness. Can use electroless nickel.
• Lugs blued and checked for full contact.
• Firing pin protrusion, .045-.055”
• Spring pressure, 28 lb, lubricate if not a using Bellville washers
• Polish etc bolt shroud, bolt bent
• Check for sloppy bolt race. You can build up with chrome, sleeve or use a top action
• Grease lugs
The Barrel
• Use a cut barrel, less stress factor
• Get the straightest barrel you can
• Ensure the barrel twist rate is constant for the entire length
• Internal bore/groove barrel size optimised for projectile for whole length
• Lap barrel for appropriate surface finish to minimise friction. Dead smooth barrels will foul.
• Minor taper lap, then lap lands to sharpens edges
• Minor re-profile with grinding between centres to assist id and od concentricity . Removes/decreases bend/belly if possible. Do not use a positive rake cutting tool which probably caused the problem. It has to be ground, because it leaves no distortion like a tool bit. The tool bit while profiling between centres tends to bend and cut more in the middle of the barrel than the ends of the blank
• Bore chamber and cut shank threads and shoulder in alignment with bore
• Polish threads with v shaped icy-pole stick at 60 degrees with lapping grit in the lathe
• Draw up to 120 ft lbs
• Set up in vertical plane
• Use minimum headspace, unless you want more kernels. The cases will fire-form to your headspace.
• 11 degree crown aligned with bore, with 40-45 degree chamfer nip on bore. You are indeed lucky if the groove is at 6 o’clock as this also protects rifling damage at the crown from a cleaning rod
• Run in until copper disappears with mild loads
• Remember it will take at least 100 rounds to work harden stainless, don’t jam projectiles too early
• If you are going to use a tuner, put a light variable tuner on with the variable weight sitting just past the muzzle on its tube. Important. This will only be adjusted after load development for group shape at 300 yards or environmental changes such as mound or atmosphere. This tuner is not a fundamental weight for compensation but works on minor vibrations. A fundamental weight on the muzzle can be used later in redial cases.
The following tests can still be done in this conditioning process.
The case
• Even after the second firing, the case maybe about 20% below capacity which of course you would use a lighter load to maintain a specific velocity. Spring back is the factor which affects velocity spreads by altering the air-gap, length of powder column and combustion. So the ideal test case is on a 4th firing as is for match with shoulder bumped ½ to .001”. This minimises vibrations and is just enough to pre-tension barrel and action joint threads. This also makes primer tests more accurate. So for a new barrel, you need a fire forming barrel cut with same reamer to fire form and ideally custom dies to match the chamber.
Primer Test
• Do primer tests with two different powders and pet loads or OCW powder load. But load .010” off the lands. This will give a better indication of primer spikes which will show in elevation; whereas 20+ thou jump will absorb some of the spike and mislead you in selecting the best primer. That could be six tests at ten shots or a match string number. Run with the one that gives firstly, the least vertical at 140 yards and secondly lowest ES.
Compensation Tests
• Do a 25 yard compensation test with 3 rounds about a grain apart for .30 cal. Use less for small cals. Try to reload one at normal load, one above and one below. Use your OCW powder load for normal load and keep .010” jump. If that is near your fastest load you have to induce a velocity spread by at least 30fps between loads below your fastest load by adjusting powder. So that’s a normal load, a second load less 30fps for middle and a third load less 30 fps again for the low load.
• Do this a couple of times to be sure, as we only want a general indication of compensation direction at this stage without burning out the barrel.

Adjustments
Positive or neutral compensation
• If the group is positive, or neutral, proceed to nodal tune load development as you would normally with powder charge and free flight. Note: A single tiny group that forms at 3 o’clock when all other groups are a bit bigger at 6 o’clock is just a teaser that will bite you if you try to use it. Focus on finding a load that is reliable and forgiving – because that is “standard” for what that barrel will do – not the anomalies as has been said by others. It is also the basis for your OCW principles. It is still best to find that load near the left of the peak of a sine wave to allow compensation for cold slow low shots in the morning and be right on the node as the day heats up. Do this testing at 140 yards to avoid transitional yaw as the bullet goes to sleep. Load in .3 grn or .2 grn amounts above and below using round robin method. If you are getting lateral, reduce load. If getting vertical, increase load. Or you can reduce neck tension for lateral or increase neck tension for vertical. Then further tweak jump 2-5 thou. Note: jamming bullets can give lateral if just touching in sloppy throats.
• Next, you can play with your tuner now for group shape at 300 yards. Try to take the lateral out in preference to elevation. Elevation super centre height is acceptable, but a nice round group that is tight is the best. But only move tuner in small amounts.
• Next go to 500 yards, use your tuner to reduce elevation spreads or group shape further, but by ¼ to 1/3 turns. Tiny amounts. On cold days move tuner in from your test reference point. This allows the barrel to rise more and toss that slow shot higher on a positive barrel. Later in the day move it out because you don’t want it to toss the bullet too high. The rate is .0005” per degree F. Or, for humidity move .001 out for every 10% increase or vice versa according to another bloke. It worked for me. This depends on weight of tuner variable weight and the thread pitch.
• If more adjustment is needed, I play with .005” increments in free flight. Powder charges .3 grn with a max of .6 grn seem to equate to small tuner movements which are really working on those secondary and tertiary vibrations to get you close to a node. Plotting elevation against velocity on a graph will tell you which way to move. A hint of positive compensation near the node should hold elevation out to 1000 yards and work with your skills and not against them.
• If it is negative in the first instance, proceed to negative analysis because the further you go in distance the more this barrel will work against your skills.
• Remember don’t test in a fishtail wind at 500 as it interferes with elevation due to Magnus forces, a cross wind is better. However, a left wind will lower and a right wind raise. Its not too much to worry about. But a sudden large drop off to zero winds can cause the bullet to go low as relatively speaking there is not enough wind to generate lift to hold the bullet up and overrides the left wind right wind concepts to some degree. Sometimes that’s why grouping in windless conditions can be so hard. So it is more important to do testing in the late afternoon when temperature is less likely to cause low shots and mirage has abated so light errors are reduced. This means testing will form a true baseline which will help you judge the real effects of environmental factors and so you will learn how much to move for those factors in the future.
Negative analysis
• Jump solution: Once on a negative trend, its usually a long downward slope unless the barrel is thick and shorter before it starts to run up to a peak again. So we need to jump bullets over a long distance to find an area to work with to refine things. To jump test, shoot 5 shot round robin groups at 0.010 jam, 0.040” jump, .080” jump, .120” jump separated by aiming off in the horizontal if you are concerned about wearing the barrel out. If not, 5 shot groups in .015” increments out to .090”. You may do this in reverse order to condition the bore because jamming tends to wear throat early on. Don’t just kiss lands as it induces variable pressures with carbon and variable ogives. Draw lines on highest and lowest groups to form a channel. I don’t use a ladder test for this. It’s just habit. If you find a rising trend in the groups look for the peak or an indication where it might be to refine loads to the left of the peak for positive compensation or on the peak for nodal tune as you would normally. Complications can arise if we are compressing powder or an upward trend in the groups cannot be detected. (You just might be lucky and find a peak in the range you can accommodate). So go to a powder charge solution next.
• Powder Solution for negative - Round Robin Testing 5 batches of 5 shots ½ grain differences for 30 cal. Use .010” jump. This will identify if we can find an upward trend of the sign wave or whether it keeps trending down. If you find a peak proceed to fine tuning about that load in .3 grn or .2 grn amounts above and below. If you are getting lateral spread reduce the load. If getting vertical, increase load. Or you can reduce neck tension for lateral or increase neck tension for vertical. Then further tweak jump 2-5 thou increments. If that does not work go to a change of powder.
• Change powder and use the primer that performed best with that. If this still does not cure the negative trend try the next procedure.
• Jam the bullet in with distances between .010” or 0.030”. This can induce barrel lift by altering the pressure curve.
• Rather than go in with the throat reamer in .010” increments or steepen the leed angle, the solution is to turn the barrel 180 degrees like re-indexing it. This calls for a chamber or shoulder nip up and a start again with original compensation test at 25 yards. On the other hand a negative compensator can still be used at the shorts and you have to look at the nodal tune with the least extreme spread. I have found tuning in the trough rather than the peak for the shorts to work well. It can still work at the longs if your extreme spread is in single figures but it is still working against super centre counts.
• You can add a fundamental weight to the muzzle until the right compensation profile is reached, but weight and bag handling may be a problem.
• You can replace the stock so the bore line is higher and or lower your butt plate.
• Another solution is to cut off the barrel ½ inch at a time and test at 25 yards.
Barrel indexing
• Sometimes due to torque effects on the rifling on some barrels the muzzle can twitch a bit more like your wrist can flap on your forearm. You may reduce this with a faster powder because the pressure curve is pushed back to the breach and so not activating wobbles towards the muzzle. Groups tend to arc with this condition. However you may see groups form on the diagonal even with a vertical plane barrel setup due to torque. You re-index the barrel by the angle of the group which is usually before top dead centre of the muzzle. Groups that form with a bias at 6 o’clock in the vertical are the best for nodal tune. However depending on where you want the compensation bias, reversing the curvature in the vertical can be beneficial. Certainly reducing the extreme spread of reloads will reduce the amount of tinkering you have to do.
Scope
• Must be level
• Lap scope rings, so there is no tension on the scope
• Ensure barrel is aligned vertically to minimise adjustments for zero
• Don’t wind your elevation or windage screws to maximum or minimum settings it will damage the mechanism over time.
• Store your scope with windage and elevation settings in the middle of their range to reduce tension on the springs
Williada 28/082014

Brad as always, some great info from David as always, need time to sit down and absorb it.

Cheers
Danny

I appear to be behind the curve here. There are various references to the sine wave, what's the origin of the wave?

A sine wave is just a representation of a simple kind of oscillation, or periodic motion. If you took the up and down component of a circular motion, say, and traced it out in time it would give a sine wave. Real motions, such as the pattern traced by the muzzle of a rifle, are more complicated but this is a simplification. When you fire a bullet the shock wave in the steel barrel moves about 10 times faster than the actual bullet so by the time that bullet gets to the muzzle the vibrations are well established and it will be deflected slightly. When you get a hollow group, or a strung out group , to some extent it is the result of this vibrational pattern. But its not just up and down, it can also be side to side at the same time...

Yes agreed the amount of information here is totally amazing.

Tim L- to me I took the reference to sine wave as the wave formed when plotting shot position and velocity during OCW testing- maybe David will be able to expand on that a little for us. Basically it would allow for compensation to keep those slower shots in (on the upward curve) while the faster shots are right on (plateau).

Things like this are the entire point of sharing what he told me. I must point out too, that he was more than happy for the information sharing and Im not doing this for any other reason apart from helping others who are a little confused understand what I was struggling to comprehend.

http://en.wikipedia.org/wiki/Sine_wave

Tim. The origin of the sine wave is from when the primer ignites, then to the detonation in the chamber, and, everything that happens until the projectile has exited the barrel.
There are too many reasons for the varying sine waves seen in different barrels to list here, but, if a TR shooter occasionally rests his finger on the barrel on the barrel while shooting, even that is enough to affect the shot to shot sine wave(and, consequently, point of impact).
Article and graphs below pinched David's post in "Compensation tuning......." here;

viewtopic.php?f=5&t=6104

williada wrote:Below are graphs of Bisley Blue Box Ammo and Brown Box Ammo circa 2003-4. The first three are Blue Box and the last three are Brown Box. There was only a short delay in the test for Brown Box from Blue Box and so the barrel had not cooled sufficiently. What I want to highlight is the rhythmic nature of the sine wave in the velocity spread in Brown Box. The tests for velocity, barrel temperature and pressure were simultaneous.

Thanks guys, I get what a sine wave is but where does the quoted sine wave come from. ie how would I plot it? What am I looking at?

What is the evidence that tells me where the barrel is on it's oscillation? What's the x-y.

If it just group height (compared to other groups) against velocity, I get it. I fear there is more to it though

Yes its simple. Plot height on the Y axis (vertical) or say velocity of each shot(or groups in a round robin test equally spaced by your aim-off) on the x axis (horizontal) for instance.


Trivia

It’s named after a mathematical function called sine of which the sine wave is a graph. Its plotted on the X, Y axis of the graph. You started studying this concept in trigonometry in year nine mathematics. Remember when started looking at a right angled triangle and had to work out the length of one of its sides. You may have applied the concept of sine to solve the problem which is a ratio of two sides of a triangle, the opposite side and the hypotenuse side. Rather than teach the maths, can you imagine a right angled triangle being made out of sticky tape and being stretched along the outside of a cylinder with the pointy end stuck on the left end of the cylinder and the right angled end stuck on the right hand end of the cylinder. Now if you turned (rotated) that cylinder around and around so the short side or the triangle wound up on the right hand end of the cylinder, you would find the longest side which is the hypotenuse would wrap like a spiral around the cylinder.




We can visualise this in another form as rifling if it was done on the inside of the cylinder or a helix or like a thread pitch on a bolt. If we slice that bolt down the middle, end to end, we will see it in a two dimensional shape, where we can clearly see a wave the thread makes if we look at the top of the bolt when we hold it up in the light.

Sine waves are useful in mapping harmonics. We are mapping the barrel vibrations as they move forward in a similar way in a two dimensional sense. In the context in which it is used in our analysis, is in terms of the frequency oscillations of the projectile induced on the exterior ballistics by the forces of the internal ballistics as it is pitched forward up the barrel. It’s like looking a stretched spring. In other words the internal barrel vibrations are reflected in the coning motion about a trajectory of the bullet. But the trajectory it takes also depends on the angle that projectile is tossed out of the barrel which depends on its position in its swing.

We try to time the bullet exit where the elevation forces are minimised about the peak or the trough of the vibration as represented by the sine wave. The group pattern we see on the target is in fact a reflection of the barrel vibrations plus exit angle caused by the vibrations. When we map or plot our groups in the horizontal we tend to visualize something that mimics a sine wave pattern. Our sine wave works like a guitar string being plucked and released. The recoil and bullet motions pull the rifle barrel to a new shape creating barrel vibration. Other small high frequency vibrations are superimposed on this barrel deformation. Things like a scope position, bolt or less than optimum armouring or reloading can slightly alter where the muzzle points before the bullet exits. When the barrel rises vertically and can travel no further it will correspond to the peak of a sine wave and when it falls to the bottom it will correspond to a trough of the sine wave. The tops and bottoms of the swing are stationary moments compared to the distance between them where the barrel is rising or falling. Because velocities vary, inaccuracy will be accentuated when the bullet exits while the barrel is moving between peak and trough, revealing itself in a general elevation string. We use the characteristics to find our best macro tune in the elevation context. Of course we can use a muzzle weight to dampen the size of these waves to a ripple rather than a breaker. Because the vibrations (and there is probably about nine waves of vibration that mix together), a micro tune is used to sort some of the actual group shapes i.e. reducing circumference, diagonal, vertical, triangular, lateral etc. primarily by powder charge and free flight in the first instance. However in doing this the exit timing and bore angle are also effected so as to change the trajectory. So we find the tightest spot on a long set of sine waves while taking note of the general trend of the barrel swing. Edit: Often the bottom of the sign wave can give a tighter group if you are nodal tuning up to middle distance. Perhaps the major vibration has moved through before bullet exit. If the bullet exit is timed on the secondary vibration for instance, it has less energy and cant lift the muzzle up as high and gravity dampens this lift more so. I have observed this as tighter elevation with a six o'clock group in barrel indexing tests. I have to give it more thought.

In the case of OCW (optimum charge weight), it is claimed that vibrations return down the muzzle to the breach before the bullet exits thus holding the muzzle in a static state, so it will group well as the bullet exits the muzzle. The OCW is used for pet loads that are generally good for other rifles with the same setup. They claim they are tolerant loads for a range of atmospheric conditions because they determine the best node where most of the projectiles group with variable velocities. I just use them as a starting load for further compensation tuning. Look up how to do this test on the internet.

A couple of extra thoughts:

If you are going to lap a barrel, practice on about a dozen old ones. You then re-chamber and re-crown. Don't ever attempt it on a new one that has been professionally lapped unless you are up to a professional standard. It is an art and you have to use both lead and copper laps which you make. The length of the lap is critical to feeling those small variances in barrel diameter.

Don't polish your chamber. The cases have to grip the walls of the chamber. You don't want variable thrust forces on the bolt head. So make sure that chamber is wiped squeaky clean with metho before you shoot. A very slight residue of oil in the barrel won't hurt if it is wiped out once and some us used to leave a bit in the Shultz barrels to make sure that first shot did not go low out of a cold barrel. Similarly, you can over polish your cases in a tumbler and introduce more variables. I clean the outside with steel wool or use my ultrasonic cleaner. A slightly rough surface assists the gripping of the walls of the chamber.




David.

A few more pictures to illustrate the macro tune, not the micro tune.

This is all fantastic, and very timely, info, as I'm basically about to start form scratch with a couple of 308 barrels for F-TR. By chance rather than planning one is 1:12 and the other will be 1:10. I had a plan, but in light of this info, I am putting a new one together.
I'd like to thank all the contributors to this thread and "interpreting your results".
These 2 threads have slotted so many things into place it's been a "eureka" read

Brad Y wrote:If flying, powder takes on moisture at 30,000 ft and takes at least a fortnight to settle. In the meantime velocities can be all over the place and you might not perform well. Most international teams ship ammo in or have it made on site by arrangement because of this problem.

I'd be interested in an explanation of this.

Barry

Barry, as a meteorologist you might be the bloke to give an exact answer. This phenomenon came to my attention at the Winchester Geelong factory about 2002 when Peter P and I were having discussions with them about the production of the Palma ammo for the team. It arose from a discussion about sealant with them. They had removed a bitumen compound from the ammo at the request of shooters in the past because of fouling issues, but found that testing in their underground range that groups did not meet their standard. So they went with a new compound. It was then we were informed about transporting powder by air.

Whether this happens at 10,000 ft or 30,000 ft I don't know. I too felt the need to have this confirmed, so I asked some seasoned international team shooters whether this was the case and was told that USA ship their ammo for this reason. This was again confirmed at the Bendigo Practice Shoot for the team publically by a serving Army Officer who was also a pilot.

This is only a hunch, but could the other volatiles in the powder and the retardant be affected too? When you open a can of fresh powder, you can smell the ethers come off, but not so much with the older cans. Graphite which is a retardant, is also hydrophilic and if these changes are happening would the kernel size or in the case of some powders with the the middle hollow section with a bigger surface area which are seen in slower burning powders be more affected? If the vapours are coming out of the powders, could they condense on the inside brass walls of the case leaving powder in direct contact with the brass walls a little different to the inside of the powder column? Or vice versa with the powder absorbing vapours? Could this lead to variable ignition? As I have been told it takes a fortnight to settle or be re-absorbed. Just maybe it won't be quite the same stuff as when it was first produced.

I have also noticed my own gear encased by foam to have condensation on it after flying interstate. I don't think the cargo compartments are pressurised.

If this is the case, and it seems likely and particularly if our reloads don't have any sealant whatsoever, the powder being pretty dry stuff would absorb moisture as it gets pretty cold up there. David.

williada wrote: I don't think the cargo compartments are pressurised.

If this is the case, and it seems likely and particularly if our reloads don't have any sealant whatsoever, the powder being pretty dry stuff would absorb moisture as it gets pretty cold up there. David.

Pressurisation could be the crux of it, I was thinking the holds WERE pressurised although I'm far from certain about that. Marty L would be able to set us straight on the matter. It's certainly cold up there. I've little idea how long this takes to permeate into the suitcases other than once having a couple of stubbies of beer travel home from Perth with me without ill effects. I should stick the kestrel in my case some time and check. I note that James Corbett seems to be able to fly to a Queens without experiencing ammo problems but then a couple of hours to the other side of the country is not the same as 20 hours to the other side of the world.

It might be informative to stick a few rounds in the freezer for a day and see how they shoot compared to unfrozen rounds from the same batch. I'll add that to the list of things to be done when I've got the spare time I never have.

Cheers

Probably all changes again depending on the atmosphere you seal inside the powder tin or the loaded ammunition at the time.

0% to 100% relative humidity on the day will dictate the amount of condensation inside a container.

I can't image loaded rounds really liking the constant vibration causing the powder to compact within the case at a certain angle perpendicular to gravity. Then having any significant amounts of moisture condensing onto the compacted powder inside a case. Even after salt dries out, you still have to give the shaker a good smack to loosen everything up again.

In terms of powder compacting, we all know that differences in burn rates between funnel, drop-tube and swirl loading powder exist. Nobody would recommend running loaded ammunition in a vibratory tumbler let alone a barrel tumbler. So while your scotch isn't walking off your folding table in the passenger cabin, I doubt noise and vibration controls are as significant in either the pressurised or unpressurised section of the cargo hold.

Aaron, I think you might have hit the nail on the head. It would be interesting to hear from other shooters who have observed in big competition which rifles performed better or worse in just the 7 mm class and .30 cal. I'm thinking its not just bucking the wind factors. Anecdotes like JK's observation at Belmont are important if you can collect snippets to analyse will help establish the big picture later.

Coming back to related area, in the ammunition neck. This sort of effects that neck tension. We all know that altering neck tension effects powder burn rates and pressures therefor group size. We all know that inconsistent neck tension is a big no no. Winchester demonstrated to me the actual benefit of sealant that in effect made neck pressure consistent, higher and prevented contamination of the powder by environmental factors. We can of course through reloading get a better result without the sealant. But we are not serving a mass produced market, where their testing and statistical solutions have minimised the problems associated with mass production.

However it would seem to me that with light tension on the neck could pose accuracy problems if the ammunition was stored for any length of time in bad environmental situations. Further changes, supposing you don't lubricate the necks, (which may resist moisture takes place over time) can occur: firstly the brass gets harder over time and so the neck tension will change. Secondly, there is a difference between cupra-nickle jackets or whatever material is used and brass. Two dissimilar metals over time will be subject to electrolysis, so a bonding of materials may take place. This too will alter neck tension.

If neck tension is dissimilar from testing then the results may well change.

I note, it is common practice in benchrest shooting that they load on the spot overseas.