#45 Postby williada » Mon Apr 13, 2020 3:09 am
Alan, I hope I have cured you. I thought I had quit long posts.
Yep, I love to do compensation tests at 25 yards and once posted results revealing shocking chrono results with Waites’s old saum that at distance was still a sound rifle. In essence, such testing is best a guideline when more sophisticated methods form benchmarks which have far reaching implications for the rifle system and its application in a variety of conditions when testing variables one at a time.
Discovering a barrel’s compensation profile is a guide to what type of tune best suits it i.e. an OCW tune, nodal tune, compensation tune etc. and whether super low SD’s are achievable because barrels are individuals or have system faults through poor smithing, recoil management etc. that have to be worked around. I have found 140-yard testing soon reveals the strength and weaknesses of the macro vibrations in the system before I lean on the practical application of coning theory and then work with micro vibrations to refine tune further.
Coning theory allows for analysis of an actual deflection when the bullet first encounters the atmosphere and is first exposed to a crosswind which the latter can be described as cross wind aerodynamic jump. I use 140 yards as a benchmark distance to analyse rifle input variables for group and muzzle lift so I can discount other factors such as wind, light, mirage and drift both vertical and horizontal down the track to evaluate the rifle’s true performance.
In laymen’s terms, one-hundred-yard testing simply does not expose the boat tail projectile to all yawing disturbances before the bullet goes to sleep. Many commentators say this occurs around 200 yards. This in turn adds uncertainty and may add bias to decisions about group size, shape, trend and muzzle lift so important for advanced compensation analysis and or nodal analysis. This is not the territory of beginning and intermediate shooters. A decent ballistic background is required to put things into perspective otherwise it requires a small group activity on the range as part of a development squad to understand what is happening.
But new shooters can gain some benefits from doing testing at 140 yards. The dispersion is generally greater at 140 yards compared to one hundred yards and enables us to track its cause rather than accept the random. It fits nicely with a 1/8 moa grid over several aiming marks in the horizontal for accurate analysis of spreads. At one hundred yards, a one-hole group with boat tail bullets does not tell me enough about what the bullets are likely to do at longer ranges but they will open up sufficiently to give insight at 140 yards together with groups beside it in charge tests of incremental load. From the many analyses I have done with elite shooters submitting groups to me in the development squads of the past and individually, tight groups are often on the razor’s edge in relation to conditions. Groups either side in a charge test of the tight group are a better indicator of the real strength of that tight group if you are relying upon a neutrally compensating barrel and low SD and ES. Ramifications for a barrel could mean that the tight group test distance is the compensation range and thereafter everything turns to shite.
The boat tail is sometimes far from perfect. Carefully look at a handful of bullets and observe the intersection of the body with the boat tail to see if all are even at the junction. This junction forms the base the gases push up against. A bad base is effectively the same as a bad crown which increases bullet yaw and dispersion. It is easier to discriminate normal shots (which have gone to sleep, relatively speaking), from the outlier rather than just accepting the group size and shape on face value. The outlier can be discounted. It is important to index any bullet runout for 140-yard testing and load a marked case the same way. This helps start the projectile same way each time and reduces the random directional distribution of shots. So important when determining compensation profiles i.e. vertical position for a given velocity but less important if you are a nodal tuner.
As I mentioned on posts a few years ago, we also want to reduce the effects of wind and mirage on groups by shooting up short because long range benign conditions are not always available at 1000 yards. An anomaly will be explained later. Unless you understand where overturning moments lie, then testing at mid range can be deceiving if you do not know your compensation profile and what to expect when doing to vertical ladder testing. By testing at 140 yards we know what to expect mid range etc. from the macro profile for further fine tuning. Positively compensation barrels may perform well at 300 yards and be tight at 1000. What matters in between will reveal greater spread unless the positive compensation is mild. So also, beware of ladder testing at 300 yards if you do not know the compensation profile of your barrel. You can get away with a bigger window of SD in a mild compensator across the course otherwise positive compensation is range specific. A neutral barrel has to rely on tight SD’s or it will struggle with elevation at 1000 yards. A negative compensator will be done at 600 yards. Remember Alan’s stats on Queens where scores across the board blew out past 700? Now where was that overturning moment?
A few years ago, I posted a series of groups from Ecomeat’s testing of a tuned load over many distances to establish that a group shape is retained but maybe enlarged with distance. It may breathe in and out to some degree depending on the compensation profile but should hold if SD’s are kept tight and velocity is maintained well above the transonic range. By keeping records of plots at long range the effective discounts for mirage and drift can be evaluated and learned. It is important to keep copies of your past plot on your shooting bag to realize when tune has dropped off or a condition should have been discounted for. This is particularly valuable when learning how to use a tuner. My analysis of rifle stock design forms part of a pinned commentary was based on testing at 140 yards where benchmarks were established for comparison. By all means test at other distances to confirm results, but beware of the wear and tear on a good barrel but be confident you will hit the spot when the occasion arises from sound 140-yard group analysis. I have seen too many top barrels worn out by over testing. Better to learn to read wind.
If conducting short range testing and you want to predict down range trajectory you need to find the distance where that projectile is stabilized so the drag curve and the BC matches the G7 norm which is applicable to long range. This then gives meaning to the ballistic programs out there. Otherwise, marginally stabilized bullets maybe more representative of G1 BC according to Brian Litz. We have moved on from G1 thinking with new drag models with the advent of Doppler radar enabling theory to be closer to actual. Pardon the pun, but a further spin off is an accurate calculation of spin drift both horizontal and vertical. This factor should be used in windage calculations and calculation involving mirage along with elevation. With sound grouping at short range, you can then examine the effects of light, particularly on those cumulous cloudy days.
The 140 yards was result of work I did from1980-1985 for a barrel manufacturer and more specifically with Project Penumbra for two years 2002-3 for the NRAA. Thousands of rounds have been fired with numerous barrel twists, different internal dimensions and profiles with both projects from machine rests. As I have said many times before. Pretty sure I got the statistical significance right being a former maths teacher many moons ago. My short-range testing was done at home from a machine rest which was transported to Rosedale for 1000-yard testing. I was trying to match a barrel configuration to best suit issued .308W ammunition for marginally improved performance. I was also experimenting with leade angle, free flight, and later bloop tubes for muzzle blast and variable tuners. I simply extended the test range out at home to find a distance where projectile groups from many barrels were more consistent and found 140 yards to be the most practical for the .308 Palma round in 2002 in a wide range of weather conditions over several years of regular testing. I could not see on the target significant changes past that distance due to coning which in theory flutters a little further out. Of course, I like to do testing in a right crosswind with a RHT barrel and use that as a base for discount where conditions change. In recent years Craig supplied me with a left-hand twist barrel which I tested over a couple of years to gain significant data and test some theories with regard to drift, jump and group, and won a couple of 1000-yard open competitions in good company. The base line for this barrel was established at 140 yards and I had no reason to doubt the normalized precession would be significantly interrupted past 140 yards by the Magnus force at the overturning moment or in the last ebb of its flight given the stability factor. The take home is why would you expect the bullet flight to change past 140 yards outside optimized parameters?
For the most part, people run with an SG of 1.4 determined by Miller, for a particular bullet in a given atmosphere. It appears this was what the industry ran with across the board but a paper by his assistant revealed more insight. I think a higher figure is required which I have since applied to other barrels with success based on trial and error experiments in the 1980’s and Graham Mincham’s suggestions to me for short range and long-range testing during Project Penumbra based on his army resources. Given that a bullet is capable of being dynamically stable, its scientifically proven yaw instability may run out to 200 yards in both the vertical and horizontal. I have the paper but can’t find it at the moment, but yawing instability was documented in a US Army paper circa 2004, which I sent to DaveMac but sourced from Norm which clearly demonstrates 140 yards is on the money and confirmed what I had been doing at home years before and with project Penumbra was correct. There is a further confirmation on page 156 of Applied Ballistics, Litz 2009 where there are two illustrations of this revealing the instability out to approximately 200 yards. I have copied and posted that before as well as charts from Arrow Tech. Also, read of Rifle Accuracy Facts, where Harold Vaughn Chapter 10 explains external ballistics. But his coning theory is flawed according to Boatright in relation to the projectile nose pointing out which has been accepted theory. Jim Boatright of which I am a huge fan has written several academic papers on coning effects. He also points out new and validated approaches that differ from Vaughn, Litz and Miller's SG of 1.4 which are more recent with new equations in different papers.
Yes, tuners and long-range testing can mop up results to a point. Over a long series an individual may have better weather judgement than competitor with a more accurate rifle. Not so in team shooting where coaches take control. The vertical ladder test gives little away with regards to group shape but at 1000 yards it does give insight into the velocity required to maintain elevation as per pyramid testing, I once posted of Ecomeat’s long range groups. Rosedale experiments revealed the effect of wind above ten mph and the formation of elliptical groups at extreme range. It wasn’t mirage because I used the machine rest. It is an open range. These were cross checked with 140-yard groups where knowledge of group formation was critical for analysis. This has obvious ramifications for group centering if critical velocity is not maintained and was a revelation.
In relation bloop tube and tuners and I looked at vibration analysis which has benefited a few elite shooters in this country such as Craig to make the most of less than perfect gear to win a Queens. The 140-yard testing has held up. Over the last couple of years, I have mentored Albert. His WA results speak for themselves but he achieved them. No doubt others build on what I believe and so it should be.
Its easy to quote recognized achievements of others but with new knowledge we can see where old accepted theory and practice does not reveal all. It takes nothing away from their achievements. Depending upon culture a good story often gets in the way of the search for the truth as Peter so rightly demands. So, with smarts you can put two and two together from snippets you get and go figure the gaps. The journey is most satisfying when you can observe and learn for yourself. I have been fortunate to have great mentors in my formative years. As I age I like to keep the young guns on their toes on my day.
To conclude, 100 yards is a good distance to teach the approximate value of MOA i.e. 1” = 1 MOA.
Boatright 2012 “Each observable ballistic phenomenon of a spin-stabilized rifle bullet can be explained in terms of the acceleration of gravity and the total aerodynamic force acting on that bullet. In addition to the coning motion itself, Coning Theory explains the spinning bullet’s aerodynamic jump and its steadily increasing yaw of repose together with its resulting spin-drift. The total aerodynamic force on the bullet comprises its drag and lift rectangular components and produces an associated overturning moment acting upon the rigid bullet. The coning motion of the bullet includes two distinct but synchronized aspects: 1) the well-known gyroscopic precession of the spin-axis of the bullet, and 2) the previously little-known orbiting of the center of gravity of the bullet around its mean trajectory with the nose of the bullet angled inward toward that trajectory. New equations are developed governing the orbital motion of the CG as a circular, isotropic harmonic oscillation driven by the lift and drag forces as they revolve together at the gyroscopic precession rate. Standard Tri-Cyclic Theory governs the uniform circular precession of the spin-axis driven by the overturning moment acting on the spinning bullet as a free-flying gyroscope. The synchronization of these two motions is the defining principle of Coning Theory.”
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