#21 Postby williada » Wed Jun 22, 2016 6:43 pm
Few people have put a great amount of shots down the range at 1000 yards from a machine rest with a seriously large sample size over different seasons and years. That range is special and on the margins of ballistic performance which absolutely tests the normal distribution models. Absolutely you need a tuned rifle capable of small groups to improve score.
What I discovered was the group shape changed in winds over 10 m.p.h. became elliptical and their axis was also linked to muzzle timing, was more pronounced with left wind, that right winds produced tighter groups and how critical small velocity changes were in regard to your twist rate and group size. This is very close to when mirage is blown away, so you have to be careful to discount that if you are not using a machine rest. There were quite a few other findings that I will reserve for development programs because they depend on developing people’s understanding and cannot be given justice to in these threads trying to bring some folks up to speed. Just as when you first start shooting you have to learn to read wind, there are advanced lessons to be learned when you have mastered the basics.
Granted a computer model can impute changes to a very small degree that humans could not even visualise and that is a problem for most. We have to deal with the actual. In that sense, the ellipses are skewed. You have to then ask yourself then, how applicable is a standard deviation of velocity to a skewed distribution with what you are testing? You can shoot hundreds of shots and the skewed results will pop up due to conditions we have no control over.
My solution has been to reverse engineer from the groups on the target. Some groups such as a competitive shoot are small compared to a significant sample from a test I have done, but my thinking is based on the large sample. When I overlay many smaller groups, I soon see the trends. Just as cross checks do with other tests.
A casual glance at a small group in isolation is meaningless unless you see the whole process. Just like the advice Matt received regarding SD numbers on a 10 shot group from Dave, but his thinking like mine was based on larger numbers with tuned rifles. Similarly, computer models not based on tuned rifle parameters are meaningless and random because of course you can hide sections of the whole spread to look like small groups and then show the big picture to reveal it’s not small it is huge. That does not represent a tuned rifle but is selling a concept that you have to be aware of before refinement can be made for judgement. It is also the same conceptual process the university teachers on .248Win’s You Tube link did describing Magnus force and gyroscopic force on the ES SD thread did. It does not quite happen like that in reality with projectiles. Teachers do this, then in the next stage they would introduce high order analysis and synthesis skills to refine and make judgements. It’s about getting the idea first. The thing you need to do is to see if the groups are repeatable if they are small.
My solution to short range testing, where we can produce normal distributions easily, raises another problem i.e. finding the tune from which we can draw comparisons for normal distributions. Normal distributions have outliers we classify as random. But if we have enough knowledge we can find causes of the outlier and where possible reduce its impact to find a tune. I do that by examining frequencies and harmonics to identify different frequency paths including group centres and over time I have discovered causes. There are a few people in Qld, who sent me groups through the development process where I was able to identify the problem and fix it. The methodology has not been made public and only a few have been exposed to it with great success. I dare say others will improve on it as we have built on other people’s work.
Many groups of significant sample size remain skewed at long range and it is due to a greater influence of the Magnus force at the over tuning point and way down the track with constantly changing centre of pressure which also contributes to greater yaw of repose, of which spin drift yaw is a part. Two uneven forces one vertical and one lateral will form an ellipse. The Magnus force although small is linked to vertical and the spin drift has greater links to the lateral but remains fairly constant, the Magnus force does not. Pulses are set up which tend to cluster shots and disperse others by tiny amounts. If your velocity is not matched perfectly to the twist rate these groups will be bigger, so the timing of bullet nose oscillation as it hits the target can change impact point significantly at long range. We cannot see this minor, but still significant stuff in conditions.
Being truthful who can read wind to 2 mph and account for different wind zones over 1000 yards or different air densities regularly, which are affecting the centre of pressure on the bullet? We can measure it on the target and take note that in these general situations, particularly with a left wind over 10 m.p.h. an ellipse will form. Or if it’s too big there is a velocity issue. Frankly, you have to learn to master a big group at 1000 yards as Tony was demonstrating. You have not got much to play with if your group size averages ¾ minute. That’s a 1/8 either side as buffer. That margin can be enhanced knowing your group shape. The hardest part is seeing that ellipse in all the shots if your sight are moved based on position as well as wind. That is the art of reading the tea leaves.
Why wouldn’t you centre your group properly based on an ellipse when taking wind changes into account? This is about getting to know what to look for, not reading something into it; what your rifle will do and that is different to what it does at short range compared to long range. That means also taking into account a barrel’s compensation profile or a mechanical or tuning issue that loses odd shots. Still the ellipse will underpin it whether large or small at 1000 yards if winds are greater than 10 m.p.h. It is a coaching issue. Many people have just learned what the actual apparent aiming mark movements do too, and they will benefit by applying this knowledge. Surely you would want to know if you are winding the sight the right way.
The Project Penumbra and extensive work over 5 years in the 1980’s did determine the right twist rate and bore size for .308 with factory and reloaded ammunition for the Sierra 155. This examined not only static gyroscopic stability which most texts refer too, but considerable work with dynamic stability which relates to yaw of repose which relates to centre of pressure and centre of gravity with Magnus force and gyroscopic spin. Of course the different twist rates we used in paired barrels determined the tractability i.e. the ability to stay on the bending trajectory path. There were also experiments with transient yaw that dampens with distance such as nutation (javelin –like wobble) and coning to find that 140 yards was a suitable test distance. This was produced before a paper was written in America confirming the procedure that Norm supplied me with.
Graham Mincham also confirmed with me in discussion and in writing in 2002 that led me to conclude there should be different barrels matched to different distances in terms of dynamic stability and tractability. This is why we tested paired barrels from 1-12 to 1-15 with different bore sizes.
Finally, while a tuned rifle with low angular spread at short range is a great all rounder, that 1000 yards is special and on the margins. However, a one rifle setup limits the scope setup. By that I mean you are disadvantaged by not having the optical centre set for 1000 yards. This adds a real problem when you want to reduce parallax errors which you can detect by moving your head up and down, left and right. With a scope set for 1000 yards it is less likely to run out of windage and not develop erector tube problems because the internal springs have not been cramped. I have seen scopes suddenly bugger up in the VRA team championships in successive years and would you believe my old Nightforce has just done that because I have to screw down to 300 yards and cramp that spring. We don’t have much long range shooting in my current club.
Anyway those have been my experiences for better or worse that have shaped my thinking. I always stand to be corrected. David.