Vertical dispersion at 600 yds to 1000 yds

[williada]

Thanks for posting Tony. There could be a few more possibilities here with a lot more data. Could a figure 8 pattern be completed? If so would that be the total movement of the barrel from top to bottom of its swing? Or do those nodes represent positions of top or bottom of barrel swing? Could they be reversed? With the nodal tune you can see the effect of wind on the bullet in terms of aerodynamic jump.

[plumbs7]

Look at this weird group with an ultra clean barrel ! That's 9 shots at 300 yds ! A little crazy !

[plumbs7]

Some real rad barrel harmonics is happening there ! After the 5 th shot it was all business ! Crazy ! This is where coming over from.308 to 7 mm I've struggled .
.308 was 2 Fowlers and let's shoots 1000 yards !

7 mm is lets shoot a club meet then shoot a 1000 yds ! maybe just me!

[AlanF]

Graham,

I suspect its not so much a 308 vs 7mm thing as it is the "personalities" of those particular barrels. You might just as likely have had the same situation in reverse i.e. the 7mm barrel settling sooner. Every individual barrel is different. Even the best barrel manufacturers are unable to make barrels so uniform that they perform identically. Its because even the tiniest variation in dimensions and metal properties can affect how a bullet leaves the barrel due to the extreme forces involved when a round is fired.

Alan

[.248WIN]

Three videos on the Magnus Effect,Or wind blowing sideways across the bullet.


https://www.youtube.com/watch?v=PM9vnEWmA1c

https://www.youtube.com/watch?v=OCA1njSxgv4

https://www.youtube.com/watch?v=23f1jvGUWJs

[Normmatzen]

Malcolm,

I think you just caused an epiphany for me!
I shoot at the Sacramento Shooting Center in northern California. The long range course is noted for complex winds. I have kind of found it is dangerous to shoot in a head wind or tail wind. We have mounds all the way from 200yd to 1000 yd with a gap at 100yd. This gap is replaced with an opening in the very high berm on the left of the course causing unknown winds through that draw. I say unknown as there are no flags at 100 yd!
Your explanation of the up and down drafts caused by these mounds has just caused me to always watch for head and tail winds!

[williada]

Great work .248W. Its been hard to convince some over many years about Mr. Magnus. You nailed it visually. There are of course a few other forces at play with a bullet. We will get there in the end and debunk the myths. Thank you. David.

[DaveMc]

One thing I would like to point out here (and have done a few times in earlier threads) is the 10:00- 4:00 slope we see in projectile vertical displacement due to crosswinds is NOT due to MAGNUS FORCE or the Bernoulli effect. It is in the opposite direction. If it was due to the Bernoulli effect we would see an 8:00 to 2:00 slope.

Harold Vaughn picked up on this in his text and spent a lifetime fighting people about it (He was the head rocket scientist at Sandia Laboratories) and so did Litz. I do not want to get into a massive physics discussion here but will point the readers to the necessary vector diagrams for them to think about themselves

Here is a diagram of the Magnus/Bernoulli vector forces and is the same in the shooting video link above.

[DaveMc]

The effect we see is due to a combination of forces (including a very minor Magnus force in the opposite direction) but the major force is a gyroscopic one as per the vector diagram pinched from Wikipedia here.

200px-Gyroscope_wheel-text.jpg

[williada]

I really agree with you Dave, because it is the tension I mentioned between the the Magnus force and the gyroscopic force pulling in different directions that creates the pulse that is influential in the ellipse shape of the group which I think is more realistic than the straight line depicted by earlier models and has more relevance to the fliers with a left wind down around 4 o'clock. That's why I feel Magnus forces can't be dismissed as trivia. The Magnus moment also has a role in stability balancing the centre of pressure with the centre of gravity. I was very pleased someone had actually brought people forward and acknowledged the Magnus force and my comments indicated bullets had other forces at play. Its role just needs refining. Also have seen the axis change many times and while I feel the major factor is gyroscopic as I indicated on my diagram, I feel the Wikipedia model has not fleshed it out fully to be applied to the pulsing frequencies which seem to come into play past 700 yards and form oval groups. That is the significance of the odd shot that can really kill your score on left wind tough days.

[plumbs7]

I really agree with you Dave, because it is the tension I mentioned between the the Magnus force and the gyroscopic force pulling in different directions that creates the pulse that is influential in the ellipse shape of the group which I think is more realistic than the straight line depicted by earlier models and has more relevance to the fliers with a left wind down around 4 o'clock. That's why I feel Magnus forces can't be dismissed as trivia. The Magnus moment also has a role in stability balancing the centre of pressure with the centre of gravity. I was very pleased someone had actually brought people forward and acknowledged the Magnus force and my comments indicated bullets had other forces at play. Its role just needs refining. Also have seen the axis change many times and while I feel the major factor is gyroscopic as I indicated on my diagram, I feel the Wikipedia model has not fleshed it out fully to be applied to the pulsing frequencies which seem to come into play past 700 yards and form oval groups. That is the significance of the odd shot that can really kill your score on left wind tough days.

I was trying not to come back to this forum before the Queens to have a media , I phone black out ! But have failed ! The above is very good info and if I'm reading correct confirms what I have experienced !

Above Daivd W is a group I shot at 1000 yds a few months ago and which has been seen before ! David I believe you had already drawn lines on it showing the effects of manus effect( I think that's what the lines are for) and it would also be tree top turbulence ! Wind was from memory 7:00 O clock area to 9:00! At about 7-10 mph .
Is the above an example of magus effect ? Regards Graham .

Looking at the wind vectoring diagram must have been coming from 9-11 0'clock not the above !

Edit: sorry Magnus effect is spin drift . The above is I think ? Aerodynamic Jump! ???

[plumbs7]

Graham,

I suspect its not so much a 308 vs 7mm thing as it is the "personalities" of those particular barrels. You might just as likely have had the same situation in reverse i.e. the 7mm barrel settling sooner. Every individual barrel is different. Even the best barrel manufacturers are unable to make barrels so uniform that they perform identically. Its because even the tiniest variation in dimensions and metal properties can affect how a bullet leaves the barrel due to the extreme forces involved when a round is fired.

Alan

Yes that is correct as every barrel has its own personality! The one I have now loves being fowled and shoots its best at about 25-30 rounds . Was just thinking is the extra bearing surface of the long 180 Gr pills causing some wild barrel harmonics while there isn't any carbon ( lube ) in the barrel ? Bit like a cold start engine before start up! The pistons are at first in metal to metal contact !
In a barrel the chattering must be a lot more as it showed at 300 yds in my case ! Maybe different for others ?? Anyway that is another thread !!! Another time ?

[jasmay]

Ok, I'm totally lost on the Magnus 10-4 stringing and how you work it out.

I have looked at dozens of plots, and there would be an equal amount that string 2-8 as there is 10-4, how exactly are you discriminating the data on this one?

[DaveMc]

RE: AERODYNAMIC JUMP: PLEASE DO NOT KEEP CALLING THIS MAGNUS FORCE! - Magnus force acts in the opposite direction. The comparison I use is someone doing 100 km/hr in 5th gear in a small car and jamming feet on the accelerator and brake at the same time - Of course the car will stop as the braking force will far override the torque of the small motor in top gear. BUT then would you say - "Jeez lucky I hit that accelerator or I would never have stopped."???? Both act on the car but did you stop because of braking or acceleration?? In reality the acceleration slows your braking speed but not the reason you stopped. The reason we see the 10:00-4:00 slope is due to gyroscopic forces. Magnus works against this to reduce the magnitude and is only confusing the matter. Shooters should ignore it completely unless looking at higher level ballistic applications - It will just confuse the hell out of you.

(What Williada is saying is far more complex and talking about overturning destabilizing forces between the centre of gravity and centre of pressure - gyroscopic acting on CG, Magnus acting on CPressure - A very complex model that would take a HUGE amount of data to demonstrate . - I have a great amount of respect for David and what he has done and have no doubt there is a lot behind his statements.

Jason, You are absolutely correct - all explainable with the sampling errors thread that I have started and will gradually add to. Yes - I also have some perfect examples of 8-2 groups as well (and most shooters would) but these are due to the random fall of shots within a population. The larger the vertical variance and the lower the wind variation the harder it is to flesh it out and the more sampling errors you will have. A common mistake we make is to post the "perfect" group that shows what we want to see when it is really due to the random fall of numbers. Once again the random plot generator I was working on for the sampling errors thread demonstrates this VERY clearly. You can get any shape you want if you press F9 enough.

THE TAKE HOME MESSAGE IS NOT TO READ TOO MUCH INTO INDIVUDAL GROUPS!!! - use them for working out where you should investigate further. Load some more and go do it again. Repeat, Repeat, Repeat. Or as Peter Smith would say "That's great now go and do it again". You will also hear this in a different form around the range when some of the older more experienced guys say "I don't take too much time testing I let the results on the range tell me what is working"

Drawing lines through individual (3,5 and 10 shot) groups is fraught with sampling errors. The above group does not demonstrate anything on its own - especially so with such high vertical variance amid the horizontal.


So how do we test for Aerodynamic Jump (the 10:00 to 4:00 slope we see as shooters)

1) We must either shoot at the same aimpoint or have software that unwinds both vertical and horizontal aiming adjustments (Peter Smiths does but it can be written into spreadsheet models easily).
2) You must shoot in high enough wind variation so that the vertical displacement due to horizontal wind change is clearly visible amid the vertical variance due to rifle shooter/load accuracy. (The more accurate your setup (lower vertical VAR/SD) - the clearer these patterns become as does shooting in good sighting (low mirage situations)
3) You need a large number of shots - ideally 30 + at the extremes and not tempted to draw lines through the outliers!!!

It is rare in everyday shooting to have enough data points to show this accurately.

We had some good examples of this in the plotting from the world championships in Raton (and in a few other team events) where the wind was generally 3-5 minutes on either side and changing often (10 minute between extremes). With 8 shooters doing 15 rounds each 3 times a day we had a significant amount of data points in the right conditions with accurate rifles to demonstrate the effect reasonably clearly but still with a large amount of scatter and a low correlation coefficient.

It was remarkable to me that when I ran McCoys calculations how close he was to our observed results. Litz also has some good approximation formulas that fit with our observations.

[plumbs7]

Ok , deep breath! Gyroscopic torque precession vertical! This is a theory only and from a dumb Plumber at that ! But it seems to explain the phenomenon that I've been experiencing over the past few years and most notably at Squad and practice over the past 10 months. In the past Mr Litz has said ( and I think a lot of Mr Litz!) that head and tail wins don't amount to much vertical! About 2" at 900 yds !

One of the wonderful things with a really accurate Rilfe is there is a good chance of patterns developing . Yes some of my vertical is me , and in the earlier days when I use too bag my Teamie Ecomeat about his OCD reloading practices ! My loads May and I'm sure they weren't good as they should've , as the x ring just seemed elusive even at the best of times ! So some can be blamed on reloading and sd or a poor compensating tune !

However there has been a pattern developing of heads winds drop rounds low and tail winds lift rounds high in relation to the strength and direction of the wind !
So Mr Litz says heads and tail winds vertical are nothing to worry about ! But experience and head scratching says something else !
Leads to the Question " What the fire truck is going on!)

Let's examine ( from a plumbers view) the forces at play!
Magnus is a minor effect ( rh wind ) and to some effect in a right wind right twist barrel goes against aerodynamic jump as it creates downward lift . As aerodynamic jump is more powerful and wins out and creates lift from a right wind .
Remember the pill is doing about 300 000 rpm and is creating lift !

Ok if aroedynamic jump was what was causing a 2-8 clock slide it would be creating the most lift from a full value right wind and would likely to producing groups in an upside down v .
But it's a linear 2-8 slide ! Left wind from 8 - 10 clock produces a 10-4 slide was and is explained by aerodynamic jump!


But what's happening the other direction!???
First up let's look at the below YouTube video and see the force in plain action !
http://youtu.be/GeyDf4ooPdo
Your bullet is doing the same thing and is pointing slightly nose high and a little right as the metplate is chasing the tourque . If it were possible to insert a shaft into the base at 300 000 rpm into 180 Gr vld pill the metplate would want to chase the torque I the direction it's spun . This is part of the force of spindrift and Magnus does the rest . Which is worth in the Southern Hemisphere with coralis effect helping us , at 1000 yds Brisbane Lat about 1/2 Moa from a 100 yds zero 180 Gr 7 mm pill 1:9 twist.

Back to Rh winds and head winds ! When the winds come from 12 clock the pill is induced under load and lifts the nose slightly higher and causing or changing the BC lower and drag higher . Thus causing the round to go low .
Is the effect linear ? Yes , to a point until the gyroscope loads up enough and says no more yaw ! So from light to moderate winds the effect is most noticeable! Length of the pill and the leverage that can loaded up too can have an effect to a point . But our 7 mm pills are quite efficient compared to a 155 .308 and time of flight is shorter so it's not as noticeable ! A good argument for a Saum !


The opposite happens when the wind is behind and makes the round go high with a slightly more nose down attitude and more aroedynamic flight !

Wind from 2 clock tends to drop rounds at 8 clock and rounds that are shot in a 4 clock wind tend to drop rounds up in the 2 clock area . The effect seems to be more unpredictable from a right wind with aroedynamic jump fighting against the wind and torque .
Left winds seems to be a little calmer but similar effect !

Like I said theory from a plumber so most likely rubbish.

Lastly I've shot the ton that much this year if I said 30 times or more I think that would be about right thanks to squad and has pushed me to hopefully be a better shot and person ! Getting off topic !
There seems to a speed , twist relationship to dynamic stability at 1000 yds . Meaning even if you have a perfect load , sd etc at ranges even up to 900 yds . At 1000 yds it's just crap!
Basically stay above that area of dead speed zone ! and it can even be a fair way above the transonic region !
Look I really loves this topic and I love this sport because of the physics!
This is a theory so take it as Mills and Boone type reading !
Better than watching Neighbors! Hehe!