Does this help?Let's assume that a given parcel of crosswind is exactly identical in the first 1/3 as the last third of a bullet's trajectory and similarly the same regardless of the bullet's drop (probably a statistical impossibility, but let's go with that for simplicity's sake). The only other meaningful variable that I can think of is projectile velocity--which being faster after muzzle exit I would think would mean less "dwell time" of the projectile in the crosswind parcel of air than there would be at a slower velocity towards the end of the trajectory path; hence more time in the parcel would result in more drift.
What am I missing here?
Do Test Groups' MOA Size Change With Range?
- Thread starter Bart B.
- Start date
stagpanther
New member
I usually try to be respectful as I can in order to try to educate myself--my apologies if it didn't come across that way. But if you look at the magnitude of the drift from 0 to 300 vs the magnitude from 700 to 1,000--it looks roughly three times as much at the distant third??
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You were respectful. And right in your reasoning.
I used Sierra Bullets software that allows two bullets be compared and different winds in several range bands. With the bullet velocity slower at 667 yards than zero, it spends more time in the last third than the first third.
A table with wind above LOS is attached.
I used Sierra Bullets software that allows two bullets be compared and different winds in several range bands. With the bullet velocity slower at 667 yards than zero, it spends more time in the last third than the first third.
A table with wind above LOS is attached.
Attachments
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Bart, sure they can break records but to win match is more than just one 5 shot or 10 shot group.
It's no secret that record group is smaller then the other groups and that short yard they do shoot 5 targets in 5 relay same day. You have to write program figure that.
F-Class seems to make big deal on records
https://www.6mmbr.com/gunweek088.html
https://www.rifleshootermag.com/editorial/tubb-wins-nra-long-range-rifle-championship/84431
https://www.ssusa.org/articles/2017/1/9/king-of-2-miles-extreme-long-range-competition/
It's no secret that record group is smaller then the other groups and that short yard they do shoot 5 targets in 5 relay same day. You have to write program figure that.
F-Class seems to make big deal on records
https://www.6mmbr.com/gunweek088.html
https://www.rifleshootermag.com/editorial/tubb-wins-nra-long-range-rifle-championship/84431
https://www.ssusa.org/articles/2017/1/9/king-of-2-miles-extreme-long-range-competition/
What am I missing here?
Cross wind accelerates the projectile sideways. The projectile keeps the side-way speed even the cross wind disappears. It will slow down but it will keep on moving sideways till impact. The cross wind happens nearer to the muzzle, the more time it will have to travel sideways. When calling the wind, you want to put more weighting on wind near you.
-TL
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stagpanther
New member
Unless you're obtuse like me. 
I truly had no idea that a crosswind vector could add it's own inertial energy vector to a projectile's path. (intuition tells me this is impossible--otherwise a projectile would become hopelessly destabilized by every bit of mixing air it encounters).
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The spin keeps the bullet stable.
Once you get something moving in a particular direction, it continues moving in that direction until something stops it. Newton's first law of motion and all that.
Motion is velocity and velocity implies momentum (momentum is mass times velocity), so if the bullet moves sideways, it must have momentum in that direction. If it has momentum in a given direction, Newton's first law says it will keep moving that direction until something stops it.
If you roll a ball across the floor and blow on it to change its direction, it doesn't immediately resume its original direction once you stop blowing on it--it will keep rolling on the new path until something else causes it to change direction again.
So once a bullet is (moved sideways by/gains sideways momentum from) a crosswind, even if the crosswind goes away, it will still keep moving sideways until air friction stops it. Since it's moving sideways relatively slowly, there's very little air resistance in that direction to speak of and it's not going to be in the air long enough to be slowed significantly in terms of crossrange velocity.
When looking at position plots (like the one under discussion), straight lines mean velocity with no acceleration. Curved lines mean acceleration is being imparted.
So, on the plot, the parts of the traces that are curved are the areas where the bullet is being accelerated sideways by wind. The wind is applying a force to the bullet (or you could say accelerating it, since acceleration = force/mass) which is increasing its velocity in the direction of the wind. The straight lines (whether slanted or horizontal) show the areas where there is no acceleration (force) on the bullet due to wind. A horizontal line means no sideways velocity, a slanted straight line means a sideways velocity.
It may seem a bit strange to think about sideways velocity being measured in inches per yard, but if you think about it, the horizontal axis could just as easily be labeled in terms of time (time of flight) and then you'd have inches per second which is obviously a measure of velocity.
Once you get something moving in a particular direction, it continues moving in that direction until something stops it. Newton's first law of motion and all that.
Motion is velocity and velocity implies momentum (momentum is mass times velocity), so if the bullet moves sideways, it must have momentum in that direction. If it has momentum in a given direction, Newton's first law says it will keep moving that direction until something stops it.
If you roll a ball across the floor and blow on it to change its direction, it doesn't immediately resume its original direction once you stop blowing on it--it will keep rolling on the new path until something else causes it to change direction again.
So once a bullet is (moved sideways by/gains sideways momentum from) a crosswind, even if the crosswind goes away, it will still keep moving sideways until air friction stops it. Since it's moving sideways relatively slowly, there's very little air resistance in that direction to speak of and it's not going to be in the air long enough to be slowed significantly in terms of crossrange velocity.
When looking at position plots (like the one under discussion), straight lines mean velocity with no acceleration. Curved lines mean acceleration is being imparted.
So, on the plot, the parts of the traces that are curved are the areas where the bullet is being accelerated sideways by wind. The wind is applying a force to the bullet (or you could say accelerating it, since acceleration = force/mass) which is increasing its velocity in the direction of the wind. The straight lines (whether slanted or horizontal) show the areas where there is no acceleration (force) on the bullet due to wind. A horizontal line means no sideways velocity, a slanted straight line means a sideways velocity.
It may seem a bit strange to think about sideways velocity being measured in inches per yard, but if you think about it, the horizontal axis could just as easily be labeled in terms of time (time of flight) and then you'd have inches per second which is obviously a measure of velocity.
I have known for decades that sometimes more than 5 targets are used in a match. Sometimes 10 shot groups are used. Same with 600 and 1000 yard events; for example:
https://www.nbrsa.org/short-range-group-world-records/
https://www.nbrsa.org/600-yard-world-records/
https://www.nbrsa.org/1000-yard-world-records/
Writing a program is not needed. Most people can use grade school mental math with paper and pencil doing addition and division averaging a few to several numbers. They did that in the 1950's and 1960's at benchrest matches. A hand held calculator can also be used; that started in the early 1970's, but only to speed up the process.
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Missed this earlier.
The bullet is moving slower during the 700-1,000 yd phase of its flight, so for a given amount of sideways acceleration, you will get more sideways motion relative to forward motion than if you impart that same acceleration earlier in the flight where the bullet is moving downrange faster.
Also, the bullet is spending longer in that region since its moving slower and therefore the wind has more time to act on the bullet.
It's a combination of both of those things.
stagpanther
New member
Thanks guys.
Roadkill2228
New member
As far as wind affecting things more if encountered in the first third than the last, yes the bullet spends more time getting pushed around in the last but a plain English way of saying this is that if it gets off to a bad start it affects the whole trip.
Even in the absence of wind groups often open up more rapidly than what a purely linear relationship between group size and distance would suggest (meaning lots of guns and loads can do half inch groups at 100 that wouldn’t even come close to doing 5 inches at 1000). One thing that really contributes to this might be imperfections in jacket thickness and bullet concentricity. At close range these don’t matter all that much (visibly deformed soft points shoot lights out to 200 - 300 yards in my 243) but over distance and thousands of rotations it shows. Think of a washing machine that for whatever reason ends up loaded heavier on one side. At first you can’t tell, but as the cycle continues and it spins more and more it gets shaky and clunky and loud and sometimes even moves around where it isn’t intended to be. Laundry ballistics. This lines up with real life observations a heck of a lot more than the theory about spiral shaped trajectories and what is statistically observed on targets with many shots fired.
Even in the absence of wind groups often open up more rapidly than what a purely linear relationship between group size and distance would suggest (meaning lots of guns and loads can do half inch groups at 100 that wouldn’t even come close to doing 5 inches at 1000). One thing that really contributes to this might be imperfections in jacket thickness and bullet concentricity. At close range these don’t matter all that much (visibly deformed soft points shoot lights out to 200 - 300 yards in my 243) but over distance and thousands of rotations it shows. Think of a washing machine that for whatever reason ends up loaded heavier on one side. At first you can’t tell, but as the cycle continues and it spins more and more it gets shaky and clunky and loud and sometimes even moves around where it isn’t intended to be. Laundry ballistics. This lines up with real life observations a heck of a lot more than the theory about spiral shaped trajectories and what is statistically observed on targets with many shots fired.
The laundry center of mass moves around in its shape.
The bullet center of mass is fixed in place.
Roadkill2228
New member
I must be missing something then. Wouldn’t it be the case that if
The bullet is not perfectly concentric/uniform then its center of mass wouldn’t be perfectly, well, centred?
The bullet is not perfectly concentric/uniform then its center of mass wouldn’t be perfectly, well, centred?
Yes. Very few of a given lot are perfect in that regard. Which is why 100 yard benchrest 5-shot group records are well under 1/10th inch but several groups average for aggregate records is much larger with the biggest group near 3 times larger. All the hundreds of other groups average bigger.
Some have spun bullets several thousand rpm checking balance. Perfect ones shoot most accurate.
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Jim Watson
New member
I used to read about the Vern Juenke Machine that provided a worthwhile sort on bullet uniformity. His son plans to make more.
Have a collet made to hold your bullet, then put it in a Dremel Moto Tool that runs at least 30,000 rpm
Connect an ampmeter between the Moto tool and power outlet.
Put a bullet in the upward pointed collet, turn the tool on.
Bullets out of balance will draw more current. Those way out of balance may fly out of the collet. Best balanced bullets draw least current, they are most accurate.
Connect an ampmeter between the Moto tool and power outlet.
Put a bullet in the upward pointed collet, turn the tool on.
Bullets out of balance will draw more current. Those way out of balance may fly out of the collet. Best balanced bullets draw least current, they are most accurate.
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Bart B to your question . When I was looking for that pet load I only shoot at 100 yards . Then I found what I was looking for , I fired 3 shots dead center one hole , didn't have the balls to shoot the forth didn't want to spoil the look , kept the target . The load I'm using is the same to this day , never shot that tight again , now I only shoot 200 yards , for me shooting even 3 shots in one hole like that is going some . I'm still trying . In a 10 shot group some would be close but shooting one after another in the same hole , maybe if the rifle was bolted down , perfect conditions and without a human in the mix. Most of the other stuff , you guys are playing in deep water , way over my head . I'm an old trigger guy .
The analytical answer to the OP's query is this: group size factors that operate linearly should not cause an MOA delta based on range, while those that operate non-linearly will have such an effect.
This answer, though, isn't very helpful unless one knows what the group size factors are, knows which ones operate linearly, and knows what the coefficient of contribution of the various factors are.
Now, to revert to the real world. Wind can affect group size if it varies from shot to shot, or if it varies at different distances from the point of shot to the point of target impact. Or, as in the real world, both.
Comparing 100 yards and, say, 500 yards, wind variability is almost certainly a non-linearly variable. Even on a calm day.
But let's go one step further. Let us assume hypothetical conditions in which the direction and magnitude is both known and perfectly constant over the full 500 yards. Will the wind's contribution not be linear here? No. The influence of our hypothetically perfectly constant wind over any range is a function of bullet time of flight. Time of flight is not linearly related to range. As the bullet travels downrange, it is continuously shedding velocity, on account of drag.
So the time it takes the bullet to go 500 yards will be greater than exactly 5 times the time it takes the bullet to go the first 100 yards.
Did someone mention over thinking?
This answer, though, isn't very helpful unless one knows what the group size factors are, knows which ones operate linearly, and knows what the coefficient of contribution of the various factors are.
Now, to revert to the real world. Wind can affect group size if it varies from shot to shot, or if it varies at different distances from the point of shot to the point of target impact. Or, as in the real world, both.
Comparing 100 yards and, say, 500 yards, wind variability is almost certainly a non-linearly variable. Even on a calm day.
But let's go one step further. Let us assume hypothetical conditions in which the direction and magnitude is both known and perfectly constant over the full 500 yards. Will the wind's contribution not be linear here? No. The influence of our hypothetically perfectly constant wind over any range is a function of bullet time of flight. Time of flight is not linearly related to range. As the bullet travels downrange, it is continuously shedding velocity, on account of drag.
So the time it takes the bullet to go 500 yards will be greater than exactly 5 times the time it takes the bullet to go the first 100 yards.
Did someone mention over thinking?
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