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Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Len Backus, Mar 30, 2003.
Is there a computer program available for estimating this factor?
Not that I know of, but, in a right twist barrel, figure about a 15 Degree upper left to lower right dispersion.
Can you give me a couple examples, please?
I think the 6DOF does, Sam and Harold Vaughn are the only ones I've heard that use it though. Harold says it's pretty complicated, but I bet I could figure it out with enough time, which I have way too much of lately. Sam must know a little more about computers than he lets on, ya think?
I'm not sure what you mean by "Examples"?
I'll try the best I can.... I shoot at williamsport. Last year was a rather windy one. In fact, by looking at the scores and group aggs at the end of the year as compared to the last several years, I would say the worst year in the last 6 or 7 though I've not shot there during those. The world open was shot in a hurricane morning to night both days.
We have on average about 110 shooters each Saturday and Sunday. In EVERY group I looked at all year long when someone had pronounced wind effect (given it was someone with a gun that actually shoots) the shot/shots that blew out of the group went low right or high left, and at approximately a 15 degree angle from horizontal. Now, I'm not gonna get into a equasions war with anyone and I'm sure as hell not gonna go try to scientifically prove the angle to be 15Deg, 0 Minutes, 0 Seconds.
In a right twist barrel your bullet is turning clockwise as viewed by the shooter. Just like a pitcher throwing a pitch that curves, it is the air pushing harder on one side of the ball that provides the mass to disuade the ball on its path. Well, your bullet is spining down on the right side, that means the air "pushes Up". If the wind comes from the left, the air pushes the bullet down. Yea Yea Yea, the bullet is pitched a little also, nose up, and yawed slightly, whatever. Go shoot paper at 1000 in varying wind waiting a while between shots and see where your group forms. If I had a digital camera, I'd send ya photos of targets, perhaps Darryl can send ya some. he's probably got lots. Although, since he has some weight up there, he probably never has to shoot the relays with wind!
Many people up there like to think that wind has a recognisable vertical component that "Blows" the bullets high or low. Some think headwind and tailwind cause the vertical thay see. Well, I basically I think that my handloading sucks as compared to some of the shooters up there and my gun doesn't work as well, so, I'm not gonna blame the air. If your gun shoots 10", and you shoot in a wind, Your group WILL BE within a 10 inch band across the paper.
Lets see the flames fly from this one!
Do you have a copy of Harold Vaughn's book "Rifle Accuracy Facts" already? If you don't I suggest you get your hands on a copy, you'd love like it alot. It has a way to test what the vertical component actually is with your bullet. I won't bother if you already have read it though.
15 degrees from horizontal is close to his example, his was 17 deg for a 68gn 6mm match bullet with a GS of 1.38 (gyroscopic stability factor) at sea level.
To give you a little example, he shot a group at 200 yards with wind at varying intensities coming from 3 o'clock to produce a line of impacts from the wind drift that stretched right to left the harder the wind blew. The group looks to be in the order of an inch and a half, but, also the harder the wind blew, the higher the shot impacted because of the vertical component. It has alot to do with the GS, wind speed and range. The higher the wind speed and or the higher the GS and the longer the distance the more vertical wind drift component will be.
I can post a pic if you like to show you an illistration if you haven't read the book and seen it yet?
I've been gone and missed some of your responses. Thanks, all.
Brent, yes please post the target image and I will get that book.
The vertical component of wind drift is not a result of pressure differential from aerodynamic forces. It is in fact a result of gyroscopic precession.
Harold Vaughn and I differ on only a few things. The vertical component of wind drift is one of them. If you look at Harold's example at 100 and 200 yards you'll notice he tries to make a case for the vertical component to be linear. The examples that he gives for shooting in a varying crosswind component and then doing a least-squares function and plotting the resultant angle to estimate gyroscopic stability is valid. My six PPC with a 14 twist barrel shoots right at 18 degrees with a 67.5 gr. bullet. The Wolf with an 11 twist barrel will shoot about 26 degrees when a least-squares function is plotted at 100 yards.
As we all know, wind drift is a squared function, not linear. If one were to continue Harold's theory well past 200 yards, let's say to 1000 yards, there would be a massive reduction in the angle plotted because Harold believes the vertical component is linear and wind drift is exponential.
The truth is, in practical observation, we see a significant angle (15-20 degrees) at 1000 yards, very similar to what 4MESH describes. Mathematically, we describe gyroscopic precession with an exponential curve, yaw of repose is one example.
I believe the observations of 4MESH are correct, I don't believe he has identified the actual forces involved. There are several explanations that I would offer as to why Harold missed the mark on this element. One, he did not have the equipment and a controlled environment to test at extremely long ranges, if he did he would have noted that the vertical component was not linear. Second, much of Harold's research was done with the equipment that fired at significantly higher launch angles than the typical 1000 yard rifle. To understand this issue from a practical sense, you must have a rifle that will shoot with almost zero vertical error. Without this tool, any attempt to measure will be lost in the noise of other overlapping error sources.
Remember that Harold offers data only to a very short distance relative to what most guys on this site are shooting. His ascertion that the angle is strongly correlated to gyroscopic stability is correct.
I have observed on many occasions past 1000 yards a vertical component relative to the actual wind drift in a ratio of 1 to 4, with bullets on the edge of stability, 1 to 6.
[ 04-08-2003: Message edited by: S1 ]
Sam, thanks for your insight. Looks like some more testing's in order now. Here's a teaser from Harolds book. You might have to save the images to be able to zoom in closer to read the text.
S1, so you contend that the bullet changes direction because of gyroscopicly applied/affected forces. I have another set of conditions to run past you but I'll have to get back on that.
Brent, for those people without the book, you're gonna have to post that in a higher res. Blowing it up is pretty problematic.
I saved it and enlarged it on Microsoft Picture and Fax Viewer "OK", but it is a little hard to read, but manageble, the other programs didn't work as well.
When I send the images to Sony Imagestation so I can post them they are obviously reduced in size automatically, the original was 896 x 1408 at 244KB, now it's 286 x 450. I don't know how to do it any better than that. I can email you them if you need me to, they should stay at the higher res.
I've got the book so I don't need them. Thanks anyhow. What'd ya think of the collet dies? Did ya notice the marks where the taper changed so the one resized the neck on a taper? That other case you saw was Ken Markles invention. Pretty cool huh. That's a 416-6.5 that shoots pretty darn good at 1000.
Sorry I forgot all about it. It does look like it was alot of work. Why size the neck on a taper? The short little funky shouldered fella there looks interesting! Nice lookin rifle you got there too!
You sure are a handy kind a guy, the can do type, I like that!
How much vertical rise or drop due to tail or headwind? Examples at 1,000 yards?