Gyroscopic Precession and Bullets

[MasonInBend]

I'm currently re-reading Applied Ballistics for Long-Range Shooting in small bits but I can't find an answer to the following question. How does gyroscopic precession effect bullets? Gyroscopic precession is a known phenomenon that simply put states, an outside application of force onto a spinning object will have a response 90 degrees later in the rotation of the object.

Coming from an aviation (helicopters) background I've often wondered where gyroscopic precession (GP) shows up in shooting. I thought I found the answer when Hodnett mentioned "crosswind jump" in his DvD series, and again when I read about aerodynamic jump (page 77 from Applied Ballistics). However, both of these concepts attribute the deflection to pitching and yawing gyrations? Secondly, both of these explanations state the bullet is deflected 90 degrees PRECEDING the application of the wind, be it from a 9 or 3 o'clock. Wind from the 9 o'clock to LOS deflects down and right; wind from the 3 o'clock deflects up and left.

Lastly, because bullets weathervane in the wind slightly, deflections from GP should act opposite in terms of the vertical component but might be amplifying the horizontal vector? Obviously aerodynamic jump has to have a greater effect because it is the observed deflection.

PS Has anyone completed the Litz Challenge (a rifle that shoots better at farther distances than close ones)?

Thank you for your time in advance,

Mason

 

[allenn]

I have never seen a 6mm bullet that I thought had gyroscopic precession. Spin, yes, precession, no. And I have spun a lot of them. I also have never been able to find a bullet that would spin on it's centerline. See video link.

https://www.youtube.com/watch?v=uOUqy9G8s6Q

 

[Bryan Litz Ballistics]

Good questions Mason,

Aerodynamic jump (same as crosswind jump) results from the pitching/yawing motion of the bullet as it realigns its axis to the oncoming air vector. Your observation about the direction is very keen, you would expect it to go the other way. Here's what's going on. The axis of the bullet does initially move in the direction you would expect, but that first initial motion is small. By the time the axis swings 180 degrees, it reaches the 'first max yaw' cycle. It's this 'first max yaw' cycle that causes the steering/jump of the bullets flight path and results in aerodynamic jump. It's 180 degrees from the direction you would expect because it takes that long for the initial motion to grow into the max yaw.

Spin drift is another aspect of bullet flight that results from gyroscopic action.

Some terminology if you're interested;
pitch and yaw refer to the direction of the angle; pitch being up and down and yaw being side to side. Sometimes yaw is used to describe the magnitude of the angle made by the bullets axis and it's velocity vector.

Precession and nutation refer to the fast and slow frequency oscillations of the bullets axis. Precession is the slow mode and nutation is the fast mode. In other words, looking at a pitch/yaw plot, precession would be the big circles and nutation the smaller circles inscribed along the orbit.

Here's a video I made showing the motion in both angular and trajectory reference frames.

Some have tried the Shoot-thru-target challenge. Thier results match ours which is that groups scale linearly with range. There will be a more thorough reporting of this in the next volume of 'Modern Advancements in Long Range Shooting'.

allenn,
That's a very interesting video and machine! I've been wanting to build one of those for years and it's still in the plans to do so. Have you observed a correlation between the read-out of your machine and how bullets actually group? What is the range you're able to measure, meaning how much difference from best to worst?

-Bryan

 

[Joe Salt]

Dumb Question will it spin the bullet point first, that's the way it will be going in the wind?

Joe Salt

 

[allenn]

Joe, there are no dumb questions. Yes it will spin them point down. It doesn't seem to matter if I spin them point up or point down, they still all spin with a wobble.

Bryan, sad to say my testing has been very inconclusive. The spinning range is usually between 48 to 54 hertz. In this range there doesn't seem to be a difference at the target. Out of that range it's hard to tell. At a higher range accuracy does seem to improve. But I'm not sure of my shooting abilities. One time after sorting 4 or 5 hundred bullets I found 3 that spun in the 60 hertz range. When I shot them at 200 yds, it was the smallest 3 shot group I ever shot. I wish I could find more of those so I could find out if it was a fluke. I recently got some Bart's 105 6mm bullets and want to do some testing with them. All of my testing to date has been with Berger 105's. Shot in a 1-8 twist Krieger 17 pound 600 yrd type rifle.
I believe Franklin Mann was correct in thinking group size could be reduced with bullets that were balanced better.

 

[Bryan Litz Ballistics]

Thanks for the info allenn.

We agree, the balance of a bullet is paramount to precision. Walt Berger shot his way into the hall of fame with bullets made on jackets that were individually turned on a jewlers lathe to a uniform thickness. Uniform jacket thickness is necessary for balanced bullets. Understanding the importance of balance on precision led to the creation of J4 jackets, which are made with extremely uniform wall thickness and enable the bullets made on them to be capable of high precision.

-Bryan

 

[Joe Salt]

allenn have you played with different air pressure, I'm sure you have! Or maybe a longer tube? That is a great test that you came up with, keep trying to improve on it I think you may have something.

Joe Salt

 

[allenn]

It takes a certain amount of air pressure to keep the bullet afloat. To much pressure and you blow the bullet out the tube. To little and it sinks and doesn't float.
A longer tube doesn't work as well as a short tube. I think a longer tube dampens the spinning of the air as it goes up the tube.
Imagine dropping a bullet into a tornado. You have air going up and spinning.
My thoughts as of late have been how to make a better bullet. I have a few ideas and will try to get some pics up Friday or Saturday if your interested.

 

[Joe Salt]

allenn everyone wants the perfect bullet, but what is the best way to determine which ones are bad? You are definitely on to something. But the other thing that I think about is what is a bad barrel doing to a good bullet that a good one doesn't? What I guess I'm trying to say is the bullet you are spinning has no lands or grove marks, try spinning one of those! There was a guy on here that I think was shooting into water and was doing something with them, can't remember. may have been you.

Joe Salt

 

[rayjay]

Shoot a good RF BR rifle at 50yd with good flags and you will see that the vertical component is real. I have a fast twist bbl that I have shot some [ prior owner was a dedicated experimenter ] and the horizontal drift is about the same as a 16t but the vertical is much greater.

CF spends so little time being affected by the wind that it's difficult to see the vertical unless you shoot longer distances. Then you have so much ground not covered by flags that it's difficult to see what's really going on.

Then you throw in the bad bullet issue and ARGGG ! I bought a 30BR from an excellent gunsmith that also made his own bullets. I eventually came to realize that it wasn't the gun that was so amazing it was his bullets.

 

[Joe Salt]

Bullets and Barrels I always said!

Joe Salt

 

[allenn]

One of the problems in the making of bullets that will lend to there being out of balance is when the bullet is pushed into a die and a point is put onto it.
The 6.5 Grendel picture shows a bullet (on the far right) that has a deformed spot on the inside of the bullet at the point. How do we know if we get a uniform fold of the jacket on the inside of the bullet when it is formed in the point die. I believe if the bullets were first crimped with a six point crimp as shown in the second picture it would form a more perfect bullet on the inside.
I do not have a set of dies to make bullets to test this theory. I would though, like to work with a bullet maker to make a few of these bullets and test them.

Hey Joe, it wasn't me shooting into the water. It is an interesting idea though. What if I shoot a target and from there the bullet drops into a big cattle tank filled with water. The bullet is then recovered, loaded and fired again. I wonder if it would go into the same hole it made the first time it was fired.

 

[Joe Salt]

Maybe someone knows who that was that was shooting into the water and recovering the bullet for his test. But I was wondering how they would spin with the grooves in them? I think he was using his swimming pole! ;D

Joe Salt

 

[Joe Salt]

Wish I had a place to shoot 2000 yards, now that sounds like fun!

Joe Salt

 

[michaelbz]

You have an interesting test there. Have you tested your device with a plug gauge or any sort of precision ground rod to determine a baseline, and what were the results?

 

[allenn]

Good idea. I never tried testing with anything other than a bullet. I might have to make something and see what happens.