Twisting it right...

RiverRider

Handloader
Dec 9, 2008
1,425
53
I hope you guys find this worth reading...


I wanted to try and explain this with accompanying graphics, but I was unable to easily find suitable images and my skills with drawing software would probably lead to more confusion than clarification. So, I will try to simply use words.

Starting at the beginning...bullets are spun by rifling in order to lend them gyroscopic stability. We ALL know that. I'm not sure I know anyone who has never messed with a gyroscope in some form or another. They behave in curious ways. One of the odd things about a gyroscopically stabilized object is that when something pushes it in a way that would move it out of its gyroscopic plane, it reacts by moving in a direction 90 degrees away from the applied force. The phenomenon is known as "gyroscopic precession."

A bullet that is spun by rifling has a certain amount of gyroscopic stability. The faster it is spun, the greater the gyroscopic force.

A bullet in flight meets aerodynamic resistance. The sum total of the aerodynamic resistance can be expressed as a vector called the "center of pressure." If said bullet is flying in perfect attitude, the center of pressure would be right on center of the meplat. If the bullet gets tipped ever so slightly, then the center of pressure is no longer at the tip of the meplat. For instance, if the nose tips up ever so slightly the center of pressure would now be slightly back from the tip and on the underside of the bullet. All this is stated simply to give an understanding of what the center of pressure is.

A bullet has a center of mass, whether in flight or not. It is a physical property of the bullet and does not change as long as the bullet retains its shape.

So now, imagine a conventional spitzer bullet fired from a smoothbore. Somewhere, and likely not far from muzzle exit the bullet would begin to tumble. This is because the center of pressure is somewhere near the nose of the bullet (at least initially) while the center of mass is further back toward the base of the bullet. The center of pressure will act as a lever and tip the nose in one direction or another. Once it begins to happen the bullet tumbles. It's all about the distance between the center of mass and the center of pressure. Think leverage.

Next consider a spitzer bullet fired from a rifled barrel, only in NO atmosphere. Since the bullet is being spun, the bullet inherits gyroscopic stability. In the absence of atmosphere there is no aerodynamic resistance and consequently no center of pressure. That's just interesting to ponder.

In the real world, our bullets DO encounter aerodynamic forces. A bullet may have a near prefect attitude as it leaves the muzzle, but perfection just does not exist. A center of pressure will form somewhere very near the tip of the near-perfectly formed spitzer bullet, but it is going to be somewhere other than the very point of that wondrous Ballistic Tip (makes a fine example, I think). Since this center of pressure is going to apply a force that is some distance from the bullet's center of mass, it will have a certain amount of leverage and try to tip the bullet in one direction or another. For the sake of illustration let's just say the center of pressure appears at the underside of the nose and wants to tip the nose of the bullet UP.

Since the bullet has a certain amount of gyroscopic stability, it will react to the force exerted on the underside of the bullet's nose trying to tip it up by deflecting the nose of the bullet to the right or the left, depending on the direction of the bullet's spin (it doesn't really matter). Let's just say the nose wants to move to the left instead of up, and it does. As that movement is taking place, the center of pressure has moved to the left also and now the bullet reacts through gyroscopic forces by moving the nose DOWN slightly, and once again the center of pressure moves simultaneously to the top side of the bullet which tries to drive the nose of the bullet down. The gyroscopic force will simultaneously deflect the nose of the bullet to the right.

So if you can follow that, you realize that it's much like a cat chasing his own tail. He can never catch it. In effect, the gyroscopic forces acting on the bullet are playing a never ending game of "keep away" from the center of pressure.

If the gyroscopic force is insufficient, then the leverage of the center of pressure will tip the bullet about its center of mass in one direction or another and the bullet tumbles.

To repeat and amplify: it's much like a cat chasing his own tail. He can never catch it. In effect, the gyroscopic forces acting on the bullet are playing a never ending game of "keep away" from the center of pressure. There is always a bit of error, and it is constantly being addressed by gyroscopic precession. When there is the correct balance between the two forces, the bullet will fly nose first with the axis of its spin tangent to the trajectory.

IF gyroscopic force is much greater than necessary, then the center of pressure's effect is effectively absent. This was something that had to be taken into account when spitzer-shaped gyro-stabilized artillery shells were developed. When rate of twist was too high and the shell was fired at high elevation, the shell would not travel the entire path to the target in the proper nose-first attitude. If you think about that, you realize that this changes up the achieved ballistic coefficient and the calculated trajectory becomes useless. Not only does the shell impact improperly (they are designed to function hitting a target nose first) and fail to perform as designed, but the target is most likely missed all together.

Personally, have had many exchanges with shooters who simply repeat what authorities say, which is "overstabilization of bullets does not exist." I can imagine more than one reason these authorities might say that. One that comes to mind is that the rate of twist required to overstabilize a typical spitzer bullet is many times faster than anyone would dream of using...what that rate would be, I have NO idea. Another reason could be that the effects of overstabilzation (as in missing targets because of unpredictability of trajectory) won't be apparent unless we're trying to hit a target 2500 yards or more distant. I think another reason is that guys like Litz really just see the subject as a can of worms they don't want to open because they don't want to explain it.

I'd like to understand the subject better myself, but the internet seems to be more of a hindrance than aid. Searches turn up all kinds of arguments on internet forums, and NEVER a good explanation of how the whole thing works. If anyone knows of an available AUTHORITATIVE resource that goes deeper, I'd love to know about it.
 
Well. Three days up, 80 views, and not a single comment. I have to wonder if what I've said makes no sense to anyone but myself. Maybe I failed to explain some key point. Did I leave something important out of the explanation??
 
I guess I had a hard time understanding your theses. Was it that over stabilization does exist? If that was your point, you kind of discounted it by highlighting you would have to be in these "out of boundary" conditions, like super fast twist (1:4 or 1:5 maybe) that no one would use. Or extreme long ranges (2500 yards in your example) which is a-typical even for most long range shooters.

Good description of the forces acting on the bullet though! It was just hard for me to understand where you were going with this.
 
It's just - not something I'm concerned about.

I'm happy as all get out to take game out to 300 - 400 yards.

Will gladly take shots at coyotes and other vermin at longer ranges.

Used to compete in prone competition at 600 yards mostly. Sometimes farther.

Helped train USMC Scout Snipers. We never worried about 1,000+ yards. We'd call for mortars, artillery, naval gunfire, or air support out there. The 7.62 NATO was almost useless out there...

Was a SWAT/Law enforcement sniper for many years. Never worried about over-stabilization. Did real well in tests and competitions. Confined myself to 600 yards and in. Figured I'd NEVER have to make shot past that, and was right. Never had to take "the shot." Thankfully.

So... Your info is kind of interesting, but only in a theoretical way. Am 60 years old now. Been shooting and hunting most of my life, at some fairly high levels. NRA "high master" in long-range prone is about the best example I can give. And NOTHING in what you posted mattered at all to me and my shooting.

It's interesting, but pretty much irrelevant to a lifetime of shooting that included wearing out rifle barrels...

Therefore - I've got nothing to add to your post.

Regards, Guy
 
Made me wonder if gyroscopic precession is the reason for certain rifles outshooting the predicted trajectory of their load or inexplicably drifting in a certain direction. I have 2 of those, both military rifles.

One of my K-31s seems to shoot unnaturally flat. This is a load I have a lot of experience with in various rifles. 155 CC at 2600fps. Elevation calls differ by 1MOA to 300m from other rifles with the same bullet at the same velocity until about 600 yards where it seems to stabilize or not be significant enough to notice. Others on here have noted similar phenomena.

I also have a mosin nagant sniper that has about 1.5 MOA of unnatural left "wind" to 600 yards.
 
I agree with the thesis, and external stabilization forces certainly exist in ballistic trajectories. Certain missiles have a ballistic flight/trajectory for a substantial portion of a mission flight, but those are different applications.

For a bullet that is well constructed/balanced I don't think you can over twist. Over the course of 600 yards (farther than I would shoot at an animal) I don't think the trajectory is much affected by the extra bullet rotations. One might note an accuracy difference in bench rest applications, but I would posit you wouldn't see any difference in the field.


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Interesting stuff to read and a good explanation.

A bit nuanced and not terribly relevant to me, because a load either shoots well to 500-600 yard or it doesn't.

Since I am not a target shooter, the interesting stuff starts for me after a bullet hits the animal. i am more interested in the terminal side of things.
 
The purpose of the post is to attempt to explain why a bullet will fly with its rotational axis tangent to the trajectory arc following a comment on the thread Fotis started.

It IS pretty much just a theoretical discussion. Those who attempt shots at the ranges required for over stabilization to manifest are equipped with the right stuff to begin with, so it's not something that's commonly observed. Even so, one of the only ways I can think of that you could verify over stabilization would be to shoot at a target at sufficient range and observe bullet holes showing a consistent nose-up attitude, which is very problematic in that you'd have a very hard time hitting the paper given the unpredictable trajectory. As I've said, I have no idea just how fast you'd have to spin the bullet to make that happen.

If you think about it, we've all seen the same effect, though, in a different venue. If you watch football you'll see a nice pass thrown to a receiver where the nose of the football follows the trajectory of its arc and the ball ends up right where the QB intended. We've also seen long passes come floating down where the spin axis of the ball does NOT remain tangent to the ball's trajectory. Too much spin on the ball.

I know some guys don't care to ponder this stuff, but I am among the other group. While I have absolutely NO interest in banging steel at 1000 yards or tagging a goat at such ranges, I do find certain questions fascinating and I enjoy learning the answers to certain mysteries. Each and every one of us has a unique approach and outlook to this hobby. The last thing I'll look at in selecting a bullet is its BC, but certain aspects of external ballistics are too intriguing to ignore. The same goes for internal and terminal ballistics.
 
My only experience with overstabilization, in terms of the immediate and very noticeable failure of the bullet to withstand the forces involved, was with a Swedish M96 6.5x55 and the 123 grain Hornady Amax. About 10 percent wouldn't make it to the 200 yard mark, disappearing into a poof of smoke and a spiderweb of contrails at 100-150 yards. Interestingly, the other 90 percent shot very accurately and didn't leave any evidence of pending failure on the target or cardboard backer, but did violently fragment before impacting the berm some 40 yards behind.
 
Those bullets may or may not have been overstabilized. They were overspun, for sure.
 
So what you are referring to is instability in the pitch axis versus either roll axis or yaw axis?
In other words the roll axis is fine or stable but the pitch axis can still be unstable despite stability in the roll? Thus the nose up profile?
Just a question. Wouldn't increased roll conceivably help to stabilize the pitch?
Also wouldn't stability in the pitch axis decline almost exponentially at a given distance with decreasing speed? Even though the roll axis, while slightly decreasing in speed, would not become unstable as quickly as the pitch?

I'm assuming that due to distance required to make the pitch axis unstable, this is not accounted for in stability calculators? Only the roll axis?

While I agree with Guy that it is pretty much a non-issue at the distances I shoot, it is interesting to ponder. Thanks for the post!
 
Well, there is no roll axis. The bullet's spin IS the roll axis, if you want to exercise liberty of imagination. The nose of the bullet can be in error in either pitch or yaw, or a combination of the two.

In search of an illustration, I ran across a webpage I had never seen. I think it might help clarify some stuff:

http://www.cheytac.com/2015/11/17/balance-flight/
 
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