RiverRider
Handloader
- Dec 9, 2008
- 1,450
- 103
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.
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.
