Uncovering the Mystery of Bullet Flight: Investigating the Helix Theory

[DRice.72]

I have been involved in a discussion concerning the behavior of a bullet during flight. It seems there are long distance shooters who expirence a better MOA at 300+ yards than at 100 yards. The theory pit forth is that when a bullet leaves the muzzle, it is not stable in its flight. This lack of stability causes the bullet to fly in a helix type pattern. This helix occurs around the "true" trajectory. Over time, as the initial velocity decreases the bullet settles into a proper spin, or what these distance shooters refer to as "goes to sleep". Is this possible? If so, where might I find some scientific articles, or other types of research I may be able to read?

Thanks in advance for your response.

 

[Bob S]

I believe a long range shot will pull to the right with right-hand rifling, and to the left with left-hand rifling. I also recall that the coriolis force "pushes" the bullet to the right in the northern hemisphere. Some guns and cannons have left-hand rifling to improve accuracy in the northern hemisphere. Google " coriolis force left-hand-twist rifling"

Bob S

[added] The coriolis force of course doesn't "push" the bullet; the Earth rotates underneath.

 

[tuoni]

There are three separate variables that describe a bullet's stability:

• static stability — also called gyroscopic stability; a bullet stable in this regime will not tumble as it flies through the air, and is primarily dependent on angular velocity, i.e. bullet spin. This is why rifling twist and muzzle velocity is important for accuracy.
  
• dynamic stability — as the bullet exits the muzzle, an angle of yaw is induced, which for a dynamically stable bullet is dampened over time, and is primarily dependent on bullet shape. An easily recognised characteristic of a yawing bullet is that the entry wound is elliptic rather than circular. NOTE: The presence of yaw does not necessarily indicate instability.
  
• tractability — a bullet that is tractable is able to turn along and follow its trajectory, i.e. if fired at an angle, the bullet will be able to turn its nose down when the trajectory begins to descend. An intractable bullet will travel base first.

http://www.nennstiel-ruprecht.de/bullfly/stab.htm
http://www.nennstiel-ruprecht.de/bullfly/fig12.htm
http://www.nennstiel-ruprecht.de/bullfly/fig21.htm

The helical flight path is not an indication of instability, and is experienced by all bullets. It may be what makes it easier for them to shoot with greater accuracy at longer ranges. However, for most properly designed bullets the yaw usually becomes insignificant at around 5000 calibres, for a 9 mm bullet, this would be a distance of 45 m (or 49 yd).

It is also worth noting that velocity is not the primary factor, but radial rotation, i.e. spin. Muzzle velocity is important because it determines bullet spin, however, once the bullet is spinning, the angular velocity decreases far more slowly than the bullet velocity. So velocity is not that important during flight, as long as the bullet is statically stable at the muzzle, one can assume it to be stable for the rest of the flight.

 

[DRice.72]

Thanks for the responses!

The helical flight path is not an indication of instability, and is experienced by all bullets. It may be what makes it easier for them to shoot with greater accuracy at longer ranges.

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how do we know this? This helical path is at the core of the debate. It is said by shooters to cause the increased accuracy at longer ranges. This is the core of the debate. I like to see any scientific articles related to research on this.

 

[tuoni]

As the bullet leaves the muzzle, it is in a transitional state, where it no longer is inside the bore but still being propelled - and otherwise affected - by the escaping gases. This transitional state is where yaw and the helical path is induced. It is also worth noting that this is why rebated boat tail bullets are more accurate and regarded as superior to flat or boat tail bullets. Rebated boat tail bullets can also gain up to 10% in muzzle velocity.

The dynamic stability is primarily dependent on bullet shape and construction:
http://www.nennstiel-ruprecht.de/bullfly/dynacond.htm

As I said, for a dynamically stable bullet, this helical path and angle of yaw is dampend over time, so it will be the highest at the muzzle. For a dynamically unstable bullet the helical path and angle of yaw increases, and it may or may not be the lowest at the muzzle.

This is know through empirical experiments and rather advanced aerodynamic theory and calculations. I don't have any scientific papers that describe bullet stability in great detail, so you'll have to find that on your own. Such advanced topics might be difficult to find on the net though. It would be better to go to your library, ask professors/teachers are some university, or try to find books which describe ballistics in greater detail.

 

[DRice.72]

Thank you very much! I'll keep looking, all of your responses have been very helpful!