Spin drift question

[Mikelarry]

Will spin drift take the round to the right or left? Right hand twist right drift?

[danmellinger]

It can take it in any direction depending on your orientation relative to the spin of the planet.

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[Mikelarry]

It can take it in any direction depending on your orientation relative to the spin of the planet.

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Coriolis and spin drift are two different effects.

[WolfInSheeple'sClothing]

The direction of gyroscopic drift (spin drift) depends on the rifling of the barrel. If the rifling is clockwise the bullet will drift to the right, or for counterclockwise rifling the bullet will drift to the left. This is do to the yaw induced by the by the axis of rotation. Since the axis is tilted up and to the right for clockwise (right hand rotation) the airstream will push on the leading edge of the bullet causing it to veer off to the right. Picture the bullet as being stationary and the air as moving to help visualize it.

ETA: Drift caused by the Coriolis effect depends on the hemisphere in which the shot is taken. Since the earth rotates counterclockwise, a shot the Northern Hemisphere will drift to the right. The opposite applies in the Southern Hemisphere.

[Mikelarry]

The direction of gyroscopic drift (spin drift) depends on the rifling of the barrel. If the rifling is clockwise the bullet will drift to the right, or for counterclockwise rifling the bullet will drift to the left. This is do to the yaw induced by the by the axis of rotation. Since the axis is tilted up and to the right for clockwise (right hand rotation) the airstream will push on the leading edge of the bullet causing it to veer off to the right. Picture the bullet as being stationary and the air as moving to help visualize it.

ETA: Drift caused by the Coriolis effect depends on the hemisphere in which the shot is taken. Since the earth rotates counterclockwise, a shot the Northern Hemisphere will drift to the right. The opposite applies in the Southern Hemisphere.

This can't be true a shot taken below the equator will not result in the round going left but not as far to the right if it was a clockwise twist. The round wants to spin right like a top.

[50 Shooter]

How far are you shooting?

Less then 1K it wouldn't enter my mind or coriolis either, extreme distances beyond 1,500 then you might. The biggest thing I would be worrying about is wind.

[danmellinger]

Very true. Might have been a little drunk while browsing.

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[Dead*Reckoned]

warning* this post includes actual science

spin drift really only becomes apparent at the longer ranges, because the angle of attack on the bullet is higher at those ranges. the bullet will always arc, but the nose of the bullet may not drop towards the far end of the arc. This nose high attitude exposes the underside of the bullet to the airstream more so than on the top. With a clockwise twist, the bullet will want to roll to the right. It's pretty simple actually.

Spin drift has a lot to do with the bullet design ( will the nose drop at the far end of the arc?, due to center of gravity and pressure relationships.) Once you you get past the effective range of the certain cartridge you are shooting, and you loose energy the bullet will bottom out and begin to drop at increasing rate, especially if the bullet drops subsonic or transonic

[Cactus_Tim]

Sheesh, and I always thought spindrift was spray blown from waves by gale force winds.

Oh, I see, you're discussing the Magnus effect.

The Magnus effect can also be found in advanced external ballistics. Firstly, a spinning bullet in flight is often subject to a crosswind, which can be simplified as blowing either from the left or the right. In addition to this, even in completely calm air a bullet experiences a small sideways wind component due to its yawing motion. This yawing motion along the bullet's flight path means that the nose of the bullet is pointing in a slightly different direction from the direction in which the bullet is travelling. In other words, the bullet is "skidding" sideways at any given moment, and thus it experiences a small sideways wind component in addition to any crosswind component.

The combined sideways wind component of these two effects causes a Magnus force to act on the bullet, which is perpendicular both to the direction the bullet is pointing and the combined sideways wind. In a very simple case where we ignore various complicating factors, the Magnus force from the crosswind would cause an upward or downward force to act on the spinning bullet (depending on the left or right wind and rotation), causing an observable deflection in the bullet's flight path up or down, thus changing the point of impact.

Overall, the effect of the Magnus force on a bullet's flight path itself is usually insignificant compared to other forces such as aerodynamic drag. However, it greatly affects the bullet's stability, which in turn affects the amount of drag, how the bullet behaves upon impact, and many other factors. The stability of the bullet is affected because the Magnus effect acts on the bullet's centre of pressure instead of its centre of gravity. This means that it affects the yaw angle of the bullet: it tends to twist the bullet along its flight path, either towards the axis of flight (decreasing the yaw thus stabilizing the bullet) or away from the axis of flight (increasing the yaw thus destabilizing the bullet). The critical factor is the location of the centre of pressure, which depends on the flowfield structure, which in turn depends mainly on the bullet's speed (supersonic or subsonic), but also the shape, air density and surface features. If the centre of pressure is ahead of the centre of gravity, the effect is destabilizing; if the centre of pressure is behind the centre of gravity, the effect is stabilizing.

[WolfInSheeple'sClothing]

This can't be true a shot taken below the equator will not result in the round going left but not as far to the right if it was a clockwise twist. The round wants to spin right like a top.

Spin drift depends on the rifling of the barrel (right or left hand twist). The Coriolis effect (vertical) depends on the latitude at which the shot is taken. You would have to take into account both effects to get the net drift of the bullet. These effects only matter at extreme long range. There are also many other "effects" to take into account as well, but they are very insignificant and lead to a very complicated equation of motion. This isn't something I'm pulling out of my butt. I did my senior project/thesis for my physics degree on external ballistics. It can get messy, this is just a generalization.

[Mikelarry]

How far are you shooting?

Less then 1K it wouldn't enter my mind or coriolis either, extreme distances beyond 1,500 then you might. The biggest thing I would be worrying about is wind.

1000+
At 1000 it will drift 8"

[Mikelarry]

warning* this post includes actual science

spin drift really only becomes apparent at the longer ranges, because the angle of attack on the bullet is higher at those ranges. the bullet will always arc, but the nose of the bullet may not drop towards the far end of the arc. This nose high attitude exposes the underside of the bullet to the airstream more so than on the top. With a clockwise twist, the bullet will want to roll to the right. It's pretty simple actually.

Spin drift has a lot to do with the bullet design ( will the nose drop at the far end of the arc?, due to center of gravity and pressure relationships.) Once you you get past the effective range of the certain cartridge you are shooting, and you loose energy the bullet will bottom out and begin to drop at increasing rate, especially if the bullet drops subsonic or transonic

Thanks

[WolfInSheeple'sClothing]

1000+
At 1000 it will drift 8"

Yeah spin drift is a much more significant effect than the Coriolis effect. However, everyone always thinks of the Coriolis b/c they talk about it in movies and games. Other considerations include the Magnus effect, the curvature of the earth, warmth of the powder, transonic effects and much more. This is why we have ballistic cpus, and why one should always keep a range book. The full equation of motion for a spinning shell has 10 coefficients (30 before symmetry is taken into account), can only be solved numerically, and takes hours for a desktop computer to solve.

[WolfInSheeple'sClothing]

especially if the bullet drops subsonic or transonic

All hell breaks loose when a bullet goes transonic!