Bullet drift?

[johara1]

A 1 mph wind that you can't see can move the bullet 5" at 1000, spin drift in a very accurate rifle means nothing, barrel quality means a lot more..... jim

 

[JEFFPPC]

What happens to that spin drift if I go to a slower twist barrel, or a faster twist barrel? How much is it affected.

 

[johara1]

Hey Jeff I wonder if you have canted lands does it cut down on the spin drift..... ?? ...jim

 

[rwj]

I screwed up my math above, my spin drift at 1k is 0.75 moa, or 7.5 inches roughly and not 1.5" .

For the 3 oclock or 9 oclock scenarios, the spin drift gets blended into the numbers and ignoring it works fine in most disciplines. But if I have to make a cold bore shot at 1,300 yards, my spin drift is about 12 inches. If I don't calc that in along with the wind, I am that far off to the right. If I get to shoot sighters and walk it in, it doesn't really matter. In tactical and ELR you really need to center up on shot 1 so its a necessity.

Interesting... is spin drift only a factor in windage?

 

[JEFFPPC]

Ask the OP.

 

[FeMan]

Interesting... is spin drift only a factor in windage?

Coriolis has both vertical and horizontal components but I believe spin drift is a horizontal phenomenon only. I have heard people claim there is a vertical component to it but considering the physics involved the claim of vertical doesn't make sense to me.

 

[rwj]

Coriolis has both vertical and horizontal components but I believe spin drift is a horizontal phenomenon only. I have heard people claim there is a vertical component to it but considering the physics involved the claim of vertical doesn't make sense to me.

What would limit a spinning bullet to influence only one plane of its flight? Coriolis is the influence of the earth’s rotation on a bullet’s point of impact.

 

[338 dude]

Spin drift and coriolis effect are not the same thing. Coriolis has both vertical and horizontal components but is negligible at the ranges most of us shoot. I love shooting matches in switchy head winds where people don't dial spin drift

Research something else called the Magnus effect it to contributes to vertical and horizontal changes

 

[FeMan]

Spin drift is created as the bullet starts to drop during flight. As it drops and accelerates downward from gravity, the spin results in air at the boundary of the bullet on the right side to be is traveling faster that it is on the left side in a right-hand twist. Because of the spin of the bullet, the right side is going against the air flow as the bullet drops and the left side is going with the bullet. The comparative higher velocity at the bullet jacket on the right creates lower pressure compared to the low velocity on the left side. Although it is a little different than a wing on a plane, it is similar where the split air flow on the top of the wing results in the air traveling faster than it does on the bottom and thereby creates lift. This pressure gradient from spinng in the air as it drops is what causes the bullet to start moving right. In terms of a vertical component, since the air on the boundary layer at the top and bottom of the bullet as it drops (in a no-wind condition) is the same there is no vertical force imparted on the bullet from spin.

The Magnus effect is similar but it is less and is caused by wind instead of the acceleration of gravity. The Magnus effect gives vertical instead of horizontal movement to the bullet. Similar to spin drift, a left wind passes the top bullet in the same direction as the spin and the wind is opposing at the bottom of the bullet. This creates a pressure gradient in the same way as spin drift, but it is vertical. In a left wind, Magnus causes the bullet to hit low and in a right wind it will hit high (again, it right twist barrels). It is relatively minor but can cause you to have to add or subtract a click you otherwise wouldn't need at 1k.

 

[gcabrera]

Spin drift is created as the bullet starts to drop during flight. As it drops and accelerates downward from gravity, the spin results in air at the boundary of the bullet on the right side to be is traveling faster that it is on the left side in a right-hand twist. Because of the spin of the bullet, the right side is going against the air flow as the bullet drops and the left side is going with the bullet. The comparative higher velocity at the bullet jacket on the right creates lower pressure compared to the low velocity on the left side. Although it is a little different than a wing on a plane, it is similar where the split air flow on the top of the wing results in the air traveling faster than it does on the bottom and thereby creates lift. This pressure gradient from spinng in the air as it drops is what causes the bullet to start moving right. In terms of a vertical component, since the air on the boundary layer at the top and bottom of the bullet as it drops (in a no-wind condition) is the same there is no vertical force imparted on the bullet from spin.

The Magnus effect is similar but it is less and is caused by wind instead of the acceleration of gravity. The Magnus effect gives vertical instead of horizontal movement to the bullet. Similar to spin drift, a left wind passes the top bullet in the same direction as the spin and the wind is opposing at the bottom of the bullet. This creates a pressure gradient in the same way as spin drift, but it is vertical. In a left wind, Magnus causes the bullet to hit low and in a right wind it will hit high (again, it right twist barrels). It is relatively minor but can cause you to have to add or subtract a click you otherwise wouldn't need at 1k.

Now I’ve got the drift
Seriously FeMan, great explanation. Thank you.

George

 

[rogina]

What would limit a spinning bullet to influence only one plane of its flight? Coriolis is the influence of the earth’s rotation on a bullet’s point of impact.

Gravity?

 

[rwj]

1. Spin drift is created as the bullet starts to drop during flight. As it drops and accelerates downward from gravity, the spin results in air at the boundary of the bullet on the right side to be is traveling faster that it is on the left side in a right-hand twist. Because of the spin of the bullet, the right side is going against the air flow as the bullet drops and the left side is going with the bullet. The comparative higher velocity at the bullet jacket on the right creates lower pressure compared to the low velocity on the left side. Although it is a little different than a wing on a plane, it is similar where the split air flow on the top of the wing results in the air traveling faster than it does on the bottom and thereby creates lift. This pressure gradient from spinng in the air as it drops is what causes the bullet to start moving right. In terms of a vertical component, since the air on the boundary layer at the top and bottom of the bullet as it drops (in a no-wind condition) is the same there is no vertical force imparted on the bullet from spin.
   
   The Magnus effect is similar but it is less and is caused by wind instead of the acceleration of gravity. The Magnus effect gives vertical instead of horizontal movement to the bullet. Similar to spin drift, a left wind passes the top bullet in the same direction as the spin and the wind is opposing at the bottom of the bullet. This creates a pressure gradient in the same way as spin drift, but it is vertical. In a left wind, Magnus causes the bullet to hit low and in a right wind it will hit high (again, it right twist barrels). It is relatively minor but can cause you to have to add or subtract a click you otherwise wouldn't need at 1k.

   
   Considering that for approximately half of the distance to the target the bullet is accelerating upwards to its apogee, the differential pressure will act in the opposite direction than during its downward flight to the point of impact. Without actually calculating values, it would seem that the net effect on the POI will be negligible...

 

[FeMan]

Considering that for approximately half of the distance to the target the bullet is accelerating upwards to its apogee, the differential pressure will act in the opposite direction than during its downward flight to the point of impact. Without actually calculating values, it would seem that the net effect on the POI will be negligible...

There is an initial upward trajectory imparted by the angle of the muzzle but no upward acceleration. There is only downward acceleration, which starts as soon as the bullet leaves the muzzle.

To quantify drift and get a feel for it, go here http://www.jbmballistics.com/ballistics/calculators/calculators.shtml and open the "Trajectory - Drift" calculator. Insert all your rifles data, set the wind speed to zero and click "Include Spin Drift" at the bottom. This will calculate your bullet trajectory in a no wind condition. The resulting Windage in the table is your actual spin drift. With my 284, I am 6-inches right at 1k, 12 inches at 1,300 yards and over 40 inches at 2k.

It is absolutely negligible at first but it grow exponentially at long ranges.

 

[JFrank]

@johara1
How do ya judge the wind ya can’t see ?

 

[Mulligan]

@johara1
How do ya judge the wind ya can’t see ?

That’s what whiskers, ear hair, and nose hair is for!
CW

 

[JFrank]

I been using the invisible wind flags, so now I know the secret
Thx Clay

 

[johara1]

That’s what whiskers, ear hair, and nose hair is for!
CW

thats about where you are and nothing else, and seeing the mirage in your rifle scope shows you the wind at the target. now learn how to see what counts on the way down to the target is the answer..... jim

 

[JFrank]

thats about where you are and nothing else, and seeing the mirage in your rifle scope shows you the wind at the target. now learn how to see what counts on the way down to the target is the answer..... jim

Jim
I need a wind coach, what’s your schedule look like ?

 

[johara1]

Going in for an operation in September and I should either be able to see or get a long white cane...... Going to Reade to shoot tomorrow and if it weren't for SSSV ED Sightron I couldn't have shot at all this year plus a spotting scope to see flags and mirage..... it's hell to get old, ..... jim

 

[JFrank]

Old Farts Rule..