Spin Drift calculations

[Richard H Brown Jr]

Who knows, maybe someone will now come up with a question on how a badly manufactured bullet with it's center of gravity off the center line of the bullet, will effect the path of the bullet out at 1,000yds or more. "Shooting! Bam! Uh, Short and Left, Come up 2 and right 3! Shooting! Bam! Hit! Leg below knee! Shoot Again!"

Of course with a 155mm, you just have to get withing CEP. Blast and Fragments due the rest.

[Johnny P]

Years ago my son was getting old enough to want a hunting rifle. He picked out a Winchester in .308, and I bought him a supply of .308 ammo from a local commercial reloader to practice with. Our first trip to the range was a disaster, as after bore sighting the rifle grouped in a 12 inch circle at 100 yards. Some groups were slightly smaller and some were larger. We called the store and they told us to bring the rifle back. We traded for a Ruger heavy barrel in .308 but the results were the same. It finally occurred to me that it could be the ammo, so we fired three shots with commercial ammo and the bullet holes were almost touching. At the time I remembered seeing an ad in the Shotgun News for FN bullets at a ridiculously low price in lots of 5000, and the reloader admitted to buying several thousand of them. It turns out that the jackets were not uniform and the result was a pattern instead of a group.

[PhillipM]

Years ago my son was getting old enough to want a hunting rifle. He picked out a Winchester in .308, and I bought him a supply of .308 ammo from a local commercial reloader to practice with. Our first trip to the range was a disaster, as after bore sighting the rifle grouped in a 12 inch circle at 100 yards. Some groups were slightly smaller and some were larger. We called the store and they told us to bring the rifle back. We traded for a Ruger heavy barrel in .308 but the results were the same. It finally occurred to me that it could be the ammo, so we fired three shots with commercial ammo and the bullet holes were almost touching. At the time I remembered seeing an ad in the Shotgun News for FN bullets at a ridiculously low price in lots of 5000, and the reloader admitted to buying several thousand of them. It turns out that the jackets were not uniform and the result was a pattern instead of a group.

I was given some bullets that possibly came from that batch. I was in disbelief my rifle and I were shooting trash can sized groups at 100 yards!

[rickgman]

Hi Herschel, Check out the table below for cal .30 M2 Ball ammo. As you can see, until the range gets to be substantial, spin drift is zero or very small. So for most shooting, one can neglect spin drift - there are much greater sources of error tha must be dealt with like wind drift. Nonetheless, the phenomenon does exist. Rick

Range Elevation Spin Drift
Yards MOA MOA
100 2.4 0
200 5.1 0
300 8.1 0
400 11.5 0.3
500 15.5 0.3
600 20.3 0.7
700 26 0.7
800 32.4 0.7
900 40.2 1
1000 49.3 1.4

[rickgman]

Let me try that table one more time.

Range Elevation Spin Drift
Yards MOA MOA

100 2.4 0
200 5.1 0
300 8.1 0
400 11.5 0.3
500 15.5 0.3
600 20.3 0.7
700 26 0.7
800 32.4 0.7
900 40.2 1
1000 49.3 1.4

[Johnny in Texas]

The Coriolis effect is only present while shooting down the bath tub drain at 1000 yrds.

[Rick]

But you would have to admit the water spins counterclockwise. So a left hand twist barrel would make less splash. Going with the flow always causes less turbulence even if its not the right thing to do.

[Herschel]

rickgman,

I don't doubt that spin drift is real. My point I was trying to make is that when you determine your 600 yard "no wind" zero the wind drift factor is accounted for when you obtain this zero. The wind drift will be the same from shot to shot. Changing bullet weights or types, I suppose, could change the wind drift. Do I make sense?

[rickgman]

Herschel, Yep, that makes sense. Rick

[Johnny P]

At 600 yards it probably wouldn't make enough difference to adjust for. The rotational drift doesn't start until the bullet reaches the top of it's arc and starts to nose over. The air under the bullet tries to keep the bullet nose up, and the bullet drifts in the direction of rotation.
Here is one that it would affect at 600 yards. The rear sight of the LP.08 "Artillery Model" was adjustable out to 800 meters. As the rear sight is raised it moves to the left to compensate for the rotational drift, which was apparently quite a bit with the short round nose 9mm bullet. The transverse pin in the front of the sight is drilled eccentric, causing the sight to move to the left as raised.

[fkienast]

True. Drift occurs and it is always in the direction of the rifling. The drift varies with velocity and is a function of range i.e. barrel elevation. Drift is accounted for with a windage correction. When firing the total windage correction includes corrections for drift, weather and Coriolis azimuth effect (but not the rotation range effect). These corrections will be minimal due to the short range. I always understood that drift occurred at the summit of the trajectory and it was transitioning into the descending branch of the trajectory.

[Sunray]

"...spin drift is the least of your worries..." It's the least worry anyway. A wee bit of wind will move a 150. 1,000 yard bullets they ain't.

[rickgman]

fkienast, It is a common misconception that spin drift only occurs when the peak of the trajectory is passed. It is imparted by increasing projectile deceleration coupled with the spin of the projectile. Increased deceleration caused the projectile to nose over to a greater extent. A projectile begins decelerating immediately after leaving the barrel but the rate of deceleration increases substantially at incresased range. Rick

[fkienast]

I believe it occurrs while the projectile is still oriented nose up, at the summit of the trajectory, and a cushion of dense air forms under the rotating projectile as it is trying or beginning, to descend, and at this point, the projectile rolls in it's direction of its spin. At least that is the way it was taught at the artillery school in the '90's. At any rate, it is usually minimal for a direct fire weapon.

[Johnny P]

It is explained in Hatcher's Notebook, but might take a while to find it.