Spin Drift calculations

[Marine A5 Sniper Rifle]

150 grain spitzer at 2800 fps at 1000 yards in a 1903 Springfield will have a spin drift of 8 inches to the right. Right or wrong? If wrong, what is the correct answer?

Jim

[Richard H Brown Jr]

Day, night, desert, open farm land/prarie, jungle, swamp, mountains, urban, city, summer, fall, winter, spring, dry, drizzle, rainy, thyphoon, light snow, medium, heavy, avalanche, still, light winds, gale, hurricane? Or going with scientificially impossible, free space, null gravity?

[Richard H Brown Jr]

Did a quick google search and came up with this page and links. http://www.snipercountry.com/HotTips/Spin_Drift.htm INCLUDING a calculator program.

http://www.povn.com/~4n6/index.htm 404's out. http://www.nennstiel-ruprecht.de/bullfly/index.htm scientific study, and you need basic physics background.

First post was just to start conversation.

RHB

[Marine A5 Sniper Rifle]

Thanks, Richard. I was wondering for a moment, because none of the things you mentioned directly effect spin drift, but maybe indirectly if they effect velocity, angle of yaw or spin rate. If the poster is correct, and Spin Drift is 0.25 to 0.50 MOA, it is a force with which to be reckoned at 1000 yards, yet I never see it mentioned in discussions on external ballistics. It does appear to be less than I originally assumed, but still a correction that needs to be made.

Thanks again.

Jim

[Maury Krupp]

For starters a 150gr bullet going 2800fps (which is about all you'll get out of the 24in barrel of an M1903) will go subsonic before it gets close to 1000yd so spin drift is the least of your worries.

I shoot a 175gr bullet out of an M1 at approx 2640fps and don't see any spin drift off my normal no-wind zero. Maybe because it's down within the noise of my hold but none of the good 1000yd shooters I know ever appear to calculate it or even mention it as a factor.

Those good shooters are able to pretty much stay in the 2MOA (20in) 10-ring with a fistfull of Xs (10in). If you're looking for more precise shot placement than that then spin drift might be more of a factor; otherwise forget about it.

Maury

[pmclaine]

Just a thought. If you measure the compensation of the slide on the 1903 sight could you extrapolate what the designers used for spin drift when they cut the ladder. Than again was the ladder cut for the 30-03 and the calculations would not apply to 30-06.

[Rick]

This is so far over my head but just for discussion you would have to think of the coriolis effect. It spins one way in the Northern Hemisphere and the opposite way in the Southern. So in the our Northern Hemisphere it would make a difference if you were shooting East or West. A left hand spin barrel would work best in the Northern Hemisphere and a right hand best in the Southern Hemisphere.

[Johnny P]

The Model 1873 Trapdoor with the Buffington sight also had the compensation for rotational drift built into it. The slow moving, large round nose bullet had quite a bit of rotational drift.

http://i53.tinypic.com/2n8xa4o.jpg

[Liam]

"A left hand spin barrel would work best in the Northern Hemisphere and a right hand best in the Southern Hemisphere."

Hmm....my 1910 dated Norwegian Krag has left-hand twist. I'm going to shoot it this weekend!

http://i164.photobucket.com/albums/u23/Liam_068/kra_0021-16.jpg

[Rick]

In theory you have a leg up as you are spinning with the Coriolis effect instead of fighting it. On the other hand there is a good chance that I have no idea what I'm talking about. Hopefully someone with more knowledge will post.

[Johnny P]

If you were shooting several thousand miles the coriolis effect would effect impact, but not shooting a few hundred yards. In effect the rotation of the earth has moved your target by the time the bullet gets there due to the long flight time. If you were standing on the North Pole and going to shoot straight at Cuba, you would probably hit somewhere between Cuba and Yucatan.

[rickgman]

Gentlemen, Spin drift and the Coriolis effect are two different phenomenons. The Coriolis effect is as Johnny mentioned the effect that occurs when the target moves due to the rotation of the earth while the projectile is airborne. This is significant when dealing with artillery but not small arms. Spin drift, on the other hand, is the effect of gyroscopic action on a spin stabilized projectile in flight. The projectile in flight assumes an attitude in which the nose is oriented slightly toward the direction of spin and slightly downward. Also, the nose of the projectile orbits in a very small arc. Due to the projectile orientation combine with its orbiting motion, the projectile movers further sideways on the down swing of the arc that on the up swing of the arc. This effectively causes the projectile to drift in the direction of the spining motion. Spin drift is definitely more pronounced as range increases. Hope that short explanation helps. Rick

[Liam]

This makes my head hurt. I'm still going to shoot the Krag this weekend. ; )

[Herschel]

Back in my hipower shooting days, with M1 and M14, I knew my 600 yard zero. That would have taken into account the spin drift to get the zero. Wind and light would have a greater effect than the spin drift. I shot well enough to earn the Distinguished Rifleman Badge and rubbed shoulders with some really good shooters. Never heard any mention of spin drift from any of them. On the few occasions when I shot at 1,000 yds I just moved the elevation up without any change in the windage, except adjusting for wind and light as mentioned above.

[Johnny P]

I believe Hatcher said it was only seven inches at 1000 yards for the service round.

[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.

http://i54.tinypic.com/zlyvyt.jpg

[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.

[rickgman]

Gentlemen, It is necessary to distinguish between a projectile's attitude relative to the horizon vs. its flight path. In flight, the nose of the projectile is actually very slightly high relative to the flight path (but not necessarily above the horizon). The result is that air pressure not only works on the front of the projectile but slightly from underneath. Since this point of action is forward of the center of gravity, it tries to force the nose even higher. However, the gyroscopic action induced by spin on the projectile forces the nose in an arc downward and toward the direction of rotation. As the nose of the projectile swings in a small arc, it moves further toward the direction of rotation on the downswing than it moves in the opposite direction on the upswing. Thus, spin drift is created. Rick