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Old July 7, 2013, 04:50 PM   #1
tobnpr
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Compensating for spin drift...

I had always thought this effect was "negligible", meaning don't try to compensate for it in the scheme of other minor variables.

But I saw something today that stated it was as much as 1/2 minute at 500 yards- and about a full minute at 1000, for a "typical" .308 bullet.

Certainly, not negligible. Also seems that it's near impossible to calculate precisely as there's too many unknowns with bullet designs.

One recommendation was to zero at 500-600 yards. It'll shoot a few inches left at very short ranges, and a few inches right at 1000. But, while that's fine for banging steel at 600 and beyond, it would require re-zeroing if punching paper or doing load workups at shorter ranges.

How do you address this?
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Old July 7, 2013, 05:18 PM   #2
Jim Watson
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I just turn the knobs on the sight or scope until the group is centered on the target.
I keep notes on sight settings for different ranges, windage as well as elevation.
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Old July 7, 2013, 07:00 PM   #3
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That would be fine, if I could shoot as good as the rifle.
When the bullet doesn't go where intended, I need to be able to rule out whatever factors I can...
If I can't quantify spin drift, I don't know how much of the "miss" is drift, and how much was a bad wind call.
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Old July 7, 2013, 08:20 PM   #4
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Looks to me if you've done all your homework and made notes, you'd always know the spin drift vs. distance. So, what's left is the wind.

I guess I'd do spin calibrating for 500 and 1,000 yards on a very calm day or on a day when a steady wind is in my face or at my back.
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Old July 7, 2013, 10:59 PM   #5
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Quote:
I had always thought this effect was "negligible", meaning don't try to compensate for it in the scheme of other minor variables.
Definitely not negligible. Check out ballistic.zdziarski.com

Jonathan knows his stuff his program uses the JBM ballistic engine, but only
runs on iphone, ipad. You have to turn this option on and select your bullet
profile, he has a huge built in database. Be sure to also enter your rate of
twist, RH or LH, and the length of your projectile.
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Old July 7, 2013, 11:00 PM   #6
Bart B.
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'Twas either Hatcher or Townsend that proved decades ago that the .30-06 with 172-gr. FMJBT bullets have about 6 to 7 inches of spin drift at 1000 yards. There's not much gyroscopic precession that causes spin drift when the angle of bullet departure to hit at 1000 yards is about .75 degrees and the bullet's angle of fall at the target is about 1.7 degrees; the bullet tilts only 2.45 degrees during flight from its angle of departure. It won't precess very much going that far.

Which means if you sight in at 1000 yards and get a zero with bullets striking a couple inches to the right, they'll be right on in windage at about 700 yards, an inch or more to the left back through about 400 yards, then a lot less to the left back to the muzzle.

But spin drift does happen; there's web sites with calculators to figure it out. If you shoot your stuff into no worse than 3/8 MOA through 1000 yards, then yes, worry about spin drift. Best wishes reading the wind past the first 100 yards and making exact corrections for it all the way to 1000 yards, too, as the wind drift for each yard of bullet travel's a lot more the closer the bullet gets to the target. You can only measure wind speed where you are at; good luck estimating crosswind speeds at various points downrange to the target. The wind speed's faster at the highest point in the bullets trajectory above the line of sight than at the muzzle and target; how much depends on the terrain. And a 1 mph crosswind moves .30-06/.308 bullets about 10 inches at 1000 yards.

I've never changed windage zeros for a .308 Win. or 30 caliber magnums from 100 through 1000 yards. And never ever heard folks at a match getting concerned about spin drift. The subtle cross winds that are always out there mask any drift from bullet spin for all practical purposes. Besides, spin drift's the same for every shot fired at a given range. It's the trajectory in the horizontal plane.

Fewer than 10% of the best long range, top ranked competitive shooters on this planet will put their first shot down range where they've shot before, got good zeros for their ammo there, and have it strike within 5 or 6 inches of their point of aim on a target 1000 yards away.

Therefore, in looking at the realities of spin drift and its relationship to all the other things that move bullets off their desired trajectory, I consider it negligible.
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Old July 8, 2013, 05:11 PM   #7
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Gyroscopic precession, same as on a kids gyroscope, responds to any turning force applied to the spin axis by moving the axis at 90° to it. As the bullet flies, the fall due to gravity causes the trajectory to bend. If the bullet were to be pointing as perfectly straight into the path of the trajectory as possible, its axis would have to be on a line tangent to the arc. Precession, through the epicyclic mechanism, keeps working to correct the bullet point into the air stream, but with the arc constantly angling further down, it is never able to quite get there. As a result, there is always a very small upward pitch of the bullet off the trajectory, accompanied by a larger yaw angle to the side representing precession's 90° response to the overturning force caused by that upward pitch into the air stream.

The yaw in flight is called the yaw of repose. The faster the spin relative to the drag force on the nose pitch, the bigger that yaw angle is. In a right hand spinning bullet, the yaw is to the right (and vice versa). The yaw angle is typically on the order of a few thousandths of a degree, while the upward pitch is on the order of ten times smaller than that. So the overall angle off tangent to the curve is pretty small and mainly to the side. That yaw presents a very slightly sideways profile into the wind, creating drag that pulls the bullet to the right. The amount that drag moves the bullet by the time it gets to the target is spin drift. The even smaller upward pitch component creates a small amount of bullet lift, but it's so small it raises the bullet point of impact on the order of less than half an inch at 1000 yards.

As I mentioned, faster spin increases the yaw angle, so a fast spinning bullet sees greater spin drift than a slow spinning one. As Bart said, you can find calculators that take this into account. Some just assume a typical spin rate, but if you find one, like the JBM calculator, the PM Ballistics program, or QuickTARGET Unlimited, that asks you not only for muzzle velocity, but also for the pitch of your rifling as arguments, then you likely have the full calculation invoked. It's easy to test. Just declare zero wind speed and try different twists to see if the windage correction changes.

For example, the QuickTARGET Unlimited 2 DOF software says a 175 grain MatchKing at 2700 fps MV will have about 1.3 moa of spin drift between 100 and 1000 yards if fired with a 10" twist barrel, and about 1.0 moa of spin drift fired from a 13" twist barrel. It says a 155 grain #2156 SMK Palma bullet at 3000 fps from a 13" twist barrel will have about 0.8 moa of spin drift between 100 and 1000 yards, but the difference between 800 yards and 1000 yards is only about 0.3 moa, so adjustments over a typical Long Range course of fire isn't much.

Bryan Litz's point mass solver (PM Ballistics) disagrees with QTU by about 0.1 moa. It gives the #2156 about .7 moa at spin drift with the 13" twist at 3000 fps and at 1000 yards. The 175 grain SMK gets .9 moa with 13" twist at 2700 fps, and 1.2 moa from a 10" twist at the same velocity. The difference between the two programs appears due in part to how they estimate gyroscopic stability factor (they don't get quite the same number). I'd have to play with them a little more to find what else is involved in the discrepancy, but it's not much, and I think you get the general idea.
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Old July 9, 2013, 07:37 AM   #8
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If you shoot your stuff into no worse than 3/8 MOA through 1000 yards, then yes, worry about spin drift. Best wishes reading the wind past the first 100 yards and making exact corrections for it all the way to 1000 yards, too, as the wind drift for each yard of bullet travel's a lot more the closer the bullet gets to the target. You can only measure wind speed where you are at; good luck estimating crosswind speeds at various points downrange to the target. The wind speed's faster at the highest point in the bullets trajectory above the line of sight than at the muzzle and target; how much depends on the terrain. And a 1 mph crosswind moves .30-06/.308 bullets about 10 inches at 1000 yards.
Point is well taken. The effect is about the same as a 1 mph full-value crosswind- but, I think it's important to know it's there, and when combined with the Coriolis effect, drift is compounded.

For target shooters like myself, it's much less of a "big deal" because we have the luxury of walking-in our subsequent shots, automatically accounting for non-wind driven drift. But for a long-range hunter, that needs a first-round hit, seems that it's critical to understand and account for it.
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Old July 9, 2013, 07:55 AM   #9
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243 win.

At 200 yds Zero is 1" left & 2" high. Good for hits to 425 yards.
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Old July 10, 2013, 12:17 AM   #10
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But for a long-range hunter, that needs a first-round hit, seems that (spin drift correction) is critical to understand and account for it.
How many long range hunters have a sight setting table with spin drift corrections for different target range bands as well as the offsets from horizontal shooting to shooting at up or down angles when the trajectory in the vertical plane is flatter?
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Old July 11, 2013, 07:53 AM   #11
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He just needs a Springfield '03 ladder sight. Spin drift built right into the slope of the peep ways.
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Old July 12, 2013, 06:51 AM   #12
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Unclenick, right you are about that ladder rear sight. But get one that's got a tiny, gentle curve in it 'cause horizontal spin drift, like vertical trajectory, ain't linear. Being "close enough" with a straight ladder one compared to "exact and perfect" with a curved ladder one may not be good enough.

One other thing about ladder rear sights most folks don't think about is, they're easily adjustable for exact elevation settings when the target is at an up or down angle to the shooter. As long as the ladder is in the true vertical plane, the line of sight can be at any up or down angle from zero to 90 degrees; the trigonometric corrections are automatically made. So, put an extension arm out to one side with a weight attached hanging down and that'll automatically keep the ladder rear sight in the vertical plane.
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Old July 12, 2013, 03:30 PM   #13
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That curve is the result of several things interacting at once: One is that the drag causing it is a force, so it accelerates the bullet's side drift, increasing that drift velocity as the bullet gets further down range. Another is the arc of the trajectory increases as the bullet spends more time slowing and dropping, increasing the upward drag force that causes precession to produce the yaw of repose. Also, the bullet loses spin rate more gradually than it loses forward velocity, so, not counting a temporary trend reversal in the transonic range, the bullet's gyroscopic stability factor increases as the bullet goes down range, which also works to increase the yaw of repose. Both factors cause the spin drift drag to be a larger percentage of total drag, preventing that force from falling off as fast as forward drag does. Finally, as with all drift vectors, as forward velocity falls off, spin drift velocity has time to carry the bullet further during each successive range increment.

I don't think I've ever seen a sight set up to correct spin drift accurately. The problem is the correction changes not only with the bullet and muzzle velocity but with air density as well. It'd take a computerized sight with an air pressure sensor and a way to enter bullet velocity and BC to do it accurately.

I hadn't thought about maintaining sight verticality like that. You got me thinking (always dangerous) that a sight mounted on a ball radial pressure ball bearing slipped over the barrel and that spun around the bore axis, one for the front sight and one for the rear, tied together with a rod underneath to act as your plumb weight so that both front and rear sights stayed upright and at the same height above the bore axis as you canted the rifle left and right would always correct for canting. Of course, there's no guarantee the shooter taking the recoil with the butt plate at a can't wouldn't shift the POI off a little, but over a short range of rotation it would work to keep the sights right.

Hmmm. Actually, that might be made to work. A couple of float arm spirit levels with the peep through the tip of one and the front sight peep through the other. Have to think on how to keep the float angle vertex coincident with the bore line, but it's not impossible.
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Old July 12, 2013, 05:49 PM   #14
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Folks have been using spirit levels on front sights for decades. Once calibrated for how much angle the rifle cants for a given shift of the bubble to one side, it's easy to correct for wind.

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Old July 13, 2013, 08:08 AM   #15
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Concept doesn't seem to be any different than the bubble levels we all use on our scopes or scope bases to eliminate cant. I don't shoot iron sights much at longer ranges, but an anti-cant device would be no less important than on a scoped rifle.
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Old July 13, 2013, 08:35 AM   #16
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When you are in the field shooting from a mountain side using field positions, the drift will make no difference at game ranges.
For a sniper shooting at 1,000 yds or more it would, but not to a hunter.

Go to the range, shoot from a sitting position out to 500 yards and see how you do.

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Old July 13, 2013, 11:11 AM   #17
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Yeah, sight levels have been around for as long as I can recall. I'm just thinking it's an additional thing to pay attention to that could be corrected by a self-leveling site automatically. But the ruggedness and added complexity probably make the idea too unattractive.
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Old July 13, 2013, 08:38 PM   #18
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a self leveling sight? interesting.

the mechanism to level a sight could be a simple as barrel mounted bearings with enough counter weight to keep the scope vertical and enough friction to resist swinging. A LER scope would be needed so the scope could rotate around the bore's axis.

I wonder if I could stabilize a scope with a gyroscope?
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Old July 14, 2013, 09:33 AM   #19
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I suggested to two scope company reps at a gun show some years ago that they put a spirit level in their scopes. Place it so it appears at the bottom of the field of view with the bubble centered on the vertical wire when the scope was level. Both reps though it was a good idea. US Optics has one in some of their scopes.

http://www.opticstalk.com/us-optics-...opic25393.html
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Old July 14, 2013, 11:05 AM   #20
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I suggested to two scope company reps at a gun show some years ago that they put a spirit level in their scopes. Place it so it appears at the bottom of the field of view with the bubble centered on the vertical wire when the scope was level. Both reps though it was a good idea. US Optics has one in some of their scopes.
Konus has scopes in the $300 range with that feature. I'm surprised more don't.

I wrote in a similar suggestion to Bushnell a couple of years ago- but suggested it be electronic.

Three little dots... green one in the center, red on either side- indicating cant. Green one lights up when scope is perfectly level.
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Old July 17, 2013, 07:11 PM   #21
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I have never once heard at a long range match people worrying about spin drift. Too many other things involved to mess with. If I'm shoot a long range field match with my 6mm creedmoor even out to 1000yds I go with my normal wind call and send the shot. If the wind is slight or not a hard full value alot of times I won't even come off the target.
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Old July 18, 2013, 01:41 PM   #22
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It's not a matter of trying not coming off the target paper. It's a matter of trying not to come off the 10 ring. If you don't have enough (or any) sighters allowed, knocking all the error you can out of the initial calculation helps. That's why you'll find spin drift compensation included in Bryan Litz's Android app for use on the firing line.
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Old July 18, 2013, 06:29 PM   #23
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That's still my take, as I said in #8.

Whatever the "knowns", I like to rule them out- leaving as much shooter error as "the rest" as possible. We all know the wind call is the big booger, but other unknowns like velocity deviations matter too. Even with a big "miss", it helps being able to separate vertical and horizontal spread effects to try to figure out the "why's", and how much of each might have been responsible so that adjustments can be made.

It isn't always easy, but it's a nice feeling to see that steel swing through the scope after you've given thought to what caused the prior shot to miss, and made adjustments. Luck? Sure...I'll take that, too
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Old July 18, 2013, 06:48 PM   #24
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Unclenick, this is one of those posts I should have passed on. I understand what you are saying even though my brain is still trying to process all the information you gave us.

One of the reasons I have this question is that I am seriously considering a 338 Lapua Mag for a Christmas purchase. Not exactly a 50 yard rifle. So here goes what I am trying to ask.

When setting up a scope and zeroing for a specific bullet to be used only in a single rifle, does not the adjustment for zero with no wind, let's say at 600 meters take into account for spin drift for that particular bullet. And as long as the same bullet is used each time in that rifle, would the effect of spin drift be negligible between 600 to 800 meters??

Thanks
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Old July 18, 2013, 09:02 PM   #25
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If you can't shoot your stuff well inside 3/4 MOA all the way to 1000 yards, even dreaming about spin drift is a total waste of time. There's a similar change in bullet impact with a 10 mph head or tail wind; do you correct for that 15 fps change in the bullet muzzle velocity relative to the air it goes through?
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