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Old July 19, 2013, 09:28 AM   #26
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Old July 19, 2013, 09:30 AM   #27
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If you have a known, and quantifiable effect, why not take it into consideration? I could understand perhaps, if it were infinitesimal- but I don't see an effect in the area of minute as angle being so.

I do not understand your line of logic on this.
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Old July 19, 2013, 09:42 AM   #28
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Here is a novel approach.

Take your rifle out and shoot it. Get good wind calls and see what the drift may be at any range you plan on shooting.

AND WRITE IT DOWN, its part of your zero.
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Old July 19, 2013, 10:14 AM   #29
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Jim243,

The spin drift correction in moa grows with distance. It's not a constant value like the moa of your sights. If you zero a 250 grain Sierra MatchKing at 2900 fps MV at 600 m with a right hand 10" twist bore in zero wind, then you'll have to come left an additional 0.25 moa at 800 m, and another .25 moa going from 800 m to 1000 m. At 1000 m you also are very close to exactly 1 moa left of your 100 m zero.
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Old July 19, 2013, 01:01 PM   #30
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Uncklenick, good post on the spin drift curves increases with each range segment down range. One of the Sierra ballisticians (Bob McCoy?) mentioned some years ago that it's almost a tiny percentage of the trajectory curve but at right angles to it. Not exact, but close enough to see the horizontal path wind drift takes.

I believe that if your load has a 10 inch spin drift at 1000 yards, get a windage zero in still conditions at that range then move your sight 1/2 MOA to the right. Then you'll never be more than 1/2 MOA off exact at all ranges up to 1000 yards. The shorter range gets, the smaller angular error there is. And the error will pass through zero about 2/3rds downrange,then be a bit to the left back to the muzzle.

Nobody I know of can estimate corrections for cross winds faster than 5 MPH for a target 1000 yards away with no more than a 1/2 MOA error.

Use ballistic software to calculate the sight elevation settings for zeros at different ranges for different head and tail winds, too.
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Old July 22, 2013, 11:46 AM   #31
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Quote:
Here is a novel approach.

Take your rifle out and shoot it. Get good wind calls and see what the drift may be at any range you plan on shooting.

AND WRITE IT DOWN, its part of your zero.
So, you're saying to simply incorporate spin drift into wind drift, at any given range- and look at it as a single variable....

At 1000 yards it's going to vary from the firing solution from the old computer, but no reason it can't be done that way.
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Old July 22, 2013, 12:17 PM   #32
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Quote:
So, you're saying to simply incorporate spin drift into wind drift, at any given range- and look at it as a single variable....
No that's not what I'm saying at all.

Lets say you know your 1000 yard zero. You came up X clicks from 100 and your zero (windage) should be the same.

So you set your sights, check the windmeter/flags/mirage, etc. and determine the wind is blowing 10 MPH. So you adjust for your wind call, get a good call and release and find out you're still right or left (depending on the direction of twist) and you're still off. It's probably spin drift, so you add that to your 1000 yard "no wind" zero.

Next time you shoot, and put on your elevation, you add your "spin drift" corrections, THEN make the wind call and adjust from your no wind 1000 yard zero (with spin drift corrections added).

Wind and spin drift are different concerns, but both require corrections.

This is not a new concept by any means, Gen. Hatcher (then Major) taught this in 1917. He used the Model 1903 Springfield, Using the 1906 ammunition, the drift for this rifle cause by the spin of the rifling (spin drift) was 12 inches.
at 1000 yards.

So when you add your elevation while coming up to shoot at 1000 you also need to add 12 inches left, for this rifle,

AND THEN you would add your corrections for the wind.
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Old July 24, 2013, 04:27 PM   #33
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Isd there tabels and or charts that give general spin drift for each twist , bullet weight and velocity .

I would think that a 175gr smk with a muzzle velocity of 2600fps from a 1-10 twist has the same spin drift at 1k every time , Yes ?

There has to be somebody that has put all these numbers on paper .

I also here the inside of 1k spin drift is a non issue . 1moa at 800yds is 8" and spin drift is 3/4" . your still likely to hit what your aiming at even if you don't adjust for spin drift .

Or is the spin drift at 800yds 3/4 moa cus that would be huge
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Old July 25, 2013, 01:28 PM   #34
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Metal god,

It'll be in moa, not inches, but for the 175 grain .308" SMK in a 10" twist the number is bigger than you suggest. In an ICAO standard atmosphere, the most precise program I have includes Coriolis effect, which changes with latitude. It puts total spin drift, starting at the firing line as:

Code:
175 gr. SMK, 2600 fps, 10" Right Hand Twist

Range         Spin Drift      Spin + Coriolis Drift
Yards     MOA only (equator)    MOA at Camp Perry
   0            0.00                 0.00
 100            0.09                 0.11
 200            0.17                 0.21
 300            0.25                 0.32        
 400            0.35                 0.43
 500            0.45                 0.56
 600            0.57                 0.70
 700            0.70                 0.86
 800            0.86                 1.05
 900            1.04                 1.26
1000            1.25                 1.50
If you start out zeroed at 100 yards, then you subtract the 100 yard spin drift from all the other numbers in the column in order to get the correction. If you start out zeroed at 800 yards, then subtract the 800 yard correction from all the other numbers in the column to get the corrections. Same for any other starting zero. This subtraction will give you negative numbers in the ranges shorter than the zero range, which just means you need to turn the windage knob the other way (move windage right instead of left).
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Old July 25, 2013, 01:48 PM   #35
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Lets say thats my standard load .If you know the distance you are going to shoot . The first thing you do when starting your adjustments is correct for spin drift . Then bullet drop and all thats left is the wind call yes .

Is the spin drift a constant ? Meaning once you have a load worked up , will it be the same each time ?

Why would I want to ignore the spin drift and some how incorporate it in to the wind call ?

Interestingly enough my calc for that load gives me 1moa wind drift for every 1mph wind speed at 1k
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Old July 25, 2013, 08:03 PM   #36
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This is the point Kraig was making above. Roughly equivalent to a 1 mph full value wind- why bother? You've got to be damn good at your wind calls for that to matter. But, I'm still with you- I'm going to dial out the known effects, anyway.

The interesting take for me, is that the spin drift becomes more pronounced with range and drop in velocity- just as wind drift. All external effects on the flight are heightened as velocity drops and flight time increases.
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Old July 26, 2013, 09:53 AM   #37
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Metal god,

The things that would change the spin drift table are things that changes your gyroscopic stability factor or time of flight. As long as you are firing in the same gun, you won't likely have to make adjustments. If you change to a gun with a different barrel length and rifling pitch, you may. Altitude, or temperature affect gyroscopic stability, but these make very small differences. Going from -20°F to 90°F or going from sea level to 6500 ft, either one only makes a difference of about 0.07 moa at 1000 yards (<3/4 inch at 1000 yards). Changes only a benchrest shooter could normally hold. If I both drop from 90°F to -20°F and drop muzzle velocity 150 fps, then the affect grows to just under 2" at 1000 yards (0.19 moa). If I leave the atmospheric conditions at ICAO standard condition I used for the table (29.93 inches mercury, 59°F, 0% RH), leave the velocity at 2600 fps, and change the rifling from 10" to 12", then I get about ¼ moa less spin drift (2.6" at 1000 yards).

At the Long Range Firing School, Mid Tompkins told us he would correct a quarter moa by holding off slightly. He said he only puts ½ moa click sights on 1000 yard rifles because wind typically varies a ¼ moa faster than a shooter can operate the windage sight knob and get back into position. For that reason a hold-off correction is all there is time to make. From his Palma team experiences, Bart can give you better information on that than I can. But the bottom line is the influence of all the variables I mentioned are too small to bother changing your zero for.

At 1000 yards I show a 1 mph side wind moving your bullet 1.06 moa or 11.1" in ICAO standard conditions. So 1 moa/mph is a very close approximation.
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Old August 2, 2013, 06:27 PM   #38
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Good ballistics calculators include spin drfit correction

There are a few good exterior ballistic calculators out there for smart phones, I like Shooter on the iPhone (although beware it sucks batteries).

Spin drift is a real phenomena, and the reason why NRA long range shooters don't think about it is because they have known distance targets and have already "zeroed" their rifles to account for it. But move out to various distances unknown range targets and you will need to account for it after about 500 yards or so if you want to attempt a first round hit (no "sighters").
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Old August 5, 2013, 08:46 AM   #39
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I don't know of any top classified NRA long range rifle shooter who ever gets concerned about spin drift. A 1 MPH cross wind variable masks it. Nobody estimates wind corrections any more accurate than 1 MOA anyway. Maybe 5% of these folks will put their first shot at 1000 yards within 1/2 MOA of their point of aim anyway even with a good zero established half an hour earlier; thanks to the invisible wind variables down range that are usually different that at the firing point. A 1 mph error in estimating cross wind speeds will cause a 1 MOA error all by itself. And the last half of the target range is much harder to estimate cross winds for than the first half. To say nothing of the 20% to 50% spread in wind drift from the line of sight axis to the maximum ordinate (bullet height above line of sight).

I've always used the same windage zero for .308's as well as .26 and .30 caliber magnums through 1000 yards. So does virtually all other top ranked long range shooters.

Unless you can shoot your stuff into no worse than 1 MOA at 1000 yards for 20 to 30 consecutive shots, you have bigger hardware (rifle and ammo) and software (shooter) issues to fix than being worried about spin drift.

Using Unclenick's numbers above, one could get a zero at 600 yards and the spin and coriolis drift errors wouldn't be more than 3/4ths MOA from point of aim through 1000 yards. 'Twould be to the left (right hand twist) up to 600 where it's zero then to the right. From 200 to 900 yards, the error would not be more than about 1/2 MOA.

Did anyone ever calculate the miss distance with a 10 yard error in target range about 1000 yards?

And don't forget to make elevation corrections for head and tail winds, too.
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Last edited by Bart B.; August 5, 2013 at 09:09 PM.
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Old August 10, 2013, 11:30 PM   #40
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I got it all figured out. But then the deer was gone.
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