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Can't find my spin drift or coriolis.

 
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Old 04-02-2011, 01:22 PM
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Join Date: May 2009
Location: Prosperity, Pa
Posts: 373
Re: Can't find my spin drift or coriolis.

Effects of canting your gun.
One minute of angle at 100 yards = one inch.
A 5 degree(each degree has 60 minutes) radius = 300 minutes or 300 inches. What we now do is think in a vertical plane 300 feet (100 yards) high . If our vertical crosshair were 300 feet high and canted 5 degrees, our barrel would follow that canted line. When we add elevation to our scope , we are really turning our barrel up and pointing the scope down. Gravity pulls the bullet straight down from the point of aim. Back to our ratio of 300 inches in 300 feet for a 5 degree angle, that comes out to 1 inch per 1 foot of elevation, for each foot of hold over we get one inch of deviation. For a 1000 yard shot, a 7mm needs about 20 minutes of elevation which is 200 inches which is 16.67 feet. At 1 inch per foot , we have a deviation of 16.67 inches(3.33 inches per degree of cant) . If we use a .308 that needs 30 minutes of elevation or 300 inches of elevation which is 25 feet or 25 inches of deviation. When we shoot at 500 yards, the 7mm needs 7 minutes of elevation which is 35 inches. 35 inches is about 3 feet and our ratio tells us that the deviation will be about 3 inches for 5 degree cant ( .6 inches per degree). As the angle gets larger and the cant becomes more pronounced , we also begin to hit lower and lower because the vertical component is changing into a horizontal component. A level can only be a good thing !! On the bright side, canting could be used to our advantage to counteract spin drift. A lot of shooters sight in about inch left at 200 yards to eliminate part of the spin drift at 1000 yards. Another way to erase the perception of that drift would be to cant the scope to the right in respect to the axis of the gun to get the gun to shoot left by the same amount as the spin drift. For the 7MM we need about 10 inches of correction . By the above calculations, we could apply a 3 degree cant to be dead on at 1000 yards and would still be dead center at 200 yards.
How do we set our scope to a 3 degree cant? The easiest way is to get a tall target at 100 yards with a perfectly vertical line and do the ladder test. Keeping the crosshairs lined up with the line, shoot and center at 100 and then adjust the turret up to 1000, aiming at the same center point. If the new group is exactly centered, congratulations your scope is perfectly plumb ! For my 7MM ,I would like my group to fall 1 MOA ( 1 inch )to the left at the 20 MOA and right on at 0 MOA to correct for spin drift. This would equate to a 3 degree cant and requires rotating the scope in the opposite direction as the desired effect.
The angle would be slightly different for different velocities and is determined by the amount of elevation required for the 1000 yard shot.
Doing this adjustment requires loosening the scope and requires some patience but the results remove spin drift from the equation of doping for the shot.
This also explains why some will argue that there is no spin drift. They accidentally mounted their scope at the correct 3 degree angle and their gun shows no drift!
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