Can't find my spin drift or coriolis.

[Topshot]

I can see how the rig provides a solid platform to align the scope in the verticle plane but as the scope body may be miss alinged Left to Right as well as having a 20 MOA rail, I find it hard to see how he is doing anything different than having a soild rest to work from?

 

[lisagrantb]

I don't know how to attach a picture but the way it's made not only holds the scoped action steady but it holds the scope and action centerline in almost perfect alingment no more than 36 arcseconds. This puts the scope almost exactly on top of the action and then you rotate the scope for alingment. It does this with the use of a machinest level that is secured to both the action and scope.

 

[Topshot]

lisagrantb

I worked for a long time in Survey and have experience in setting up plate bubbles. The accuracy of a plate level is determined by the radius of its curved surface and the size of its bubble.

The bubble on an anti-cant level is not a particularly accurate device. It is short and dumpy with a small radius curved surface. It is certainly not as accurate as the fine machinist level that you used to set up your scope.

As a test, set up and level your rifle on uneven ground, using your anti-cant level and then check it against your machinist level. Do this three or four times and I am sure you will get different results each time you try it.

As another test. Move the anti-cant level bubble 1 or 2mm from its central position and then check out how much it puts your rifle off level using your machinist level. This will give you an idea of the sort of accuracy that you can expect from your anti-cant level when being used while shooting out in the field under real life conditions.
You should be able to measure the angular error from the deflection of the machinist level.
I would be interested to see your results.

 

[lisagrantb]

You are correct, the anti cant device isn't the most accurate device. If I were shooting without moving the crosshair the margin of error in the ACD would not make a big difference, but, when you adjust the crosshair as much as 30 moa you have just magnified your error considerably, that's why your rifle, scope and crosshair should be as accurate as possible. Try this, put your rifle on a solid rest looking at a target. Cant the rifle a very small amount then dial in 30 moa elevation and watch what happens to the center of the crosshair, it will move in the opposite direction you canted the rifle not straight down. It may only move a fraction of an inch or so at 100 yards but at 1000 it adds up.

 

[Pdvdh]

lisagrantb,

Did you ever identify some spin drift or coriolis affect out on the field range? Curious if you worked on it any further.

Here's the tool I recently purchased and use to help ensure my scopes are mounted directly above the bore. It's already paid for itself (corrected a significant scope misalignment problem) on one of my LRH rifles.

EXD ENGINEERING : EXD ENGINEERING VERTICAL RETICLE INSTRUMENT - World's Largest Supplier of Firearm Accessories, Gun Parts and Gunsmithing Tools

There's a picture of the device and some narrative explanation in this Thread:
http://www.longrangehunting.com/forums/f18/good-scope-mounting-tools-47812/

 

[lisagrantb]

Yes, I looked at the level you are referring to. I think it would be a good tool but I decided to machine one for myself (cheaper for me). I think mine is more accurate because of the tolerances I held and the level is good to .001" in 6", I don't know what the extra bit of accuracy gets me if anything at all over the one you have. And yes, the spin drift and coriolis is correct now, they exactly match the ballistic program. I guess you could use this devise and do some number crunching and deliberately cant your scope to match your spin drift and coriolis for a particular bullet and only have to correct for wind.

 

[rem]

if there is any wind at all at any point down range it will change your results.
try shooting further and maybe it will show up.

 

[paphil]

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 ½ 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!