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Canting - the right answer

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Unread 05-25-2006, 11:10 PM
Silver Member
Join Date: Dec 2001
Location: Pennsylvania
Posts: 230
Re: Canting - the right answer

Brown Dogs post looks as I understand what happens to a turret adjustable scope that is canted from bore centerline. To prevent this my rifles are 100 yard zeroed, flat forend level, then the scope line of sight is lowered 36 moa, rifle raised and another group is shot at 100 yards. If group is right or left from vertical the scope is turned in relation to the bore until the base zero group and the dialed group are exactly vertical from one another. A bubble level is mounted to the scope for reference and exact distance is measured between groups, divided by 36 and used as my "major mark" number for that scope.

When yall find the answer to this riddle it would perhaps be useful to someone trying to shoot off from his side under some pine boughs. A feature that would be desireable to a bow hunter for example. I designed an arrow rest that could be shot horizontal,along with preliminary work on a sight that matched. Could have used "the right answer" back then for sure.

Point being for a long range rifle application to estimate/calculate point of impact variations based on cant would require measurement equipment greater than needed to prevent it in the first place..... as in bubble level? Just an average joes observation.

I hope yall find "the right answer". And thanks much for a terrific Online Ballistics calculator!
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Unread 05-26-2006, 10:07 PM
Platinum Member
Join Date: Dec 2005
Posts: 2,483
Re: Canting - the right answer

I will be convinced when 1000yd competitors start zeroing at 100yds and then applying 'perfectly vertical' holdover in order to reduce their cant errors by a factor of 10.

[/ QUOTE ]For what it's worth, a friend and I have done the following with .308 Win. match rifles after rebarreling.

First, shoot the same load we've always used for best accuracy to find out its muzzle velocity in that particular barrel.

Next, shoot the rifle prone with aperture sights; the front sight has a spirit level on it. Zero the rifle and ammo at 100 yards, then check a ballistics table/program for our atmoshperic conditions to find the bullet drop from level fire at 100 yards.

Calculate the actual bullet impact change for the rear sight movement per 0.002083-inch movement per click based on the sight radius used (varies from 32 to 38 inches depending primarily on barrel length), then adjust the rear sight down an amount equal to bullet drop plus front sight height above bore axis. Sight height's usually about 1.5 inches and bullet drop's about 1.9 inches, so move the bullet impact down 3.4 inches at 100 yards. Now the rifle's sights are set to mechanical zero; line of sight is parallel with the line of fire as the bullet leaves.

Then use a good ballistics software program to find the bullet drop at 300, 600, 800 and 1000 yards. Add sight height to those drop numbers then calculate how many clicks the rear sight needs to be raised to zero at these ranges.

How well does this work? Never been off more than two clicks (about 2/5ths MOA with a 38-inch sight radius) in elevation with several barrels. Sometimes more error in windage 'cause us humans can't dope wind that well all the time.
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Unread 05-26-2006, 10:56 PM
Gold Member
Join Date: Sep 2003
Location: Blighty
Posts: 637
Re: Canting - the right answer


My sloppy language; my point should be amended to read as follows: [ QUOTE ]
I will be convinced when 1000yd competitors start using their (unchanged) 100yd zero at all ranges, simply applying 'perfectly vertical' holdover in order to reduce their cant errors by a factor of 10.

[/ QUOTE ]

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