Checking slope on a rail

[memilanuk]

Does anyone happen to know off the top of their head (or have it written down handy) how to measure the slope of a scope rail and convert it to MOA i.e. 10, 20, 30, etc.? I understand the basic idea, but converting angular measurements tends to trip me up.

I have a gun sitting here with a (supposedly) 20 MOA scope rail on it, but when I put a NF 12-42x on it, I wind up 10 or more MOA low, and have to use Burris rings with the shims in them to raise it back up. Because of the nature of the scope rail (combination iron sight base and scope rail) it's not quite as simple as just going to a 0 moa base. Trying to figure this one out as it's driving me nutty (short trip, I know)!

 

[Clark]

Slope is rise/ run
Rise is sine
run is cosine
sin/cosine = tangent = slope

20 moa = .3333 degrees

tangent of .3333 degrees is .0058

20"/ [[100 yards] [3'/yard] [12"/foot]] = .0056

Those two numbers should be the same..... an inch at 100 yards is not exactly an moa, just approximate.


So when a guy makes a scope mount with 20 moa built in, and the front a rear of the mount are 5" apart, then the spacer is [.0058] [ 5"] = .029" thick spacer under the rear of the scope mount.

 

[Artpro]

I knew that.
Art

 

[dieselten]

there is an easier formula. multiply the constant .000291 x the sight radius (distance between front and rear screw centers).

This will give you the distance of 1 Minute of Angle on the target, at the scope base

 

[petierney]

I have a related question I don't know how to figure out correctly. I need to know what MOA (30, 40 maybe) I need on a scope base for long varment shots at 1700-1800 yards. The problem as you know is running out of elevation in the scope, my scope is limited to 40 MOA up. And/or is it reasonable to shim the rear mount. Thanks.

 

[Taildrag15X]

In the old days for us without a machine center to work with we would "shim" the old Weaver 1 piece bases with pastic shim stock to get the required 20-30 moa needed for 1K shooting. I think that in todays world of milling machines in everyones circle of friends you should be able to get what you need made with out much problem as picatinny rail material is available by the foot.