brasse
02-08-2013, 11:29 PM
Took my cheap Ironsighter full length weaver/picatinny rail for short action and made it 20 MOA down in just a few minutes.
Measured the base at the front and rear, both 0.310 inches thick. Used my spare reciever as a mandrel, using sandpaper to first get front and rear of the bases as close to the same diameter as the reciever. The outside of the base bottom was high by over 0.010 inches. Then worked on front base area until is was 0.290 inches thick, finishing up with 2000 grit paper.
Copied from somewhere else
"Making 20 MOA scope mounts
The idea is that a degree is broken up into 60 minutes. The 20 minutes you wish to move your group is therefore 20/60 degrees, or 1/3 of a degree.
The tangent function is simply the slope of the angle (rise/run). No matter how large the triangle, the slope will be the same. In this case, a triangle 1/3 of a degree will rise 0.005818 units for every unit of run (regardless of what unit you're using).
So you plug into your calculator 20 (minutes of angle) / 60 (minutes in a degree) then hit "tan". It spits out 0.005818. So for every unit of run, a 20MoA angle will rise 0.005818 units.
In this case the run (base side of your triangle) is measured from your front ring to your rear ring. So you multiply that run by 0.005818 to find the rise, which is how much you shim.
In my example, if your scope mounts are 5" apart you'd shim it 30 thousandths.
The rule of thumb seems to be .001" equals 1 MOA [the Sighttron instruction sheet may have a chart showing this ]. A .020" shim under the back of the base would be a 20 MOA correction. A .015" shim would be 15 MOA. Not a lot of difference, and you probably wouldn't be able to tell the difference in actual use.
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Measured the base at the front and rear, both 0.310 inches thick. Used my spare reciever as a mandrel, using sandpaper to first get front and rear of the bases as close to the same diameter as the reciever. The outside of the base bottom was high by over 0.010 inches. Then worked on front base area until is was 0.290 inches thick, finishing up with 2000 grit paper.
Copied from somewhere else
"Making 20 MOA scope mounts
The idea is that a degree is broken up into 60 minutes. The 20 minutes you wish to move your group is therefore 20/60 degrees, or 1/3 of a degree.
The tangent function is simply the slope of the angle (rise/run). No matter how large the triangle, the slope will be the same. In this case, a triangle 1/3 of a degree will rise 0.005818 units for every unit of run (regardless of what unit you're using).
So you plug into your calculator 20 (minutes of angle) / 60 (minutes in a degree) then hit "tan". It spits out 0.005818. So for every unit of run, a 20MoA angle will rise 0.005818 units.
In this case the run (base side of your triangle) is measured from your front ring to your rear ring. So you multiply that run by 0.005818 to find the rise, which is how much you shim.
In my example, if your scope mounts are 5" apart you'd shim it 30 thousandths.
The rule of thumb seems to be .001" equals 1 MOA [the Sighttron instruction sheet may have a chart showing this ]. A .020" shim under the back of the base would be a 20 MOA correction. A .015" shim would be 15 MOA. Not a lot of difference, and you probably wouldn't be able to tell the difference in actual use.
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