Re: Height of Scope over bore
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: PSchwenke</div><div class="ubbcode-body">If you are running a 20 MOA base, where do you measure the center of your scope for Ballistic programs? Seems like the canted scope mount could add in error? What's the proper method?
I have a 5R milspec with Wotac Gen5 4-14, Burris xtr low mount rings, and EGW 20MOA mount.
Paul </div></div>
I measure from the center of the action's pressure relief hole to the center of the tube, and prefer to use the center of the Parallax or Windage turret as the tube center. Because the turrets are usually located pretty close to the midpoint on the tube, measuring at the turret center should help to minimize height measurement error between FFP amd SFP reticles. And if the turrets happen to fall at or close to the midpoint of the base the height differential between the front and rear will also be minimized.
Also, if you use the
<span style="font-weight: bold"><span style="font-style: italic">JBM Small Arms Ballistics Trajectory calculator</span></span> you can enter the a value to compensate for the cant in a base under <span style="font-style: italic">"Line Of Sight Angle"</span>. The <span style="font-style: italic">"Line Of Sight Angle"</span> is the relationship between the barrel's plane and the scope's LOS. Since there are sixty (60) Minutes in one (1) Degree, the closest approximate corresponding value for a 20 MOA base is .33 MOA (1/3 of one Degree).
Make sure you enter the value under <span style="font-style: italic">"Line Of Sight Angle"</span>, <span style="font-weight: bold"><span style="text-decoration: underline">NOT</span></span> <span style="font-style: italic">"Cant Angle"</span>. The <span style="font-style: italic">"Cant Angle"</span> setting refers to the angle of the barrel <span style="font-style: italic">in relation to the target</span>.
You'll always have to confirm the accuracy of calculations of any ballistics program by shooting, but my measurement method has always worked for me when using JBM. I've used 20 MOA bases as well as 40 MOA bases and I've never been off more than a single click of the adjustments.
Keith