Originally Posted by Dr. Vette
Now, a question - how do you "know" that you have them correctly levelled with your crosshairs? Use the plumb bob as mentioned earlier in the thread, and go from there?
I use the receiver flats where the bottom of the bolt's two lugs ride when it's opened. Others have used the sight base holes atop and on the side of receivers; put a very long shanked screw in one, then rest a small spirit level on it. to set the barreled action at the true horizontal or vertical.
It won't matter a whole lot if there's a small error. One can find out what the error is be comparing a 100 yard windage zero with a 1000 yard windage zero. Just shoot both ranges using the same position and zero wind conditions. If there's a small error, then your can correct it by slightly twisting the scope in its rings.