Do Like that in my picture here
it's actually a good question where to aim since
--- some guys would try to put the center of their cross hair right in the middle
but by doing so there is no real defined aiming point to line your crosshairs up to
you would be putting the center of the crosshair in open space and the lines whould not line up with anything.
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Also, you can aim at the corners of the square if you want - for practicing
but not for group, since if you shoot into the black, you cannot see your holes
Secondly, in my picture is how you would want to align your crosshairs onto a circle
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Notice, the X and Y planes of the crosshair just baaaaaarely hold off a hair, without the crosshair actually touching the line, you are lining the edge of the crosshair up with the edge of the circle
---and not using the center of the crosshairs
You want to put the smallest amount of white paper between your crosshairs and the black circle
since - if you put your black crosshairs, touching the black lines, you could be into the line 1/32", 1/16"???
and not know it
but if you want to shoot the teensiest groups you can, you want to hold off just the smallest amount your eye can perceive
that way you know you are aiming in the exact same spot every time
then, you adjust your dials to place POI where you want it which is usually not where your POA is because of course then again, if your POA = POI and you make a black hole,
it is hard to hold the black reticle in the center of a black hole with thousandths precision.
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the center of the crosshair is for shooting your bullets into your buddies group, thats a good time