Muffin,
Have mentioned this before, both here and in my two books on hunting optics:
The basic optical rule is that 20/20 human vision (which is average, not perfect) can "define" about one inch (or MOA) at 100 yards, which means we can see alternating black-and-white lines half an inch wide. Beyond that the lines tend to look gray--as do zebras at, say, 500 yards.
This means that an optically good 4x scope can define about 1/4" at 100 yards--meaning there's about that much optical error in aiming.
This also means, as mathman pointed out, that the aiming point must also be appropriate for the reticle.
The error can also vary with the individual shooter's eyes--and how well the scope's adjusted for parallax, which for the smallest groups needs to be checked as well.
But the basic deal is that a 4x scope can add 1/4" to the average group size, while a typical 3-10x set on 10x adds 1/10"--if the parallax is corrected, and the shooter can shoot. This amounts to a difference of about .15 inch in group size.
The math is pretty simple for 15x, 20x, 30x etc.
Last edited by Mule Deer; 04/27/23.
