Here's a typical aperture front sight with a bubble level used on NRA match rifles in high power matches:
Here's the sight picture seen through a .040 inch (1.0 mm) aperture rear sight as the shooter focuses his eye on the front sight:
The front aperture can be changed to the size best suited for each person; some want more white around the bull than this one shows. Naturally, the better eyes one has the more easily they'll see a 1/4 MOA error in alignment.
The aiming bull subtends 6.5 MOA at 200 yards, 6.3 MOA at 300, 6.0 MOA at 600, 5.5 at 800, 4.9 at 900 and 4.4 MOA at 1000 yards.
Top classified competitors will call their shots within 1/3 MOA of where the bullet actually goes (in calm winds, perfect conditions ) with 1/2 MOA ammo and equipment. With a 16 to 25 power scope, within 3/16 to 1/4 MOA.
Below is a post front sight with a 6-o'clock hold on a bullseye. It shows qute accurate what the 600 yard NRA high power target bullseye looks like as seen through a .0595" aperture on a National Match rear sight used on M1 and M14 rifles with the shooter's eye focused on the front sight.
Top classified competitors will call their shots within 5/8 MOA of where the bullet strikes. Note the bullseye looks fuzzier and maybe a bit smaller than the one in the aperture front sight and smaller aperture rear sight. The same 36 inch bullseye is used with both front sight types.
Maybe I'll make some pictures showing a thick crosshair reticule in a scope as it appears on a 100 yard target. One will show perfect alignment of the reticule dead center on the target. The other showing what the sight picture looks like with a 1/4 or 1/3 MOA error.