in the 1980s and early 90s it was fairly common to modify a GI sight with the 4 clicks per turn 1 MOA windage adjustment to a 8 click per turn 1/2 MOA adjustment by drilling the receiver for a detent ball and spring, and the using an NM windage knob with 8 detents in it, or turning off the lugs on the GI knob and then putting detents in it. Those detents had to be spaced exactly 45 degrees apart and be exactly the same depth and profile to get accurate 1/2 MOA clicks.
The pinion shaft for the sight was also milled to reduce the length of the boss on the elevation knob side to create room for a coil spring around the shaft between the ear on the receiver and the sight base. This spring was then held in compression on the shaft and kept constant pressure on the sight base to keep it against the same thread on the windage knob.
The sight cover was also modified slightly to keep sufficient pressure on the sight riser, particularly at long ranges, and / or a shim fitted under the front of the sight base so it maintained constant contact with the sight cover to prevent the riser from rocking back and forth (and thus changing the elevation adjustment.
The riser itself was then fitted to the front sight block, usually by glass bedding to remove excess play.
1/2 MOA elevation was accomplished by either using the NM sight hood that rotated 180 degrees for a 1/2 MOA adjustment, or installing a thin plate between the sight ear and the elevation knob that had the teeth for 1 MOA adjustments on the ear side that meshed with those on the sight ear, and had 1/2 MOA teeth on the elevation knob side.
The end result was a very accurate and consistent 1/2 MOA adjustable sight. There was still some hysteresis between elevation pinion and teeth on the riser, but you just had to remember that if you adjusted the riser down, to go two clicks lower than you wanted to end up, and step back up two clicks to ensure the hysteresis was removed.
Today, for an M1A you can just get an entire NM sight set for about $100.