How to adjust an iron sight
If the sights are not aligned correctly, then the sights should be adjusted to bring the line of sight to meet the point of impact. Theoretically, this can be done with a single shot—clamp the firearm into a vise, fire one shot, then adjust the sights so they are pointing at the hole in the target. In reality, it generally takes a number of shots to establish a group, then the sights are adjusted to move the line of sight closer to the group, and the process is repeated iteratively until the sights are correctly aligned.
Vertical Adjustment (Up and down):
If your shot is above (needs to be lower) your target using both sights facing directly at your target, the rear sight should be lower or your front sight should be higher. If your shot is below (needs to be higher) your target using both sights facing directly at your target, the rear sight should be higher or your front sight should be lower. Always remember FORS: Front opposite, Rear same.
Horizontal Adjustment (Left and Right):
If your shot is left from your target using both sights facing directly at your target, the rear sight should be right or the front sight should be left. If your shot is right from your target using both sights facing directly at your target, the rear sight should be left or front sight should be right. Always remember ROFS: Rear opposite, Front same.
Detailed instructions for adjusting the sights:
Many target sights have click adjustments, where a detent in the adjustment screws allows the sight to move the line of sight a certain angular distance with each click. This distance is usually specified in minutes of arc, which translate to approximately 1 inch at 100 yards. On a firearm with 1 minute clicks, then, it would take 1 click to move 1 inch at 100 yards, 2 clicks to move 1 inch at 50 yards, 4 clicks to move 1 inch at 25 yards. If click adjustments are not available, or the click interval is not known, then the distance to lengthen or shorten the sight for a given point of aim adjustment is:
D1 / R1 = D2 / R2
For rear sight adjustments:
D1 is the distance between point of aim and point of impact. R1 is range from front sight to target. D2 is the length the rear sight must change by. R2 is the distance between front and rear sights.
For front sight adjustments:
D1 is the distance between point of aim and point of impact. R1 is range from rear sight to target. D2 is the length the front sight must change by. R2 is the sight radius distance between front and rear sights.
This formula calculates the MAGNITUDE ONLY of the sight height change; refer to the instructions above to find the correct direction for the adjustment (front or rear sight, longer or shorter). Likewise, all distances must be in the same units. That is, if a change in inches to the sight height is desired, and one is shooting on a 100-yard range, then R1 (100 yd) must be converted to inches (100 × 36 = 3600 inches) before using this distance in the equation.
An example: Consider a rifle with a distance between front and rear sights of 26.25 inches, firing on a 50-yard (1800 in) range, with point of impact 5.3 inches too high on the target, having a front sight blade that is 0.505 inches high mounted in a dovetail. How much must the front sight blade height be changed by to fix this problem? (It will be assumed that the muzzle of the rifle intrudes into the range space for following typical gun range safety protocols, and the rear sight is hence 50 yards from the target.) D2 = R2(D1/R1) = 26.25(5.3/1800) = 0.077" (magnitude of change to front sight height) Since the gun is hitting too high, the front sight must be lengthened by this much per the instructions cited previously; hence, the front sight must be replaced with a blade that is 0.505" + 0.077" = 0.582" high. With this correction, the rifle will hit the desired point of impact, all other factors being equal.