Fixed Sight Drift Formula

Do you need to apply a correction to a set of fixed sights? If you know how far away from center the bullets hit the target, the following formula will let you calculate how far you need to move the sights.

MOVEMENT = ( SIGHTRADIUS * LRERRORONTARGET ) / TARGETDISTANCE
Where

• MOVEMENT = distance to move the sight
• SIGHTRADIUS = distance from rear to front sight
• LRERRORONTARGET = distance bullet strike on target is away from center, left to right
• TARGETDISTANCE = distance from front sight to target
• NOTE: All distances in the same units (such as inches)

Example

• Given a SIGHTRADIUS of 6.4 inches (my SA Mil-Spec 1911, for example),
• A 1.5 inch “bullet hits target this far from center” distance,
• And a TARGETDISTANCE of 300 inches (25 feet),
• MOVEMENT = ( 6.4 * 1.5 ) / 300
  MOVEMENT = 0.032 inches (approx. 1/32")

Note: If the bullet is striking to the right of the target center, the rear sight needs to move the other way, to the left.