Calculating Correct Front Sight Height for Zero
Principle
There is a linear relationship linking the distances between:
1. The front and rear sight distance, and
2. The change in height needed for the front sight of the rifle and the actual location of the bullet, above or below the bull’s-eye on the target.
Equation
A = the distance from the rear sight to the front sight. Units are in inches and approximate.
B = the distance from the barrel of the rifle to the target. Example: 100 yards (3600 inches), 200 yards (7200 inches), etc. Units are in inches.
C = the CHANGE in height required in the front (or rear) sight. Units are in inches.
D = the height ABOVE or BELOW the bull’s-eye of the target. Units are in inches.
Note
1. On a SMLE the distance between rear and front sights is approximately 19.5 inches.
2. On a No4 MkI the distance between rear and front sights is approximately 28.5 inches.
3. On a No5 MkI the distance between rear and front sights is approximately 23.25 inches.
4. On a P-14 the distance between rear and front sights is approximately 37.75 inches.
Actual Relationship
A is to B, as C is to D. Therefore, the formula is as follows:
A/B = C/D
Restated: A x D = B x C or C = (A x D) / B
Examples
1. A SMLE (fitted with a +.045 front sight blade) shoots a 5 shot group 8 inches above the bulls-eye at 100 yards.
C = (A x D) / B. C = (19.5” x 8”) / 3600”. C = 0.043 inches. Therefore, you must INCREASE the front sight height by 0.043 inches.
(+.045 add 0.043 equals +0.088, closest sight +.090)
2. A No4 MkI (fitted with a -.015 front sight blade) shoots a 5 shot group 5 inches above the bulls-eye at 100 yards.
C = (A x D) / B. C = (28.5” x 5”) / 3600”. C = 0.040 inches. Therefore, you must INCREASE the front sight height by 0.040 inches.
(-.015 add 0.040 equals +0.025, closest sight +.030)
3. A No5 MkI (fitted with a +.075 front sight blade) shoots a 5 shot group 5 inches below the bulls-eye at 100 yards.
C = (A x D) / B. C = (23.25” x 6”) / 3600”. C = 0.039 inches. Therefore, you must LOWER the front sight height by 0.039 inches.
(+.075 subtract 0.039 equals 0.036, closest sight +.030)
4.A P-14 (fitted with a +.060 front sight blade) shoots a 5 shot group 6 inches below the bulls-eye at 100-yards.
C = (A x D) / B. C = (31.75” x 6”) / 3600”. C = 0.053 inches. Therefore, you must LOWER the front sight height by 0.053 inches.
(+.060 subtract 0.053 equals 0.007, closest sight .0)
With the proper height of front sight to correctly adjust for elevation zero, it can now be drifted within the dovetail to correct for windage zero and staked in place.
Using surplus MkVII Ball ammunition, keeping in mind that the rear battle sight on these rifles is designed for a 300 yard zero, a correct MPI at 100 yards should be 8 ½ inches (+/- 1 inch) above the point of aim. At 200 yards the MPI would be 4 ¼ inches (+/- ½ inch). If zeroing at 25 yards the MPI should be about 2 inches above the point of aim. If zeroed in this manner it will also ensure that the range adjustable micrometer sight is zeroed as well.
The equation is valid no matter what ammunition is used, however the shooter or handloader will have to figure out his/her own exterior ballistics information to come up with the correct MPI placement at any given range.
