The distance from the bore-center-line – shown running through A and C – to the line of sight
– shown at the tops of the sights -- must be the same approximate distance or height at each sight
location. The rear sight has an adjustment range and you should use the middle of the range as your
starting point for measurements or to determine the correct sight for the rear sight.
Measure the barrel diameter at each sight location – shown at long arrows A and C in the illustration.
Divide each measurement by two to obtain the distance from bore-center-line to top surface of barrel
at each location – represented by the short arrows at A and C in the illustration. Write down and label
your results.
Measure the height of the existing front sight. Add the height of the front sight to its corresponding
center-line-to-top-of-barrel figure to obtain the distance between bore-center-line and line of sight.
The distance between bore-center-line and line of sight (just calculated at the front sight location)
minus the remaining bore-center-line-to-top-of-barrel distance (at the rear sight) will give the needed
rear sight height.
The basic principle is: B plus ½ A (short arrow) is the same total distance as D plus ½ C (short arrow)
OR ½ A + B AND ½ C + D BOTH EQUAL the distance from center line to line of sight.
To measure a rifle barrel for a missing rear sight involves taking three measurements to determine
the fourth measurement. In this example the fourth measurement is the rear sight height.
Barrel Dia. Measurement At Rear Sight .820”
.820” ÷ 2 = .410”
.410” is bore-center-line to top-of-barrel distance
800-741-0015
Determine Rifle Rear Sight Height
brownells.com
Barrel Dia. Measured At Front Sight .596
.596” ÷ 2 = .298”
.298” is bore-center-line to top-of-barrel distance
Barrel Diagram Example With Numbers
brownells.com 800-741-0015
Measure the height of the existing front sight.
Add the height of the front sight to its corresponding
bore-center-line to top-of-barrel figure
to obtain the distance between bore-center-line
and line of sight.
Measure .503”
.298” + .503” = .801”
.801 is the bore-center-line to line-of-sight distance
One half barrel diameter at front sight plus front sight height will give height of sight line above borecenter-
line. Height of sight line above bore-center-line minus one half barrel diameter at rear sight
will give rear sight height
OR
Use the formula for determining rear sight height in this example:
Need to find Rear Sight Height at B (½ C) + D – (½ A) = B
Plug in the words and numbers (½ x .596) + .503 – (½ x .820) = Rear Sight Height
Do the math (.298) + .503 – (.410) = .391 Rear Sight Height