Fixed Sight Modification Calculator

Original Program: GunCalc V 1.0 ã
Provided by Forrest S. Fleming
[Adapted to Totally Ballistics with the Authors Permission]

This Tool is used to determine how much sight movement is needed to yield a desired impact movement.

A few examples of this would be, first, if you installed a new peep rear sight, only to find that the rifle could no longer be sighted in with the existing front sight.

This tool will determine how much taller, or lower, front sight is needed to affect a zero.

Second, let's say your new fixed sighted handgun is shooting three inches low at 25 yards. How much would one have to file down the front sight to bring the handgun into zero?

This tool will calculate how much you need to file off the front sight without a lot of file and error.

Access the Fixed Sight Modification Calculator from the Calculators Menu in the Main Menu or from the List Menu.

This calculator will work for both elevation and windage adjustments, provided the shooter takes a few moments to take accurate measurements. First, you must know what distance at which you are shooting.

100 Paces is not the same as 100 Yards, and 100 yards is not the same as 100 Meters. Take careful measurements and make sure you use the correct value. Be it 15 Yards, or 200 Meters, a guess here will yield a less than accurate calculation result.

Secondly, and most importantly, you must know the sight radius (SR). The SR is the distance from the rear face of the rear sight to the rear face of the front sight. This measurement needs to be as accurate as possible. Take the measurement as carefully as you can, and convert any fractional reading to decimal. For example, say you measure the sight radius with a metal yardstick calibrated down to 1/16" inch and you come up with a sight radius of 17 3/16 inches. You would convert 17 3/16 to 17.1875, or 17.188 inches. When taking the SR and distance measurements, remember the old adage "Junk in, junk out."

"Close enough" guesses here will only lead to inaccurate calculations and possibly ruined sights.
Your guesses are beyond the control of this formula.

Impact Movement (IM) is just as it implies. Measured from the center of the group to the center of the aiming point, determine how much movement ON TARGET is desired. If your group is centered three inched down and two inches left, calculate how much you need to raise the rear sight, or file down the front sight, and make this adjustment. The group should now be on line with the aiming point, only left. Now calculate how much to drift the rear sight to the right and make this adjustment. The group should now be centered on the aiming point.

DO NOT try to do two adjustments at once, as this will only lead to error. Make one adjustment at a time and you will have your firearm sighted in much more quickly and much more accurately.

Another word of advice is warranted here. If you have installed a new rear or front sight, only to find the gun cannot be zeroed with the rear sight adjusted to it's extreme position, DO NOT calculate how much sight adjustment is needed based on impact movement with the sight set at this extreme-most position. Return the rear sight to the center of its adjustment range and shoot a group. Use this group's center to determine the impact movement desired. If you determine IM desired with the rear sight at either end of it's adjustment range, you may get the rifle zeroed, but you will have no room to make further adjustments should you change loads.

Another use for this tool is to determine how much impact movement at target you will get per click of sight adjustment. Measure your sight at its lowest position, and again at its highest position, and divide by the number of clicks to determine how much sight movement occurs per click. Take this figure and enter it into the SM (sight movement) box, enter the range at which you want to know the amount of impact movement (often 25 yards for handgun, 100 yards for rifle) and enter the sight radius. The answer will be the amount of impact movement you will see on the target per click of sight adjustment.

Definitions:

Sight Movement -
Calculation result that determines how much sight movement is needed to yield a desired impact movement. A few examples of this would be, first, if you installed a new peep rear sight, only to find that the rifle could no longer be sighted in with the existing front sight. This tool will determine how much taller, or lower, front sight is needed to affect a zero. Second, let's say your new fixed sighted handgun is shooting three inches low at 25 yards. How much would one have to file down the front sight to bring the handgun into zero? This tool will calculate how much you need to file off the front sight without a lot of file and error.

Sight Radius -
The SR is the distance from the rear face of the rear sight to the rear face of the front sight. This measurement needs to be as accurate as possible. Take the measurement as carefully as you can, and convert any fractional reading to decimal. For example, say you measure the sight radius with a metal yardstick calibrated down to 1/16" inch and you come up with a sight radius of 17 3/16 inches. You would convert 17 3/16 to 17.1875, or 17.188 inches. When taking the SR and distance measurements, remember the old adage "Junk in, junk out." "Close enough" guesses here will only lead to inaccurate calculations and possibly ruined sights. Guesses are beyond the control of this formula.

Impact Movement -
Measured from the center of the group to the center of the aiming point, determine how much movement ON TARGET is desired. If your group is centered three inched down and two inches left, calculate how much you need to raise the rear sight, or file down the front sight, and make this adjustment. The group should now be on line with the aiming point, only left.

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