Back in November, 1998, Leonard Walline wrote an excellent article for Muzzle Blasts magazine entitled "Understanding Sight Adjustment." Check with the NMLRA to see if they have back issue of that magazine; otherwise beg, borrow or steal a copy of that issue for a full explanation of how this works. In the article Mr. Walline explained how to figure out how much you need to raise or lower a front or rear sight at a given distance in order to raise or lower the point of impact. He simply stated that "Mathematically, sight adjustment is a matter of simple proportions" and then went on to explain the use of a simple (measurable) formula to figure out the correct sight adjustments.
Basically, there are four components to the formula. All measurements are in inches. The components are represented by CAPITAL letters in the formula and are as follows:
A = The required amount of sight adjustment for a given distance (or how much you need to raise or lower your front or rear sights)
B = The sight radius of your rifle (or the distance between the front and rear sights)
C = The desired change of impact at the target
D = The distance to the target
What you are trying to determine is the value of A; or how much height do I remove from my front sight to make the ball hit where I aim at 25 yards? To do that, all you need to know are the values of B, C, and D (in inches).
Here is how the formula looks:
A = (B x C) χ D
In your example, the front sight was left high so that you can file it down to make the point of aim and the point of impact coincide. You shoot three shots from the bench at 25 yards and while the group is centered windage-wise, it is 3½ low. You need to raise the group 3½. How much do you file off the front sight? Start by identifying your known distances (in inches). Lets assume:
B = That the distance between your front and rear sights = 29Ό (or 29.25)
C = That the distance you want to raise the group = 3½ (or 3.5)
D = That the distance to the target = 900 (= 25 yards)
Now you simply plug in those distances into the formula:
A = (29.25 x 3.5) χ 900
A = .114
In order to raise your group 3½ at 25 yards, you need to remove .113" off the top of your front sight.
Set the depth gauge of your vernier calipers to .100 and with it scribe a line on your front sight. Then use a file to remove material down to that line. Go shoot another 3 shot group. The group should be real close to the point of aim, or slightly below. Before you file any more material off your front sight, shoot an offhand group at the same distance and see where that falls. Depending on how you hold your gun and how it reacts upon firing, you may or may not need to remove any more material. You may also be able to increase or decrease the powder charge in order to get the point of aim and point of impact to coincide.
Bottom line: You can use the formula above to determine how much material to remove from the front sight to get the gun to shoot close to where you point. Then, follow your normal shooting procedures before removing any more material. Slightly altering the charge, your stance, or sight picture could bring your point of aim and point of impact together without filing more off the front.
I use this simple formula all the time and it saves me tons of guesswork.
galudwig
