formula for calculating front sight height.

growr · Mar 29, 2021

I have a copy of a '73 Colt SAA that I lost the front sight on during a match last summer.

My local gunsmith put a replacement on that is too short as it's POA and POI are 6" high at ten yards over a solid rest.

The barrel is a 7 1/2 inch barrel and I am firing .38 Spl with 158 SWC and 4 gr. of Unique, so it is NOT a hot rod round by any means.....

How much needs to be added to the existing front sight to get POA to center?

Randy

FlyFish · Mar 29, 2021

Good explanation with formula here: Front Sight Height Guide

DeplorabusUnum · Mar 29, 2021

Perfect algebra story problem

Measure everything in inches.
1) SR: Measure your sight radius (back of rear sight to back of front sight.
2) H: With calipers measure from the top of the sight to the top of the barrel.
3) D: Distance from gun to target
4) POI: How many inches high over POA.

Use the following formula to figure out X (how much to add to the front sight). X/SR=POI/D

Just plug in the numbers. Let's say your sight radius is 9 inches, you are shooting from 10 yards (360 inches), and your POI is 6 inches high.

X/9"=6"/360"
So X would equal 9x6/360 or 0.15"

In this example you would need to add 0.15 inches more height to your front sight to hit POA. So the sight you need is H+0.15 inches.

growr · Mar 29, 2021

Would this be a correct calculation?

6"high at 10 yards (360 ") x 7.5" SR divided by 360 = .125 additional height needed for POA and POI to be centered?

The sight right now measures .184 add .125 = .309 total height?

Randy

FlyFish · Mar 29, 2021

I believe that would be correct if you have the correct sight radius. In your original post you mentioned a 7.5" barrel. The sight radius (as DeplorabusUnum said) would be the distance from the back of the rear sight notch to the back of the front sight blade. I guess that could end up being 7.5" also, but it seems like a bit of a coincidence.

george_lehr · Mar 29, 2021

For those that are mathematical calculation challenged, Dawson Precision has a sight height calculator where you can plug in the variables and it calculates the front sight height (not unlike using Turbo Tax).

Sight Calculator

George

growr · Mar 29, 2021

I mis-measured my total sight radius....it is 10" from the top of the cocked hammer to the front sight, so it would seem that I need a total height of .392 for POA and POI to line up.

Will keep you posted on how this turns out.

Thanks everyone!!

Randy

Protocall_Design · Mar 29, 2021

On the SAA type guns, sometimes the sight needs to be taller than it would be on a similar DA revolver because the grips are different. The SAA guns tend to roll up in your hand during recoil more than the DA guns. That means the front sight may need to be taller to account for the extra vertical movement of the gun while the bullet is still in the barrel.

You may want to start with the sight extra tall, then file it down as you sight in. With light target loads any extra movement should be pretty minimal. Heavy bullets or hot loads will make more of a difference.

Benchrest1 · Mar 30, 2021

Another vote for the Dawson calculator, and they make wonderful sights also.

DeplorabusUnum · Mar 30, 2021

growr said:
I mis-measured my total sight radius....it is 10" from the top of the cocked hammer to the front sight, so it would seem that I need a total height of .392 for POA and POI to line up.

Will keep you posted on how this turns out.

Thanks everyone!!

Randy

By my math you need a front sight that's .350 inches high.
6x10/360=.167 extra height needed. Added to your existing .184 would give you .350.

formula for calculating front sight height.

growr

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