Open sight adjustment: A Formula

[1911 guy]

With hunting season coming up, I hope this can help some of us. I just went through this with my .36 cal flintlock with fixed sights, but it can be used with any open sighted rifle.

Fire at 25 yards, use as small a target as you can to get the smallest groups you can. Aim at the bull every time, no "kentucky windage".

Measure from the center of your group to the center of the bull. You need two measurements, one windage, one elevation. Measure at 90 degrees. Ex: one inch right, two inches high.

One number at a time, divide error by 900. Ex: One inch right equals 1/900, two inches high equals 2/900. Windage now equals A, elevation now equals B.

Measure your sight radius, the distance between your front and rear sight. This equals C.

A multiplied by C gives you measurement in inches how far to drift your front sight to the right (if missig to the right, drift left if missing to the left).

B multiplied by C gives you a measurement in inches to either lower (file down) your front sight or raise your rear sight (assuming a high miss, opposite if low)

The numbers given are for twenty five yards. If shooting at fifty yards, use the number 1,800 rather than 900. If shooting at one hundred yards, use the number 3,600 rather than 900.

Zeroing my flintlock, I was dead level, but 4 inches to the left. 4/900=.00444. .00444X36=.15984. So, I drift my sights .160" to the left, bringing my POI to the right

Hope this helps all of you and takes some of the guesswork out of iron sight changes.

 

[AndyC]

Now there's something I didn't know - cool stuff, thanks!

 

[Chawbaccer]

Moving my sight just a little bit at a time works too and it won't be as hard on my head.

 

[Skofnung]

Duly noted and saved to my files.
Thanks for the tip!

 

[1911 guy]

Clarify my first post.

I just re-read my initial post and it seems kinda hard to follow. Here's the smooth version.

Divide inches of error by inches of range. 25yd is 900 inches, 50yd is 1800 inches, 100yd is 3600 inches.

Multiply this times sight radius in inches. This will be your amount to drift your sight.

Bwana Johns math below is correct, but the difference (unless you have an exceptionally long sight radius) will be less than the diameter of a bullet. I was off by 4 inches at 25 yards with a 25.5 inch sight radius and the difference is .130". I just find it easier to have one formula.

 

[Bwana John]

1911 guy, your method is very close to the actual math, but you forgot a couple of small things.

Your basic premise is from High School geometry; corresponding sides of similar triangles are in proportion.

However moving the rear sight and moving the front sight require using different formulas because of the different triangles that are used when determining whether the change is in the rear or front sight.

SO,

distance to move REAR sight/sight radius = error on target/distance to target from the FRONT sight

OR

distance to move FRONT sight/sight radius = error on target/distance to target from the REAR sight

Make sure all distances are in the same units then
cross multiply and divide to find the distance to move the correct sight.

 

[wayne in boca]

Wow,you guys are smart! I go by the old way:Measure it with a micrometer,mark it with chalk,cut it with a hatchet.

 

[abearir]

WoW!, an to think I just use a empty brass and a range rock.....

 

[geekWithA.45]

I normally just tell the guy holding the target to step a few inches to the right or left, as needed.

 

[1911 guy]

I was just reading through the THR rifle "library" and saw I made it into a sticky! I'll try to not let it go to my head.

 

[cracked butt]

SO,

distance to move REAR sight/sight radius = error on target/distance to target from the FRONT sight

OR

distance to move FRONT sight/sight radius = error on target/distance to target from the REAR sight

AHA! That explains what I thought was an anomoly I had a few years ago. I was cutting a taller front sight for My 03A3 (it shot about 10" high)and used the windage marks on the receiver as a reference for how much higher I had to make the new sight (each tick on the windage scale on an 03A3 = 4MOA.) The finished result was off by < 1" at 100 yards from my expected result, but since it was close enough I didn't care and quickly forgot about it.

 

[Omaney]

Uh...I was told there'd be no math