Open sight calculations/math

[Dimner]

I gang. I have a Type 99 Arisaka that I'm going to have a receiver sight added on (Lyman SME).

I'm trying to figure out if the original military front sight will work with the new receiver sight, or if I need to get a new one. I know there is a formula on how to calculate sight height based off of how much you want your POI to move up and down. Is there a formula that takes in account moving one of the sights backward or forward?

My end goal here is to, if necessary, hand off my rifle to the smith with any necessary front sight parts that I want installed. I really don't want to have the Lyman SME installed just to find out I need another trip to the gunsmith to get the front sight removed and then a ramp installed with the appropriate bead height. I'd much rather try to get all the work done at the same time.

 

[Snakeoil]

Here is a table. No math required.

 

[Ian]

I your gunsmith is any good he probably has one of those charts hanging on the wall above his scope mounting bench.

 

[Brad]

Simple ratio problem. Change in sight height is to sight radius as impact change is to target distance.

 

[Ian]

A few different ways to figure it, including an on-line sight height calculator (keep scrolling down): http://skinnersights.com/technical_info_17.html

 

[Brad]

Watch your units if you do the math- we tend to measure sight height in inches and range to targets in yards.

 

[fiver]

A X C
------ =D
B

A= change in rear sight height
B= distance between sights
C= distance to target
D=the change in POI
up is up and down is down on the rear sight.

 

[Brad]

Look at that, fiver does math!

Exactly how I do it. Works to figure out what thickness Shinto put under a scope base too, or what offset Signature rings to use for Burris mounts.

 

[fiver]

didn't learn it for my metallic shooting.
it's how i know how much i need to move the stock on my shotguns.
btw it works out to 1/4" of eye height/left/right/down for 4"s at 35yds. with a 30-32" barrel.
and yes i can tell if it's 4"s different.

 

[Dimner]

So in all of these equations, do I want to set the change of POI value to zero, since that is my goal?

 

[Snakeoil]

Something not mentioned yet is you need to know what your ammo does at various ranges. How your ammo performs will tell you how the sights have to be set. Well, they will help get you into the ballpark.

I believe what you are calling an SME sight is a Lyman 48. So, it mounts tight to the receiver. Any adjustment to change the relationship between the rifle POI and a sight setting is at the front. And you have mentioned this in your first post. Just clarifying before I continue.

If this were me. I guess the easiest way to know what you will need is to get the original sights set to some yardage you choose, like 50 yds. Confirm the setting on a target. Now, when the smith has the rifle, he can mount the rear sight and then look thru the rear sight and adjust until he is looking thru the original sight at the front sight. He can then look at the sight setting on the Lyman and decide if he should lower or raise the front to give you more adjustment up or down.

Here is a diagram to illustrate what I'm saying.

 

[Dimner]

Thanks snakeoil. That makes sense. I was afraid I'd have to take into account the POI shift. I'll reinstall the original rear sight and go from there as suggested.

However, I'm still curious, How do custom gunmakers do this without having test fired a brand new rifle? Is it as simple as having the two sights exactly the same height above the center of the bore?

 

[Snakeoil]

My uneducated guess is they set up a factory rifle for commercially made ammo in that caliber. There is no shortage of ballistic info available on factory loads. Then it is a simple matter of figuring out a range of adjustment and building the sights to fit into that range. Look at common .22 rifles. The sights are designed for small game hunting and plinking. Doubt you could extend very far beyond 100 yds before you ran out of sight. Look at any hunting rifle with iron sights, like a 94 Winchester. The range of those sights is fairly limited. Need more range, put a receiver sight on the rifle. The .30-30 will reach out quite a ways and still be effective. You just need the sights to get you there.

The aftermarket scope guys back when Unertl and similarly externally adjusted scopes rule the firing lines at matches had the same challenge. For .22 rimfire, I'm sure there was a chart of standard scope blocks for various target rifles. But for centerfire, the load will determine the block dimensions. Experience will get you in the ballpark. For example, a Win High Wall in .32-40 shooting black powder loads with a specific bullet weight is probably a common set-up from the Schuetzen days. So, if you build a similar rifle, chances are you can use the same block sizes that another guy used on his rifle of the same basic specs.

There are standard Unertl block sizes for the 1903 Springfield. But they are based upon military target ammo. If you shoot reduced loads, which have much more of an arc to the trajectory, you are probably going to need a different size block at one end or to make a shim for one of the blocks.

 

[Ian]

didn't learn it for my metallic shooting.
it's how i know how much i need to move the stock on my shotguns.
btw it works out to 1/4" of eye height/left/right/down for 4"s at 35yds. with a 30-32" barrel.
and yes i can tell if it's 4"s different.

LOL. You're one of those buttholes who dusts a bird and mutters "Got behind that one a little" because he actually saw where the center of the cloud went or fills in a spot on a sporting clays team with a borrowed, full-choke 870 and walmart ammunition and scores a 98.

 

[fiver]

make it an out of the box winchester or browning and i could probably do pretty good.
hand me a remington [other than a 3200 or a trap stocked version of the 870] or a Beretta and i might as well be shooting it upside down.
their comb height and drop at the heel are just wrong for me.

 

[Jeff H]

For what it's worth, I used this one recently:

Sight Correction Calculator

Dillon Precision: Reloaders, Reloading Equipment, Bullet Reloading, Bullet Reloaders

www.dillonprecision.com

to back up my own numbers.

Not sure what happened, but I ended up bringing my elevation to a good nominal for 148 through 180 bullets in the 3" 357 revolver, but I ended up going as far left as I was already too far right, so I have to make another correction. This is with a fixed rear sight I made, and a front sight blade from Fermin Garza on my 3" Charter Bulldog.

Fermin is one good fella, by the way. He talked me into replacing the front sight after I replaced the rear sight and he gave me some blanks to experiment with, along with encouragement and advice.

As @Snakeoil said:
"Something not mentioned yet is you need to know what your ammo does at various ranges. How your ammo performs will tell you how the sights have to be set. Well, they will help get you into the ballpark...."

This made my particular project a bit involved. I am testing light 148 grain WCs, moderate 158s and "moderate+" 180s (all cast) in a 3" revolver, and the ammo must be at least useful in a suppressed, 18" carbine too. I sure seem to make things hard on myself. Not bad enough that I use a revolver which gets little aftermarket support on grips and holsters, but this has added some work, but it's been a good distraction.

As it's working out, the 158s are right on for elevation, the 148s shoot acceptably low and the 180s shoot acceptably high. The cool thing is that the 148s shoot as low as the 180s shoot high, and all three shoot consistently too far left, so it's sort of falling into place.