How do I compute sight height?

[Grantman]

I picked up a RMC accusporter recently from a guy that had flinchitus. The rifle had a kinda bizarre rear rear sight-essentially an exagerated buckhorn meant to be used like a peep sight. I could not get a consistent hold with this sight. So, I welded a piece from an old TC tang onto the RMC tang to give me something to mount a Lyman 57A on, since I can't see the irons anymore. Now, the thing shoots bugholes at 50 yards, but the groups are about 8" high, with the Lyman adjusted all the way down. I fired one shot at 100 and it was a foot high. If I understand the sight geometry, the front sight should be higher or the rear sight lower in order to bring the POI down. I can move the rear sight further back on the tang, thus lowering it, or replace the front sight with a higher sight. Question: How far to lower rear sight(requires drill and tap of tang) or how much higher to go with a front sight? Sight radius is right now is about 28.5 inches.

 

[Hawken12]

Thats a little bit tricky ,as you are movinn yer back sight back,but just recalc with the new number
Meas dist from back sight to front sight
Every in is = O.08333333333of a foot
example 36 inches =2.9999999988 ft.
30 incehes x0.0833333333=2,499999999999 ft.

So the formula is (for 50 yd=150 ft.)= height adjustment either front site has to go up or back site has to go down.
multiply your dist between sights x .6666666 and divide by 150 .
So if site radius is 28.5 inches x o83333333=2.374999991
therefore 2.374999991 x 0.6666666=1.583333311 \150' = 0.010555555' bout an 1\8 of an inch +\-

 

[jimmy82]

That is an interesting formula, I am curious as to where you found it.

 

:shocked2: You number crunchers amaze me.

 

[rice1817]

It is a simple geometrical relationship based on similar triangles and ratios/proportions. It works for windage or elevation. (your 8th grade geometry teachers are spinning in their graves now!) :grin: :grin:

R/B=C/D
D=C*B/R

Where R= distance to target in inches
B= sight radius in inches
C= distance in inches of impact point from aiming point
D=amount of movement needed in sight to move impact point to aim point

 

[Rebel]

HUH? I didn't have me none of that there geometry stuff till 10th grade. :grin: And don't remember any of it.

 

[Mountainman56]

Owwww.....my brain hurts. :surrender: What's wrong with the old lazy method. Buy a really tall front sight and keep filing it off till it's right? :haha:

 

[ffffg]

 

[smokehouseman]

Amen............ :bow:

 

:thumbsup:

 

[paulvallandigham]

If that forumla is right, you need a front sight that is a little more than 1/8" taller.

 

[Mountainman56]

When you get a new front sight be sure and buy one a little taller than you need. That way you can get your rear sight up off the bottom and give you some adjustment back for future loads.

 

[shortstring]

28/X = 1800/8

1800X = 224

X = .12444

Convert 50yds into inches & ratio to the sight radius. 1/8 pretty much has it.

 

[Hawken12]

usedta be a surveyor,thats how we got so much country,all that figgerin adds up after a bit.long as don't do yer notes on the bar napkins,that is.
Actually Paul the front site would be 1\8 of an inch taller than it is now.

 

[paulvallandigham]

Shortstring: If you use the 28.5" sight radius that he gave us, rather than the smaller, 28 inches, you get .12666, or " a little more than 1/8" taller! " as I said.( 1/8" = .125") 28.5 divided by 1800, x 8 = .12666. That is the amount of height the front sight has to be INCREASED, or the rear sight decreased, to zero( Move the POI down 8 inches) the load at 50 yards.

Since the gentleman did not give us the height of his current front sight, ( over the center of the bore) we can only guess. If you happen to have the same model and make of gun, with the same barrel length as he has, with a tang aperature sight installed, maybe you can guess better than the rest of us. But, its still a guess, as you don't know if his front sight has been filed down or not. I was amazed enough that he gave us the range, the distance that the ball hit from center, and the radius of his sights in inches! With those three numbers, our math whizzes gave us all the less-than-complicated formula to work this out for any gun. Thanks, guys.

 

[bwhoffman]

this thread and/or formula should become a sticky!
in the resources area...???

good info :thumbsup:

 

[bart_anastasi]

(your 8th grade geometry teachers are spinning in their graves now!)

Not mine he is lying there saying (yeah that's Bart)

On the practile side if you need a sight that is higher get one that is more than what you need to get the back sight off the floor.

 

[CoyoteJoe]

All of the above calculations are correct but there is a simpler way to put it. R/DxC, where R= sight radius, D=distance to target, and C=the desired correction, all measurements in inches. So for a 28.5 inch sight radius and a 50 yard distance it becomes 28.5 divided by 1800 equals .0158 times 8" desired correction equals 0.1266". When ordering a new sight, one must remember that the height is measured from the bottom of the dovetail. Since you have an adjustable rear sight, I'd second the suggestion that you get a taller than needed front sight to allow some room to lower the rear if you want to shoot other loads in the future. But be aware that very tall front sights are more vulnerable to being bumped. Moving the tang sight farther down the grip slope may be a better option. :grin:

 

[Dphar]

Amount the POI is to be changed times the sight radius divided by the range in inches.
Thus, as previously stated several times I think.
8*28.5/1800 gives .126". At 100 you need .063. For a 1" change at 50 you need .0158".
I would tend to lower the front rear sight if at all possible, makes for the ball being closer to the line of sight. But I don't know if thats practical or not.
If you lower the rear lower it MORE than 1/8" then you can adjust up.
If you have a spreadsheet program it is possible to put in the formula, save it then do much easier calcs.

Dan

 

[reddogge]

Owwww.....my brain hurts. What's wrong with the old lazy method. Buy a really tall front sight and keep filing it off till it's right?

That's not lazy. It's what we used to do before computers, spreadsheets, etc.. And what's wrong with moving the rear sight forward and back with tape holding it on until you can see everything clearly?