PDA

View Full Version : Technical: metric rear sight elevation increments (check my measurements)


W.E.G.
September 09, 2003, 22:30
I was curious as to how much actual vertical increase there is between each
increment of elevation (i.e. from the 200 setting to the 300 setting, etc) on a
metric rear sight.

I measured from the bottom of the lower receiver to the top of the rear sight
aperture assembly. I suppose I could have picked any other point of reference
for the measurement. But, my caliper easily spanned from the bottom of the
lower receiver to the top of the rear sight assembly.

Here are my numbers. Tell me whether you agree/disagree, or whether there is
a better/easier way to measure the difference in elevation increments.

Rich@CGW
September 09, 2003, 23:38
I Have`nt checked your math,, But since your doing this awesome leg work..Gary !

How about calculating the rise in elevation.. as per clicks/cm`s up on POI

Wasn`t it .006 = 1 inch ??

kotengu
September 10, 2003, 05:58
Well, cross-referencing to the old "2 to 200, 3 to 300, and 12 to 600" rule (rule of thumb I've heard for comeups in MOA), that means 1 MOA is about equal to 0.013" of elevation at the rear sight.

Of course - it's only 0600 and I just got up, so I could have just calculated the number of ants it takes to carry away a jelly donut..........what was the sight radius of an FAL again?

W.E.G.
September 10, 2003, 07:39
Rich and Kotengu, the answer to each of your questions is at my old barrel-math thread at http://www.falfiles.com/forums/showthread.php?s=&threadid=21880&highlight=math

But, the short answers are:

FAL standard sight radius is about 21"
(it actually differs - but not enough to matter - depending on which rear sight setting you select).

.006" movement of any sight (front or rear) on a FAL will move point of impact one minute of angle (1" at 100 yards, 2" at 200 yards, 10" at 1000 yards, etc.)


You can calculate how far one revolution of the screw for either sight will move the sight if you know the thread pitch of the screw. Info on thread pitch for FAL sights is at http://www.falfiles.com/forums/showthread.php?s=&threadid=21952&highlight=pitch

Rear sight screws (metric or inch) 36 TPI
one rev moves sight .028" (4.5 MOA)

Front sight (metric) 28 TPI
one rev moves sight .036" (6 MOA)

Front sight (inch) 40 TPI
one rev moves sight .025" (4.25 MOA)

kotengu
September 10, 2003, 08:57
But if that's true, something's screwy. From http://www.realguns.com/calc/exteriorballistics?calc

For a 147gr bullet travelling 2700fps, BC=.393 (is that right?)

Range - yds. 0 100 200 300 400 500 600
Path - in. -1.8 2.0 0.0 -8.8 -25.8 -52.9 -92.1

So in MOA, drop from 200 to 300 is about 2.93
from 300 to 400 is about 4.25
from 400 to 500 is about 5.42
and from 500 to 600 is about 6.53

From your measurements (ASS-U-M ing that the rear sight is correctly notched) that means that rear sight height change from place to place gives us:

From 200 to 300: .014" / MOA
From 300 to 400: .007" / MOA
From 400 to 500: .0055" / MOA
From 500 to 600: .0084" / MOA

I can understand the slight error in the last three (it's hard to accurately measure small distances), but the first one just doesn't make sense.

If you just take the drop from 200 to 600 (to get a round about average) you get 92.1", or about 15.35 MOA. Divide you height change in inches between the two settings by that and you get 0.155"/15.35 MOA= .010"/MOA, which is off a good amount as well.

Double-checking your trig calculations, your .006" / MOA is correct for a 21" sight radius - so why the differences?

Not trying to nit-pick - you just got me thinking. BTW - what happened to the old JBL ballistics calculator? I haven't been able to get it to come up for some time........

W.E.G.
September 10, 2003, 13:00
I'm trying to follow what you are saying, and not doing too well.

Let's start with a simple question.

Have you measured the difference in height of the rear aperture between the 200 setting and the 300 setting?

What measurement did you obtain?
(Forget about MOA and all that gobbledygook for now. Just tell me what you got with your caliper.)

My caliper says the rear aperture is raised .040" when moved from the 200 setting to the 300 setting. This aperture assembly is on a G1 lower receiver. Nobody ever said the change in height is "correct" for the cartridge. I just want to know how much it moves mechanically from one position to the next.

We can talk about where the bullet-impact ACTUALLY moves on the target as a consequence of sight movement once we come to some certain agreement about how much the sight moves mechanically.

kotengu
September 10, 2003, 14:10
"Have you measured the difference in height of the rear aperture between the 200 setting and the 300 setting?"

Not yet - I'll try and remember to take a sight or two to work Friday and I can check it with a height gage. I just worked through some calculations based on the numbers you gave and some rough trajectories.

Can anybody else take some measurements to help verify?

7.62FMJ
September 10, 2003, 22:42
I have yet to see an iron sight that is accurate at all ranges it specifies. The 200 should be good out to 300 which is battle zero. However the change should be progressively greater not from 200 to 300 it changes .04 and from 300 to 400 only changes .03, same as at 400 to 500 then suddenly shifts to .055 from 500 to 600. Doesn't make sense but also calculations should be in meters not yards.

Plot this on a graphing calculator multiplying each drop from a set amount equal to zero so that the dot change goes downward, then super-impose this over the real bullet drop by calculating MOA adjustments the same at each 100 meter increment multiplied by the amount of given change using the sight drop in MOA.

I did this once with an AK sight set for .223 and actually milled my own out of an existing sight block to the correct MOA drop at various measurements. You will quickly see the sight which slides straight on a set plain should instead be curved as bullet drop is subject to exponential decrease in velocity thus a curve sight ramp instead of straight one unless the measurement spaces are correctly accounted for. Older mausers have a curved sight ramp to account for velocity exponential loss. Such rifles were made by marksman and designed as such- the FAL is not such a weapon but could be with the right engineering.

Creeper

W.E.G.
September 11, 2003, 07:41
Indeed.

If my measurements are correct, the "come-ups" (gradations) on the FAL rear sight are questionable.

It sure would help if a few other folks would take some measurements and tell us what numbers they get. That way we might know whether the sight gradations are consistently wrong, or just consistently inconsistent.

W.E.G.
September 11, 2003, 22:08
(...sound of crickets)

Wildcat
September 11, 2003, 23:33
Gary,

FWIW: On my rear sight assembly, the sight piece travels on a 5 ramp relative to the bore axis.

I measured the distance between the leading flat of the sight to the forward edge of the rear sight base. It seems to provide repeatable measurements.

applied a bit of trig where:

Aperture Height = X * tan( 5)

Setting dX Elevation

200M: .000 .000
300M: .163 .014
400M: .189 .016
500M: .231 .020
600M: .254 .022

Does this help or hinder?

Wildcat

kotengu
September 12, 2003, 07:07
OK - this morning I brought two metric sights to work. One I will call "Austrian" (no z-spring cut out on the bottom), the other I will call "Belgian" (with cut out). I believe they're off a G1 and an STG-58, but it's been awhile.

I chucked the sights up in a vise on a calibrated surface, and oriented them so the front of the sight leaf was exactly perpendicular to the surface (hopefully simulating its orientation to the bore), and measured the height changes with a height gage (basically a micrometer on a stick, but a little more complicated than that). While I had it chucked up, I also measured the ramp angle just out of curiousity.


......................Austrian.................... ................Belgian
................Height.....Difference............. ....Height.....Difference

200..........0.000................................ .......0.000
300..........0.019.......0.019.................... ...0.027.......0.027
400..........0.041.......0.022.................... ...0.052.......0.025
500..........0.068.......0.027.................... ...0.076.......0.024
600..........0.094.......0.026.................... ...0.112.......0.036

Ramp Angle...............5.5.......................... ..............7.5


I re-measured everything twice (took sight out of vise, re-chucked, and re-measured), along with ensuring repeatability of each individual measurement because I was a little surprised they could be so different.

I guess the conclusion we can draw is that, as Gary stated above, they are "consistantly inconsistant." If my assumptions about the sight leaf being perpendicular to the bore are incorrect, it might explain how my measurements differ from Gary's, but not explain how my two sights differ from each other. Both sights had little "slop" in them, so I don't think that's the culpret either.

Hope that helps,
Matt

(edit) Well, crap - neither tabs nor spaces seems to work to spread my table out, so periods will have to do. Hope it's clear enough which measurements correspond to what.

W.E.G.
September 12, 2003, 19:23
Originally posted by Wildcat
...applied a bit of trig...600M: .254 .022

Does this help or hinder?

I'll try to answer your question if you can answer mine:
How much nominal vertical change (in thousandths of an inch please) do you obtain when moving from the 200 setting to the 600 setting?

7.62FMJ
September 12, 2003, 21:49
What someone needs to do is plot those points on a graphing or engineering calculator relative to distance (remember 1 is 200) and see if it matches the curve from the .308 bullet drop. The measurements seem right- the idiots who engineered the sights messed but this is common on almost all rifles. If someone ever designed sights that adjust to exactly the BDC at various yardage they would make a fortune.

NewtoFals
September 14, 2003, 13:12
It's official.....i'm confused. I'm going to my mommy.:cry:

Wildcat
September 15, 2003, 00:01
I'll try to answer your question if you can answer mine:
How much nominal vertical change (in thousandths of an inch please) do you obtain when moving from the 200 setting to the 600 setting?

It looks like the forum is designed to abbreviate what it thinks are superflous spaces and other things don't seem to quite show up. Building a decypherable table hasn't turned out to be too straightforward.
I wanted a little triangle ahead of the "Elevation" too but its not there.

Anyway, from the 200m to the 600m position is: .000 + .014 + .016 + .020 + .022 = .072" (includes some error from rounding -small error source- and a small amount of play in the rear sight -big error source-)
Looks to be about 12 MOA (sight radius measured at 20.93).

This seems a trifle too few if we accept kotengu's estimate on the 147's flight path; I expected it to be closer 15 MOA. Hummm!

Wildcat

Dirtfarmer
September 15, 2003, 13:56
In your figuring of "come-ups" , Kotengu, do not forget that the M1A is in yds, and the FAL is, of course, in meters...

As to the reported difference in angle Austrian vs. metric... maybe the Belgian maintained the yard tooling from the T-48!! (I don't actually think so!)

that is one weird piece of info, however.

Sometime I will check into that on some examples of mine.

Goodwill,
-Dirtfarmer

kotengu
September 15, 2003, 16:40
"In your figuring of "come-ups" , Kotengu, do not forget that the M1A is in yds, and the FAL is, of course, in meters..."

Good point - but I can't seem to find a ballistics program that will calculate in meters. I guess we could interpolate and get close, but I figure we're in the right ballpark with the yardage numbers.

You're right about the ramp angle - I wish I had more sights to measure to see if it's just something screwy in those two sights in particular, or something between Belgian and Austrian sights. Kinda makes me want to go measure the angle of the lower receivers where the sight mounts, but I can't think of a good place to reference off of. I could also go back and set both ramps at the same angle (relative to the surface) and measure again, but I don't know that it'd make that much of a difference.

I guess we're sort of reaching the "how many angels can dance on the head of a pin" argument anyway - we could factor in what kind of climate and elevation was common around the places these were designed, and maybe some higher humidty and lower elevation vs. low humidty and high elevation accounts for the differences. Maybe the Austrians all planned to shoot only to the east in the morning, or into a prevailing head-wind, or up the slopes of the alps, or with a cup of coffee in one hand and eating green eggs and ham. :D

Short version - the only way to know your actual trajectories is to go shoot YOUR gun at different ranges with YOUR ammo in YOUR conditions. Everything else is just an estimate.

Interesting to debate on a website until the next time you can go shooting, though......

Bruce Allen
September 15, 2003, 20:59
Gee, this has been a tough and interesting thread to follow.

I have attempted to understand it as best as I can.

Using Kotengu's figures and the .006" movement as 1 moa, I wanted to see what actual figures could be obtain in elevation per rear sight adjustment.

For the 5.5 degree ramp:
The total travel from the 200 meter position to 600 was .094.
.094 divided by .006" = 15.66
15.66 X 3(300) = 46.98"
....... X 4(400) = 62.64"
........ X 5(500) = 78.3"
........ X 6(600) = 93.999"

For the 7.5 degree ramp:
Total travel from 2 thru 6 was .112
.112 divided by .006" = 18.666
18.66 X 3 = 55.98"
.........X 4 = 74.64"
.........X 5 = 93.3"
.........X 6 = 111.96"

If a 147 grn .30 cal bullet at 2700 fps drops approx. 92" at 600 yards then the ramp with the 5.5 degree angle would be the correct - assuming all other things are equal.
Now I am wondering what the other variables are in the rifles/sight platforms.

My first real question is am I way off the mark here with my presumptions?
The calculations are relatively simple, I am not sure if my premise is correct, resulting incorrect results, i.e. am barking up the wrong tree.

What say you literate guys??

W.E.G.
September 15, 2003, 21:24
Bruce, can you give us your raw numbers?

I think I follow what you are trying to do...that is, jumping ahead to the change in point of impact downrange.

Think of me like your old Algebra I teacher. I don't understand your answer unless you show ALL your work.

I just want to know what difference (in thousandths of an inch) you get as you move the sight to each successive setting.

Also, tell us what method you used to take your measurements.

Bruce Allen
September 15, 2003, 21:49
I used Kotengu's height measurements.

Then I used what you posted as .006" movement = 1 moa at 100 yards.

Starting with the 5.5 degree ramp, dividing .006 into .094 = 15.66

The other - 7.5 ramp - done the same .006 into .112 = 18.66.

If that is correct from 0 (zero) yards the bullet drop is calculated at 15.66 inches per 100 yards. As was already pointed out a curve instead of a flat incline would be more practical in real use, but we'se got to use what we'se got, right.

Anyhow using the base figure of 15.66 and 18.66 I multiplied each times (100 yards) - 3,4,5, and 6.

The similarlity between my calculation of a 93.99" drop and 92" from a ballistic calculator may just be a coincidence.

Dirtfarmer
September 16, 2003, 00:26
The "curve" of the ramp would only be necessary if the intervals between "stops" were constant.

On a steadily inclined plane, ANY height change can be incorporated by putting a stop at the appropriate height.

The "curve" has nothing to do with the functionality or "accuracy" of ones elevation correction.

FWIW.

Goodwill,
-Dirtfarmer

Dirtfarmer
September 16, 2003, 00:36
So the AUSTRIANS!!! were secretly training with yardage, to confuse the Czech enemy!

...Alright, what if, in fact there WAS some sort of hold over of the yardage systems of 1954-55 that went on, at least in some blueprints, that were passed to the Austrians that went unnoticed by engineers?

Something happened here, ... things don't match yardage by accident.
....One wouldn't think so, at least. ;)

What the Hexx is going on here????

Goodwill,
-Dirtfarmer

kotengu
September 16, 2003, 06:01
Haven't you heard - it's all part of the vast right wing conspiracy. Better hush now before the media gets ahold of this :wink:

kotengu
September 16, 2003, 06:18
"Using Kotengu's figures and the .006" movement as 1 moa......"

Close, but your method would only hold true if the notches were evenly spaced, and the trajectory was a line instead of a curve. Divide the height change at each notch by .006 to get how many MOA (or convert to inches, if you want) drop there will be from your 200yd zero to whichever position.

Example:

for the 300 setting on the 5.5 ramp:

.019/.006=3.167 MOA (multiply by (3)*(1.047) to get inches, or 9.95")

Continue on down (up) the ramp and you get:

400: 6.83 MOA (28.60")
500: 11.33 MOA (59.31")
600: 15.67 MOA (98.44")

I didn't catch it until dirtfarmer's "blueprint conspiracy" post, but it is actually pretty close to the ballistic tables in yards (as opposed to meters. The worst error is at 600yds, and it's only off by about 6" (or one measley MOA).

Bruce Allen
September 16, 2003, 11:20
You are probably correct.

I had something about that nagging me last night but in my condition (tired and have had a sinus infection for about 4 days) I just could not get it to click in my brain.

Dirtfarmer is absolutely correct and I knew the distances should be meters when I posted it but did not feel like the extra math to calculate it.

Here is an interesting pic somebody posted on the Files not long ago: