Figuring width of front sight in MOA?

[Grin Reaper]

Figuring width of front sight in MOA? orig. posted by sur+plus rifle on: April 21, 2007, 07:36:04 PM

Quote from: Nyarlethotep on April 21, 2007, 06:36:48 PM
I'm looking for the formula that allows you to calculate the apparent size of your front sight in MOA, based up distance from your eye, width of the blade, etc. I'm trying to use my front sight as a range estimating device and want to be as precise as possible.


If my trigonometry is correct, the formula would be:

angle = 2 * inverse tangent ((SW/(2*DFE))

where SW = width of sight and DFE = the distance from eye 

Both of these need to be put in the same units of measurement for the equation to work (e.g. both in inches)

If your calculator outputs degrees for the inverse tangent, then you will need to convert that to minutes.  There are 60 MOA for every degree, so the conversion is:

angle in degrees * 60 = angle in minutes (MOA)

Does this help?

[Grin Reaper]

more from sur+plu rifle

Quote from: funfaler on April 21, 2007, 09:26:42 PM
Boston, in "Boston's Gun Bible" has the formula for such a calculation, and you don't even need a scientific calculator to work it. 


Ah, yes.  Forgot about that.  It is on chapter 18 page 4 in my copy.  The formula given is:

MOA = 21600/((sight radius" X 2 X pi)/front sight width")

Let's test both equations.

For the M14 with national match sight, the front sight width is .062" and the sight radius is 26.75".  So Boston's formula gives:

21600/((26.75"*2*3.14)/.062") =  7.97 or 8 MOA.

The earlier formula gives:

angle = 2*inverse tangent ((.062"/(2*26.75")) = .1328  & .1328X60 = 7.97 or 8 MOA

8 MOA is the correct answer, so take your pick of equations. 

[Grin Reaper]

Here is a formula I just came up with.

Front Sight MOA =  120 * InverseTan[ (sight width*.5) / (distance to eye) ]   NOTE:  (Inverse Tan = Arctan= Tan-1)   

Its the same as yours,basically......

In your examples, 7.16 is correct where the eye distance is 29.75" and the sight width is .062".. The formula is created by solving for 1/2 theta in a right triangle and converting theta to minutes.

However....


Quote
I'm trying to use my front sight as a range estimating device and want to be as precise as possible.

If your goal is ---  being able to calculate the range of your target using the fixed and known MOA of your front site.... then
why all the hub bub? The percentage of error when gauging your front sight with a known object at distance is subject to large errors, +/- 10% or so. That kind of accuracy is easily obtained with the simple method descibed in the above PDF.

Just stomp on the roach, Boot impulse and trajectory calculations are not required.   


If you want to know how wide your front sight needs to be, in inches, for a given MOA value, here is the Formula:

Front Sight Width (in inches) = 2*(distance to eye)*TAN(MOA/120)

OR   MOA(in inches) at a given distance = 2 * (distance in yards*36) * TAN(1MOA/120 )

example:  X = 2 * 100 yards * 36 * TAN (1/120) = 1.047197558580732439591741422812987138968

I checked it against my other formula --- It works fine

MODIFY:   and if you want to know the easy way-----   

MOA(in inches) at a given distance = Pi / (3 * (distance in yards / 100))    Note: This formula is good to 8 decimal places

Or even more simply:       MOA(in inches) at 100 yards = Pi / 3 
Pi=3.14159265

orig. posted by Truckinmike

[Whisker]

Great topic, long buried.  Just thought I'd dig it up for the masses...  Maybe it would fit better in the Rifle Knowledge Bank area?

[MCKNBRD]

Instead of all that math...why not just take a sheet of 1/4" graph paper, a ruler, pencil, and sharpie and make your own chart for use at 25 yards?

Start at the bottom and pick a centerline (preferably NEAR the vertical center of the sheet), mark it with the pencil.  Then, make a line that is 2 squares wide, centered on your centerline (henceforth to be known as CL).  Up at the top, make a similar line, lets say, with 6 squares on each side of the CL.  Now, from the edges of the long line on top and the short line on the bottom, use the ruler to make a straight line between the two.  You should have a funnel shape on your graph paper.  After you are satisfied with your "artwork", go over it with the permanent marker, nice and boldly.  (You will have to see this and interpret it from 25yds, after all!)  Now, you can make incremental marks to help ID the width.  Start at the bottom, make a BROAD horizontal line, with an easy-to-read number '2' on it.  Go up to the point where the lines you've drawn intersect horizontal lines that are 1/2" off the CL.  Another BROAD horizontal line, with a nice, BOLD "4" beside it.  You should do it for all whole-number widths; its MUCH easier that way.  What you have at this point (other than a splitting headache from the permanent marker!) is a gauge that goes from 2 moa wide at the bottom to 12 moa wide at the top.

After you've built your gauge, take it to 25 yds and assume a good shooting position.  Focusing on your Front Sight, move it up and down the scale until you've matched it to the width of the funnel.  You should be able to focus your eye on the number beside the width you matched, or at least count the hash marks and remember them.

FWIW, this is how I found out my Mini-30 has a 6moa front blade.

I'll see if I can find the picture that I scanned from JB at Ramseur eight-oh-eight!...but be warned, its flippin' HUGE!

Byrdman

[MCKNBRD]

FOUND IT!!!



Thanks, JB!

[Junior Birdman]

   Thanks, MCKNBRD!  I've given a lot of folks that thing and it finally got posted!  I just hope it comes out in the proper dimensions when printed.

   And you're right....It's HUGE!

[Geddys Uncle]

While my inner mathemagician really likes the formulae and figures, my inner farm-boy LOVES the idjit-resistant (there are no truly comprehensive foolproof, idiotproof, failsafes in this world) simplicity of the Funnel-chart.


instead of gassing yourself into discombobulation and headachery with the Sharpie fumes, wouldn't some black construction paper be more user-friendly?....or even mask the area that you want to stay "white" and spray-paint the other areas black, while you're outside......

actually, in order to not have blurriness of the black riflesight and the black paper be an annoyance, perhaps use the spraypaint trick, but, w/ Hunter Orange or some other easily-delineated color?

as the thoughts continue to stream, What about good, old duct-tape? nice straight edge to lay down right on the angled pencil-marks....contrasting color, no fumes, and, dare I say it? another darn fine use for God's Gift to handymen and marksmen everywhere.


just a couple of thoughts

[MCKNBRD]

To avoid the sizing issues altogether, JB, I recommend that folks look at your sheet and use the instructions I tried to cobble together to make their own out of graph paper...and as stated above, there are MANY ways to skin that cat.  Colors, patterns, whatever...the MOA funnel is the best way to figure out what your rifle has, thus equipping the Rifleman with another data point to help master his equipment and use that knowledge to make him more effective.

Actually, I need to make a couple of them for my own use...I gots some 'sperimentin' to do with my LTRs...

Byrdman

[Junior Birdman]

  Yup, they're just too easy to make. The only thing I do not recommend is a black wedge on a white background. (Prototype #1...not good)

   I like the black paper method, just make the thing 4 moa wide at the bottom and 12 moa wide at the top and you'll be able to cover every front sight I've ever come up against.

   Also make the "bars" which note the width large enough to see with either the Mk1 eyball or whatever glass you're using.

[Whisker]

My front sight says "NM 072." Is that weird?

[gunville]

National Match 0.072.

For round sights you can narrow them pretty easy - remove the fs, put the thread part in a drill and spin it against a file until you get the diameter your looking for - a mic or calipers help - a little black paint and you're done.

I recently did this with my AK sights - brought them to 0.050 and it made a big improvement - especially with aging eyes. With the AK's short radius it's a lot easier and quicker to form correct sight alignment and get a good sight picture.

Was that chart put-out by HP?  It looks like it's designed to sell toner and ink.