Front sight post moa math

[dart67eb]

Folks,

The instructor manual say that the front post on a garand and  20" AR is 7moa.  It seems about right.  I can't seem to make the math work, though. 

  moa = p/(((R x 2 x 3.1416)/360)/60)

  R= distance from eye to front post
  p= post width

  So, the distance from eye to front post is 22" That is the radius.  Times two is the diameter or 44"   Times 3.1416 (pi) gives you the circumference or 138.2.  Divide by 360 degrees to get .38 inches per degree.  Divide again by 60 to get .006 inches per MOA.  Standard front post is @ .072.  So divide that by .006 and get 12.  12MOA?   Where am I going wrong?  There should be a way to determine the moa of a front sight on any rifle without actually posting paper at 25yds.

[Burnett]

Haven't studied up on it at all, but...
I understand why you measured from the eye, but is it supposed to be from the rear sight?
Not sure what that would do the the calcs.
And it's too late for me to get my brain wrapped around it now.

Greg

[ITB]

Folks,

The instructor manual say that the front post on a garand and  20" AR is 7moa.  It seems about right.  I can't seem to make the math work, though. 

  moa = p/(((R x 2 x 3.1416)/360)/60)

  R= distance from eye to front post
  p= post width

  So, the distance from eye to front post is 22" That is the radius.  Times two is the diameter or 44"   Times 3.1416 (pi) gives you the circumference or 138.2.  Divide by 360 degrees to get .38 inches per degree.  Divide again by 60 to get .006 inches per MOA.  Standard front post is @ .072.  So divide that by .006 and get 12.  12MOA?   Where am I going wrong?  There should be a way to determine the moa of a front sight on any rifle without actually posting paper at 25yds.

Your math is correct.  I find it easier to do it with trigonometry.

tan θ = width/length, where θ (theta) is your angle in degrees

My Garand front sight is .069", my eye is 30" from the front post. 

arctan (.069/30) = .1318° or 7.9 MOA

So, using your example of 22" and .072:

arctan (.072/22) = .1875° or 11.25 MOA

Using your numbers and the way you're calculating it (and not rounding), it also calculates to 11.25.

I wish I knew I could apply trig to shooting when I first learned it.  Would've made it a hell of a lot more interesting.

ETA - I have a national match front post on one of my AR's - .048" wide.  That would be 7.5 MOA wide at 22 inches.

ETA II: Most online calculators do trig in radians, not degrees.  So if you use Google, add "in minutes" to the equation.  For those unaware of Google's talents, cut and paste this into the Google toolbar and see the magic:  arctan (.069/30) in minutes

[dart67eb]

Folks,

The instructor manual say that the front post on a garand and  20" AR is 7moa.  It seems about right.  I can't seem to make the math work, though. 

  moa = p/(((R x 2 x 3.1416)/360)/60)

  R= distance from eye to front post
  p= post width

  So, the distance from eye to front post is 22" That is the radius.  Times two is the diameter or 44"   Times 3.1416 (pi) gives you the circumference or 138.2.  Divide by 360 degrees to get .38 inches per degree.  Divide again by 60 to get .006 inches per MOA.  Standard front post is @ .072.  So divide that by .006 and get 12.  12MOA?   Where am I going wrong?  There should be a way to determine the moa of a front sight on any rifle without actually posting paper at 25yds.

Your math is correct.  I find it easier to do it with trigonometry.

tan θ = width/length, where θ (theta) is your angle in degrees

My Garand front sight is .069", my eye is 30" from the front post. 

arctan (.069/30) = .1318° or 7.9 MOA

So, using your example of 22" and .072:

arctan (.072/22) = .1875° or 11.25 MOA

Using your numbers and the way you're calculating it (and not rounding), it also calculates to 11.25.

I wish I knew I could apply trig to shooting when I first learned it.  Would've made it a hell of a lot more interesting.

ETA - I have a national match front post on one of my AR's - .048" wide.  That would be 7.5 MOA wide at 22 inches.

ETA II: Most online calculators do trig in radians, not degrees.  So if you use Google, add "in minutes" to the equation.  For those unaware of Google's talents, cut and paste this into the Google toolbar and see the magic:  arctan (.069/30) in minutes

Thanks.  I'm glad somebody remembers trig.  Anyway, I'm getting out the tape measure and caliper tomorrow to get the real numbers.  We may need to get Dond in on this if we are putting out bad dope.

[dart67eb]

"For those unaware of Google's talents, cut and paste this into the Google toolbar and see the magic:  arctan (.069/30) in minutes"

Now THAT is cool!

[ITB]

Thanks.  I'm glad somebody remembers trig.  Anyway, I'm getting out the tape measure and caliper tomorrow to get the real numbers.  We may need to get Dond in on this if we are putting out bad dope.

Agreed.  I always preface any of this info with "figure it out for your own rifle, every rifle is different, etc."  I knew my Garand was 8 MOA, so that is the example I usually use.

"For those unaware of Google's talents, cut and paste this into the Google toolbar and see the magic:  arctan (.069/30) in minutes"

Now THAT is cool!

Yup, all you need is your smartphone, and you can do the calcs right on the line!

[floydf]

There is also the difference in focal plane to consider.

Because the shooter cannot  keep both the front post and the target in focus, a nominal front post width of 7.9 MOA to produce an visually apparent 7 MOA sounds about right.

[techres]

For the purposes of 4 MOA shooting, I find it easiest to put up a 4MOA target at 25 meters and look at it.  I can get pretty close, say + or - ~1.5 MOA in the measurement.  That is gonna be good enough for range finding out to 500 yards.

Now the trig trick may be best for an oddity rifle like a short barreled rifle.  I am not sure what effective size it's front post is other than to say that SPQR complained that it was "like 32 MOA or something!" when he declared his distaste for it.   

Techres

P.S. That Google trick is cool!  Now I need to take a trig class to have a chance to use it.  My engineer father is still mad I evaded trig in school...

[oladcock]

I got through trig by drawing out scale diagrams and measureing what I was trying to find!  Like others, I slap a 1/4 MOA target out there and see how much my front post covers. Many "stock" front posts cover about 8 MOA but this is going to vary with the shooters geometry and sight radius. The Tech Sights on my 10-22 are exactly 4 MOA and I much prefer them over 8. Got a 16" AR and the sight radius is very short. I put a NM front post on it and it's still about 6 MOA, still too big for my likeing. I'll either buy something smaller or chuck it up in a drill press, break out the file and make my own.....O.L.

[SPQR]

I used your method, tech.  On the SBR the post appeared to be a little over 8 times the width of a 4moa square.  Paa!  Phooey.  Silly communist rifles. 

[ItsanSKS]

There is also the difference in focal plane to consider.

Because the shooter cannot  keep both the front post and the target in focus, a nominal front post width of 7.9 MOA to produce an visually apparent 7 MOA sounds about right.

(emphasis mine)

Folks, I urge you not to get wrapped up trying to figure out why your eye doesn't read the width of your front sight the same as a caliper does. 

Take a moment to think this through-

The reason you want to know the width of your front sight is so that it can be used as a range finder, and to assist in hold-offs for windage.

For both of these, you are comparing the visual appearance of a target to the visual appearance of your front sight.

In other words, it is more important to know what size your front sight appears to be, to your eye, than it is to know what the front measures, according to the math.

[dart67eb]

FWIW, these are the numbers I got:

CMP M1 Garand - 8.85MOA  (.085/33)
20" AR - 9.76MOA (.071/25)
Mosin-Nagant 91/30 - 7.19MOA (.081/38.75)

Next, I will line them up on a known sized object at 25yds. 

[dond]

Hey, what's wrong with you people?    Is this all you've got to do on Thanksgiving day?      

At first glance, what you all have done seems OK, but I'm not about to worry about it tonight. I'll check it out tomorrow, perhaps with a little different approach. It also seems that I've read somewhere that the front sights on some ARs are different.

[dond]

OK Guys,

itb has the easiest and fastest way to do this. If you have a calculator, you can figure it out in much less time than it would take to log onto Google.

But recall how I did this in the IM: by comparing the front sight to a man sized object at BSZ of 300 yards. And as a couple of you have indicated already, there is going to be some uncertainty or a approximation, here. Especially when the man sized object is assumed to be 20 inches wide.

Let me suggest an alternate way which you may find interesting. Consider a circle of radius r. Then consider a segment on the circumference of that circle of length s. The equation

s=ra

holds, where a is the angle from the center of the circle which subtends the segment, s. But, the angle, a, MUST be expressed in radians. In fact, you may recall the circumference of a circle is given by

c=2(Pi)r

From this, it is easy to see that 2(Pi) radians is equal to 360 degrees.

s=ra can be solved for a,

a=s/r

Therefore, your eye may be considered the center of the circle and the width of the front sight would be the segment, s. And the angle, in radians can, therefore, be found from the above equation. This is really neat because we can convert radians into MOA easily, almost anyway. A milliradian, or (1/1000) radians, is 1 mil on a mil dot scope, and is represented by 3.6 inches at 100 yards. Therefore, we could say that 1 milliradian is 3.6 MOA.  Therefore, you could just multiply the value obtained for the angle a times

(3.6 MOA/(1/1000)) radians

to get the value in MOA, which is what we want.

However, this will give you a value a little larger than the method obtained by itb. For example, Instead of 7.9 MOA for his Garand, you would get 8.3 MOA. This is a fairly small amount, and the difference involves the "almost" mentioned earlier. And the reason is that 1 MOA is not really represented by 1 inch at 100 yards. It is something like 1.047???. When this is taken into account, 7.9 MOA will be obtained. But in comparison to other estimates with the front sight, this will be negligible.

But I bring up the equation, s=ra, because. It is a neat equation which can be used to find the distance to a target easily.

If a target, or object, has a known size, that size can be represented by s.
And the distance to that object can be represented by r. And the equation can be solved for r.

r=s/a

If you have a mil dot scope, it is easy to find the number of mils which the target subtends. say x mils. But since this is really milliradians, or mr, and we must express a in terms of radians. The angle a would, therefore, be entered as

(x/1000) radians. And furthermore, r and s must be entered in the same units, say yards.

Therefore, the distance to the target would be given by

r = s/(x/1000)

or

r = s(1000)/x

I love mil dot scopes.

And I hope you haven't fallen asleep.

Incidentally, my Ruger 10/22 with tech sights has a front sight of 7.1 MOA.
I have an M1A NM with 8.2 MOA and a Scout with 5.2 MOA. The strange thing is that both of these front sights are listed as NM, but they have different thicknesses.

[oladcock]

"Incidentally, my Ruger 10/22 with tech sights has a front sight of 7.1 MOA.
I have an M1A NM with 8.2 MOA and a Scout with 5.2 MOA. The strange thing is that both of these front sights are listed as NM, but they have different thicknesses."

In other words, NM means nothing when it comes to thickness. Learn something new everyday! ...O.L.

[dart67eb]

So, if the front post on my AR is almost 10moa, a 20" target will cover it at 200 yds.  This is a big difference from saying you have a 7moa post which brings it to 286 yds or almost 6 clicks in come ups (2 clicks per moa).  Do we want to cover this when we teach KD?  Or am I just losing my mind here?

[dart67eb]

This is Andrew. I'd just like to say that you can find the angle measure Don's way except that it doesn't have to be in radians. The formula for finding it in degrees is s=(a)(pi)(r)/180
where s=sector length, a=angle in degrees, and r=radius. You can then divide by 60 to get MOA.
So i guess that the formula for MOA would be s=(a)(pi)(r)/10800 since 180x60 equals 10800. In this second case a would automatically equal the angle measure in MOA.

[SuperDave#2]

Please excuse my intrusion into this discussion... DonD is correct with the 20" assumption when calculating for distance...

At Volunteer Training Site - Tullahoma we use both Type E silhouette ( 40" x 20" ) and Ivan silhouette ( 40" x 20" ) targets on the Modified Record Fire Range (MRF) which is a 300 meter pop up qualification range... We also use both target silhouettes when we do a sniper match for one of the events in calculating distance using a mil-dot scope with 12 targets set out from 100 - 1500 meters ( the shooters have to determine the distance of all 12 targets and then hand in their score sheet for this part of the event ) ...

I happen to have both of those targets at the house that I use to teach TN Army National Guard Units Preliminary Marksmanship Instruction before they go to one of our Ranges at their request from Range Control...

Please excuse the mess in my Garage

Type E






Ivan


Ivan in front of the Type E ( I always ask the question to Unit during the PMI Course if they think that the Ivan is not as wide or tall as the Type E and then I show them they are the same...


Sorry I could not add more to the conversation than this...

WK2

[dond]

WK2, it's good to hear from you again. Thanks for the post. I hope to get down to Smyrna again soon.

And Andrew, you entered the angle a in degrees, but when you multiplied by
(Pi)/180 you converted to radians. When you write s=ra, the a MUST be in radians.

[dond]

Dart,

With the numbers you give above, the correct value should be 9.8 MOA instead of 9.96. But assuming 10 MOA, this would indeed cover a 20 inch target at 200 yards. But if you are zeroed at 200 yards on the target, there would only be 2 MOA required to go to 300 yards, with the AS come-ups, which would be 4 clicks instead of 6 clicks, according to your post. Or are you confusing horizontal with vertical MOA here? But there are other variables here too. Perhaps everyone doesn't turkey neck like we do. This would change the eye to sight distance. Four more inches would take it to 8.4 MOA. And then too, as stated earlier, it seems like I read somewhere that different AR sights have different sizes. I'm not that familiar with ARs, but that could make a big difference.

[dart67eb]

Don,
You're right.  It should be 4 clicks.  I was using the standard 3,3,3,4 when I wrote that instead of the specific AR come-ups.  Also, I must have fat fingered the 9.96.  It should be 9.76.  As for turkey-neck, I do and teach nose to charging handle on an AR.  Either way,  I am going to teach  the 7 moa example but I plan on explaining how to measure your front sight MOA using the pir2 way and give them the google formula as a quick cheat.  I'm afraid that if people assume a 7 moa post when they have a 10 moa post, they'll be confused.  Kind of like when you have your son help you with your homework.

Dart,

With the numbers you give above, the correct value should be 9.8 MOA instead of 9.96. But assuming 10 MOA, this would indeed cover a 20 inch target at 200 yards. But if you are zeroed at 200 yards on the target, there would only be 2 MOA required to go to 300 yards, with the AS come-ups, which would be 4 clicks instead of 6 clicks, according to your post. Or are you confusing horizontal with vertical MOA here? But there are other variables here too. Perhaps everyone doesn't turkey neck like we do. This would change the eye to sight distance. Four more inches would take it to 8.4 MOA. And then too, as stated earlier, it seems like I read somewhere that different AR sights have different sizes. I'm not that familiar with ARs, but that could make a big difference.

[dond]

I have made an additional calculation which may interest you. As stated earlier,
1 mil at 100 yards is represented by 3.60 inches. And 1 MOA at 100 yards is, in actuality, represented by 1.0472 inches. Therefore,

1 mil = 3.44 MOA,

after rounding off.

And Dart, I knew you did turkey neck and taught it as well. I was thinking of others outside of AS. I would be interested in how the shooters respond to your calculation on the front sight. I suspect it may be too complicated for them, along with everything else they are asked to learn. The simplest thing might be to just mention that it can be calculated from measurements made of the width of the sight and the distance to the eye. And then refer to comparing the front sight to the 1 inch square.

[SPQR]

 

they don't put stuff like this in the gun mags.  must not be able to fit it in between all the advertisements.

[Old Dog]

Wow, pop ups with no holes in them.  I used to have some of those.   Now I have to tape paper them to see the new holes.

Junior Birdman used to have a "tool" on an 8.5X11" piece of paper that he'd put up at 25 meters.  Everyone then got to lay down and aim their rifle at the piece of paper.  It had a tapered target (vertical with smaller width at the bottom) and you'd line the top of your sight up so the corners of your sight "touched" the sides of the taper.  There were lines horizontally across the target and they corresponded to the width of your front sight in MOA.

Not sure if that tool was ever saved on the forum here or if its in the instructors took kit somewhere or if its lost.  If junior birdman can still be contacted he might be able to e-mail it to someone or put in on the forum himself.   It was quick and simple to use right there at the range.

[oladcock]

Wow, pop ups with no holes in them.  I used to have some of those.   Now I have to tape paper them to see the new holes.

Junior Birdman used to have a "tool" on an 8.5X11" piece of paper that he'd put up at 25 meters.  Everyone then got to lay down and aim their rifle at the piece of paper.  It had a tapered target (vertical with smaller width at the bottom) and you'd line the top of your sight up so the corners of your sight "touched" the sides of the taper.  There were lines horizontally across the target and they corresponded to the width of your front sight in MOA.

Not sure if that tool was ever saved on the forum here or if its in the instructors took kit somewhere or if its lost.  If junior birdman can still be contacted he might be able to e-mail it to someone or put in on the forum himself.   It was quick and simple to use right there at the range.

That's what I've always done with our "5 spot" drill targets. Color in 2 squares, that's 2 MOA, 6 is six, etc... Works well on scopes to determine cross hair thickness or dot/circle sizes. We've got the tools....O.L.

[dond]

Junior Birdman's sheet is in the RBC manual.

[jmdavis]

Here is a digital version that BD made after the Jan VA weekend KD.

http://appleseedinfo.org/smf/index.php?topic=20553.msg161126#msg161126

It replaces a more primitive version that I had which was a scan of JB's version.


Mike

[PHenry]

In other words, it is more important to know what size your front sight appears to be, to your eye, than it is to know what the front measures, according to the math.

Roger that. For our recent RBC, I created three cardboard cutouts of a 6' man standing (walking towards the shooter), taken directly from a photo that was blown up to represent a good friend of mine (Bob) who got married is no longer allowed to attend anything (long story). If the real "Bob" were to lay out on top of this cut out (affectionately called "Bob Who"< referring to how I respond when asked "Where's Bob?) - it would match nearly perfectly.

I created the cardboard replicas for the express purpose of front site assessment and target detection. The result proves what ItsanSKS has stated above. The math, while more "accurate", becomes moot. It's all about how your front site looks when placed onto a man-sized target. I know exactly what my front site looks like on target at 100, 200, 300, 400, & 500 yards and so does everyone else that attended the RBC.

Just like standard come-ups vs. real DOPE gathered from shooting at actual distance. The info gleaned on the range in varying light levels, and at different distances is invaluable.

I will freely admit to being old-fashioned and low-tech, but sometimes it works well. I told the shooters at the RBC - what you git from the internet and guys talking at the range isn't always useful (not in those words exactly  ).

PH

That's my .02

[dart67eb]

I have a copy of that in my SB book.  I'm going to try it out next time I get to the range. 

Wow, pop ups with no holes in them.  I used to have some of those.   Now I have to tape paper them to see the new holes.

Junior Birdman used to have a "tool" on an 8.5X11" piece of paper that he'd put up at 25 meters.  Everyone then got to lay down and aim their rifle at the piece of paper.  It had a tapered target (vertical with smaller width at the bottom) and you'd line the top of your sight up so the corners of your sight "touched" the sides of the taper.  There were lines horizontally across the target and they corresponded to the width of your front sight in MOA.

Not sure if that tool was ever saved on the forum here or if its in the instructors took kit somewhere or if its lost.  If junior birdman can still be contacted he might be able to e-mail it to someone or put in on the forum himself.   It was quick and simple to use right there at the range.

[dart67eb]

I'm going to make one of these up when I get a chance.

In other words, it is more important to know what size your front sight appears to be, to your eye, than it is to know what the front measures, according to the math.

Roger that. For our recent RBC, I created three cardboard cutouts of a 6' man standing (walking towards the shooter), taken directly from a photo that was blown up to represent a good friend of mine (Bob) who got married is no longer allowed to attend anything (long story). If the real "Bob" were to lay out on top of this cut out (affectionately called "Bob Who"< referring to how I respond when asked "Where's Bob?) - it would match nearly perfectly.

I created the cardboard replicas for the express purpose of front site assessment and target detection. The result proves what ItsanSKS has stated above. The math, while more "accurate", becomes moot. It's all about how your front site looks when placed onto a man-sized target. I know exactly what my front site looks like on target at 100, 200, 300, 400, & 500 yards and so does everyone else that attended the RBC.

Just like standard come-ups vs. real DOPE gathered from shooting at actual distance. The info gleaned on the range in varying light levels, and at different distances is invaluable.

I will freely admit to being old-fashioned and low-tech, but sometimes it works well. I told the shooters at the RBC - what you git from the internet and guys talking at the range isn't always useful (not in those words exactly  ).

PH

That's my .02