Info needed on AR-15 A2 sights

[ks_shooter]

For standard A2 sights, the Bushmaster operators manual gives the adjustment per click for the front sight elevation and rear sight windage. Based upon the numbers in the manual, these adjustments work out to about 1.2 MOA per click for the the front sight post and 0.43 MOA per click for the rear windage. These seem a little odd since I would have expected something like 1.0 and 0.5 MOA. But the manual does indicate that three clicks of the windage are needed to adjust as far as one click of the front elevation. Anybody know how the military selected these adjustment values? Also, what is the MOA/click for the rear elevation adjustment? Thanks.

[Kharn]

I bet it has something to do with the Army using the metric system rather than English units for distance, but I could be wrong.

Kharn

[Steve Smith]

Based upon the numbers in the manual, these adjustments work out to about 1.2 MOA per click for the the front sight post

-Should be 1.25 per click, 4 clicks being 5 minutes. (5 minutes per rev)


and 0.43 MOA per click for the rear windage.

-Should be .5 or 1 Minute windage on a standard sight...I believe all Bushys have .5 minute windage clicks.

These seem a little odd since I would have expected something like 1.0 and 0.5 MOA. But the manual does indicate that three clicks of the windage are needed to adjust as far as one click of the front elevation.

-umm, you mean ELEVATION, right? (windage clicks can't = elevation changes on front sight)

Anybody know how the military selected these adjustment values?

-nope

Also, what is the MOA/click for the rear elevation adjustment? Thanks.

-The rear elevation should be either .5 or 1 MOA per click. Shoot 5 rounds at, say 100 yards (make it easy), move the rear sight up 20 clicks. Shoot it again. If POI changes 20 inches, then its a 1 MOA elevation sight. If it only moves 10 inches, then its half minute. If it moves 5, its quarter minute (doubtful in a non-match gun) Do same for windage. You can do this at 25 yards of course, but your POI shifts will be divided by four.


Note: No one ever said your stock gun sights were built correctly. Not only that, but many companies can't figure out how far away to space the detents so you really get .5 or 1 MOA increments. For example, RRA says that they're rear sight is 1/4 x 1/4 but put in on a dial indicator and you get 1/3 MOA elevation increments almost dead nuts. What your sight does an what it should do could be completely different. Only shooting it will tell you.

[ks_shooter]

The calculation to determine the Minutes of Arc (MOA) is given by:

MOA = (Adjustment distance @ target/Distance to target) X (10,800/PI)

A 1" change at 100 yards (3600") equals

MOA = (1"/3600") X (10,800/PI) = 0.955

So actually, if you shoot 1" @ 100 yards you are 0.95 MOA. 1 MOA at 100 yards is really 1.047".

The Bushmaster manual states that the front sight adjustment is 3.5 cm/click at 100 m and that the rear windage is 1.25 cm/click at 100 m. These values result in the following:

Elevation MOA = (0.035/100) X (10,800/PI) = 1.203

Windage MOA = (0.0125/100) X (10,800/PI) = 0.430

1.25 MOA elevation would be 3.64 cm @ 100 m (instead of 3.5 cm).

0.50 MOA windage would be 1.45 cm @ 100 m (instead of 1.25 cm).

Is the military just math-challenged or did they have a reason why they didn't use 0.5 and 1.0 MOA on the sights?

The manual also indicates that it takes 3 rear sight windage clicks to move the POI one square left or right on the 25 m Zeroing Target. It takes one click on the front sight elevation to move the POI one square up or down. So the rear windage adjustment is 3X smaller than the front sight elevation adjustment. This is what I meant in my earlier post. I actually do understand the difference between elevation and windage.

I measured the rear elevation adjustment on my A3 sight last night it moves 0.080" for 25 clicks. With a sight radius of 19.75", that results in a shift of POI at 100 yards of:

shift = (0.080/25) X (100 X 36)/19.75 = 0.5833"

MOA = (0.5833"/3600") X (10,800/PI) = 0.557

Which is about 11% greater than 1/2 MOA.

It doesn't really matter if you set your zero and leave it. But if your are trying to make use of the Bullet Drop Compensator with loads that are not Mil-Spec an 11% error is a lot.

[Steve Smith]

Ahh, I see where you're coming from now.

I have no idea why the sights were originally set up to run off of inches. The further out you go, the weirder it will get if you come from a MOA point of view. I imagine that yes, the military was math challenged and preferred to tell their troops "come up 15 inches". Of course, this has an inverse weirdness at distance, where 15 inches at say 600 yards would have to be subtended to work on their sights.

In actuallity, a great deal of the ammo will have a similar flight path to 300 or so. I'd suggest you think in MOA and forget the inches bs from Bushy. With inches you have to calculate at each distance, but with MOA you are dealing with specific increments since your sights already work by changing angles. Also, I'd suggest you are thinking too hard. Due to the nature of the sight, you can't get a fractional setting, as you know. Therefore, round up or down as appropriate. .43 MOA = .5 MOA when rounded up, and you don't have much choice.

"The manual also indicates that it takes 3 rear sight windage clicks to move the POI one square left or right on the 25 m Zeroing Target. It takes one click on the front sight elevation to move the POI one square up or down. So the rear windage adjustment is 3X smaller than the front sight elevation adjustment. This is what I meant in my earlier post. I actually do understand the difference between elevation and windage."

- Sorry about that...it just sounded weird. Not knowing how big that square is (who the hell shoots at 25 yards?) it is impossible for me to know how much that moves it in the grand scheme of things.

[Handy]

Keep in mind that all of that goes out the window if you have other than the standard 20" barrel A2 sight radius. If you have M4 "shorty" setup, each click is MORE than 1.2 MOA for the front and proportionally different for each adjustment point.

[Steve Smith]

That's very true! Of course, the "inches" method will be out the window as well!

[mons-meg]

IIRC, the miltary uses "mils" instead of degress, minutes, and seconds. There are 6400 mils in a complete circle, while there would be 360 * 60 = 21600 minutes in a circle. I seem to remember that one mil supposedly equals 1 meter lateral deviation at 1000 meters of range. I am not a trigonometry master, or I would work that up to see if it were true.. Anyway, that's what they told us in artillery school.

Another odd meters/yards tidbit: the known distance qualification range at Parris Island uses yards, where MCRD San Diego, which uses the Camp Pendleton range, uses meters. So, if you qual on the range in California, you get to add 4 points to your score since you are shooting farther.

[HKP7PSP]

I need to know how many MOA or inches at 100 yards the rear sight elevation is per click for a 20" A2.

[Handy]

Then I guess you didn't read this thread. The thread states that it varies by manufacturer, and that the author's gun is 1.25 MOA per click elevation.

[HKP7PSP]

I thought that was the conclusion he reached for the front sight.

[HKP7PSP]

Ok, I computed it. My rear moved .0061 per click. Sight radius is 19.75.

1.11 inches at 100 yards = 1.060 MOA.

[Handy]

Well, the rear sight is marked for distances and reflect bullet drop. The first couple clicks over 200 might be fairly even and you could calculate the change per click between 200 and 300. But really, rifle sights work by having an ever increasing the MOA, since the drop between 200 and 400 yards is much less than 400 to 600 yards.

So really, each click should not be an even number of MOA. Which is the reason it is not marked in MOA, but in range.

Why would one need the MOA of a range adjustable rear sight?

[HKP7PSP]

The clicks on the rear sight are fixed. To distance markings are not the clicks.

I want it so that I can predict how many clicks up I need to do to be on paper the first time I shoot at 600 yards with 77 grain bullets going 2602fps.

The answer is -- 17 clicks up from the bottom of my sight.

[Handy]

And is that number of clicks greatly different than the 600 yard setting?

[HKP7PSP]

Well it is hard to say since my rifle is not zeroed according to how one should do it to make those work.

But those are calibrated for 62 grain NATO ammo and I am not using that, so I would not really expect them to work.

But as it is now, it is 6 clicks different.

[Handy]

So you don't have a 200 yard zero?

[Jarhead Ed]