A reputable school that has many attendees using ARs points out the following:
Switching from the large to small aperture on an AR can move POI up to 5 MOA. This is caused by the aperture base being threaded with the adjustment screw. Flipping causes it to rotate through the threads slightly.
This is one more thing to be aware of with the AR along the Appleseed trail.
Quote from: AR-180 on September 06, 2007, 02:23:09 PM
A reputable school that has many attendees using ARs points out the following:
Switching from the large to small aperture on an AR can move POI up to 5 MOA. This is caused by the aperture base being threaded with the adjustment screw. Flipping causes it to rotate through the threads slightly.
This is one more thing to be aware of with the AR along the Appleseed trail.
They may be right about the Zero shift up, but, that won't be because of the threads. Therefore, don't trust what they're saying, no matter what school it is. Check it out yourself. It would be to compensate for the different zero required for 200 meters (large arpeture) to 300 meters (small arperture), and very well be made into the sight.
The windage changes slightly because of the threads, not the elevation. I'd have to look it up to see how much, but I can say it isn't too much. Certainly less than 5 MOA, more like a little over 1 MOA, as I think it's going to work out to 1.25 MOA on an A1 type rifle.
5 clicks to the turn on an A1, 1 MOA clicks. The sight would move 1/4 turn when flipping. That would be 1.25 MOA WINDAGE shift.
YEah, I've made my living with an "AR" for a while now.
It is my understanding that the elevation changes simply because the holes are not concentric. In other words, the larger diameter hole is centered slightly lower than the smaller hole. I haven't mic'd the thing, but as rear sights go, a little translates to a lot down range. You could use the sight adjustment formula in reverse to figure out how much. If you're that bored. ::)
Elevation change in inches
-------------------------- = height change needed for sight in inches
Distance to target in inches
Seems like I recall that .006" is about 1 minute difference for average sight radius's, so likely about that on an AR.
I would think that windage change is pretty close to negligable, and certainly not 5 minutes.
I just keep it simple and use the smaller aperture on mine for all shooting. Of course, none of my sights have markings on them either, you just count clicks.
One day I'll try it out for real and know for sure what it does. I don't trust anybody's opinion unless I see them actually shoot the thing and prove it on paper. There are just too many variations, kits and mixmasters out there to say that "ALL AR's do this". JB
JB, the AR has a short sight radius, so it's probably less to make 1 MOA difference.
Well, I think I'm going to be headed to the gun room shortly, and put the calipers to the AR, and settle this one.
Mine is an A2 with a "match rear sight" (that be fine thread elevation screw, not coarse). But, as I pointed out, that isn't going to make any difference.
Quote from: Junior Birdman on September 06, 2007, 08:26:57 PM
I just keep it simple and use the smaller aperture on mine for all shooting. Of course, none of my sights have markings on them either, you just count clicks.
There's no real point for using the larger sight during normal shooting anyway. The only reasons you would use the larger one are during times of lowlight or if you are wearing a protective mask. If your nose is on the charging handle, then the smaller aperture is still pretty large, and barely visible(peripherally).
Quote from: Muddogg on September 06, 2007, 08:45:38 PM
Quote from: Junior Birdman on September 06, 2007, 08:26:57 PM
I just keep it simple and use the smaller aperture on mine for all shooting. Of course, none of my sights have markings on them either, you just count clicks.
There's no real point for using the larger sight during normal shooting anyway. The only reasons you would use the larger one are during times of lowlight or if you are wearing a protective mask. If your nose is on the charging handle, then the smaller aperture is still pretty large, and barely visible(peripherally).
Close Quarter Battle (as in less than 200 meters) and low light conditions. When you're within 200 meters of an armed enemy, you don't have time to take perfect aim. Good enough is good enough.
And the small arperture is .015" taller (approximately) at the center than the large one. I just measured mine. .015" would about make the difference between a 200 meter zero and a 300 meter zero.
And I'll stand by the ignorance of the "reputable school". If they're going to teach the AR, they had better get their facts and reasons right.
That sounds about right on the measurements, Nickle. I think we have a winner! I'd still like to actually shoot the thing and prove it. JB
QuoteIt is my understanding that the elevation changes simply because the holes are not concentric.
That was my understanding, as well.
QuoteI'd still like to actually shoot the thing and prove it.
My results were more like 6.5moa
The M16A2 rear sight windage screw, around which the aperture pivots, makes 1/10 revolution per click of windage. Each windage click is 1/2 MOA.
Flipping the rear sight 90° to index from one aperture to another only displaces the sight by 2 1/2 clicks, laterally, for a total of 1 1/4 MOA. If the 'school instructor' is correct, and the factory did not take this into account when locating the apertures, then it's only a 1 1/4 MOA shift, not a 5MOA.
It's inconceivable, though, that the M16A2 design would not have compensated for the lateral displacement caused by flipping the sight- they probably built the necessary offset into the part. The USMC Operator's Manual does not say anything about imparting a windage deviation when flipping apertures, by which I infer that the design already compensated for the drift.
The manual, in summary, tells soldiers to deploy the rifle as follows:
To engage targets from <200M, set the elevation drum to 300M and flip the large diameter aperture up.
To engage targets from 300M and farther, flip the small diameter aperture up, adjust the elevation drum as needed. No mention of a needed windage correction in the manual.
As to the vertical difference between the two sights, the large aperture should be deployed from 200M or less; as one would expect, and as the manual specifies, the location of the large diameter aperture is correctly placed vertically for targets within 200M.
How does the military use the sight? Per USMC Operator's Manual TM 05538C-10/1, page 48, my comments in parentheses and italics:
"BATTLESIGHT ZERO
When battlesights are on your rifle:
a. The front sight post and rear sight windage knob are adjusted so you can hit your point of aim at 300 meters.
b. The unmarked (small diameter) aperture must be in the up position.
c. The 300-meter mark (of the elevation drum) is aligned with the mark on the left side of the receiver"
page 58,
"FIELD FIRING TECHNIQUES (CONT)
NOTE
The normal rear sight aperture (unmarked) flips forward to expose a larger aperture marked "0-2." Use the 0-2 aperture only when the 3 is aligned with the mark on the left side of the receiver and the sight is rotated all the way down. You will automatically get a zero of 200 meters. Use the 0-2 aperture when shooting at night or at close range, e.g., in a city or in dense jungle."
page 59:
"ADDITIONAL INFORMATION CONCERNING YOUR M16A2 SIGHTS
GENERAL. Your rifle sights should be set to a combat zero of 300meters. If you are told to engage a target at longer range, e.g., 500 or 800 meters:
a. Rotate the elevation knob so that the desired range mark is aligned with the mark on the left side of the receiver.
b. Aim, squeeze the trigger, and fire.
NOTE
Return the dial to 300 meters when the mission is over.
c. See conversion chart in back of manual."
Quote from: Francis Marion on September 07, 2007, 05:10:50 PM
It's inconceivable, though, that the M16A2 design would not have compensated for the lateral displacement caused by flipping the sight- they probably built the necessary offset into the part.
It may be inconceivable, but it's true, they did NOT (to my understanding, and I have well over 30 years experience with the M16A1 and M16A2) compensate for the windage shift.
It was apparently determined to be no need to compensate, as it would only affect 200 yards and under 1.25 MOA, as you mentioned. That isn't much, when you think about it.
The Army also zeroes using the small arperture, but we BSZ at 25 meters, with the elevation knob set at 300 plus one click.
When we qualify of a KD (Known Distance, in other words, paper targets) range, we only use the small arperture, not the large one. The large arperture is pretty much for combat use only. Yes, I did look this up in the current FM 3-22.9. I have a copy, and keep it current.
Quote from: Nickle on September 06, 2007, 07:37:50 PM
Quote from: AR-180 on September 06, 2007, 02:23:09 PM
A reputable school that has many attendees using ARs points out the following:
Switching from the large to small aperture on an AR can move POI up to 5 MOA. This is caused by the aperture base being threaded with the adjustment screw. Flipping causes it to rotate through the threads slightly.
This is one more thing to be aware of with the AR along the Appleseed trail.
They may be right about the Zero shift up, but, that won't be because of the threads. Therefore, don't trust what they're saying, no matter what school it is. Check it out yourself. It would be to compensate for the different zero required for 200 meters (large arpeture) to 300 meters (small arperture), and very well be made into the sight.
The windage changes slightly because of the threads, not the elevation. I'd have to look it up to see how much, but I can say it isn't too much. Certainly less than 5 MOA, more like a little over 1 MOA, as I think it's going to work out to 1.25 MOA on an A1 type rifle.
5 clicks to the turn on an A1, 1 MOA clicks. The sight would move 1/4 turn when flipping. That would be 1.25 MOA WINDAGE shift.
YEah, I've made my living with an "AR" for a while now.
The instructions for my Bushmaster tell you to sight in for 300 meters using the small aperture and the dial (elevation) setting at the 6/3 position and to make all elevation adjustments with the front sight until you have your 300 meter zero.
To get your 200 yard zero simply flip to the large aperture. To shoot at 300 yards flip it back to small. For greater than 300 meters, dial in the range/elevation knob to the distance.
This is how I did mine and it seems to work for me on 200 & 300. I have not shot it at greater than 300 meters so I can't say how well that works.
If it changes windage I can't tell it. Perhaps the apertures are drilled to compensate for this shift. Then again if you set initial zero using the large aperture and switched to the small, it would double the windage shift.
Then again I'm not a great shooter. Even from a bench set up either my 73 year old eyes, my rifle or a combination of the two are not capable of less than 2 moa groups.
I'm just too much of a rebel to do what the instructions say, and anyway, you could get kicked out of the Man Club for even looking at the instructions! ;D
I only have one AR that even has markings, and they are "wrong". I wanted to be able to set my sights as low as 100m, so I indexed the front sight so that the markings on the elevation knob were off by 200m.
In other words, if it says "3", it is actually 100m, 4= 200m, 5=300m, etc. Sounds hokey, but it works as long as I remember to dial in 200m more that the range I intend to use. This way I have a sight which is more useful at the lower ranges, and no need to hold off. JB
I like the Santose Improved Battle Sight Zero. You can find it on the Maryland Ar site.
You zero at 50 yds. instead of 25.
It calls for adjusting the elevation drum so your markings are correct for the range you are shooting.
Check it out as it is a good read about AR's and BSZ's.
Also you can use it for 25M. shooting by setting your elevation to 4 +/-
Quote from: Junior Birdman on September 26, 2007, 10:42:26 AM
I'm just too much of a rebel to do what the instructions say, and anyway, you could get kicked out of the Man Club for even looking at the instructions! ;D
I only have one AR that even has markings, and they are "wrong". I wanted to be able to set my sights as low as 100m, so I indexed the front sight so that the markings on the elevation knob were off by 200m.
In other words, if it says "3", it is actually 100m, 4= 200m, 5=300m, etc. Sounds hokey, but it works as long as I remember to dial in 200m more that the range I intend to use. This way I have a sight which is more useful at the lower ranges, and no need to hold off. JB
On many ARs, you can adjust the marking to be correct after you have sighted them in....look down into the rear apature and, if you see an allen keyed screw, you can use it to unlink the drums. If it is an A2-style drum, it should be there. You sight 300m at 6/3 or 8/3- you should have one or the other depending on gun barrel length- then you may be able to subtract click numbers down to 200 and 100. 300m should be about your 25m setting.