View Full Version : Interesting Sight In Issue
Was sighting in my Aimpoint today and encountered an interested issue. Basically, my irons and AP should both be zeroed, but when looking through my irons the dot is about half way down the FSP.
Here's the story: a month ago I used this rifle for hunting and sighted in the irons with Federal 60gr Nosler Partitions. No issues, did a quick zero and decided to throw my Aimpoint on it from my other rifle...couldn't hit a damn thing all of a sudden. I set the dot to the irons' zero, and still wasn't hitting anything. Not even on the target at 50 yards in most cases, had trouble even figuring out where to go with it. Seemed like possibly an issue with the dot or mount, I seemed to group high for a few shots, then way low, then to the right...threw it in a box and decided to deal with it later, hunted with just the irons.
Fast forward to today. Took the dot out again and zeroed at 25 yards at the indoor range. Grouped well, got my zero nailed down, no issues with the dot. Until I looked through the irons and found that the dot was damn near the bottom of the FSP. I didn't have time to bother checking again with the irons to really check into it more...either way it seems like there's an issue with either the dot (or mount) or the irons...
Any insight as to my problems here? I'm almost positive if I tried to move the FSP down enough to align with the dot I'd bottom out before I got there. FSP is already a little low.
Gear:
-BCM 14.5 LW middy w/ FSB
-DD A1.5 rear fixed sight
-ADM cantilever mount, 1/3 CW
-Aimpoint M3L
-Shooting AE XM193
Groups look good, so I don't think anything is loose or the rifle is shooting erratically.
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What kind of groups do you get with irons? and at what ranges?
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An Undocumented Worker
12-10-10, 22:51
Looking at your parts list it seems that the ADM mount is designe for lower 1/3 co witness. From what I understand this will mean that the irons sight plane looks through the lower third of the aimpoint. This also means you can't look through the Irons and expect the dot to sit right on top of the front sightpost. The irons and the Aimpoint have to be zeroed seperatly and used as seperate sighting systems.
If you want the dot to sit on the front sightpost and be zeroed you need a mount for the aimpoint designed for absolute cowitness. this would center the aimpoint right in the line of sight for the Irons.
Looking at your parts list it seems that the ADM mount is designe for lower 1/3 co witness. From what I understand this will mean that the irons sight plane looks through the lower third of the aimpoint. This also means you can't look through the Irons and expect the dot to sit right on top of the front sightpost. The irons and the Aimpoint have to be zeroed seperatly and used as seperate sighting systems.
If you want the dot to sit on the front sightpost and be zeroed you need a mount for the aimpoint designed for absolute cowitness. this would center the aimpoint right in the line of sight for the Irons.
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Since an Aimpoint is parallax-free, with 1/3 Co-Witness, both sights will align perfectly. It wouldn't be called a "Co-Witness" if they didn't co-witness each other.
Can't compare the view through an Aimpoint to that of a scope. If you can see the red dot, no matter where it is in the Aimpoint, that is where the bullet is going.
Don't know how it works, just know that it does??
If I had to guess, I'd say the OP has a rear sight problem (loose, slop, etc.) or the he doesn't shoot very well through peep sights. But that is just a guess until we get more info.
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can we please get more info?
can we please get more info?
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Looks like he started a Thread and then hit the crib. :) That's what I should have done...
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I never measure groups, but I'd say about 4" at 50 yards with irons. I'm not terribly good with irons, but I occassionally do better.
At the 25 yard range today with the dot I was pulling 1" or less groups. Usually at 50 I can also pull about 1" groups, maybe out to 2" on a bad day or if I'm in a hurry.
On my other rifle (franken-gun, Mega receivers, RRA BCG and barrel, VTAC tube) everything cowitnessed perfectly. For some reason not so with the BCM.
Everything is tight, rear sight came off the other rifle as well, actually. FSB looks OK but I don't have another on hand to compare side by side.
I really don't think it's me because I can group with either sight. Granted groups tend to suck a little with irons but still a group and still plenty easy to see where I was trying to hit. It's not like the holes are spread out all around the target. The first day out with the AP on this rifle I would still group...it just wouldn't group while cowitnessed with the irons.
Hopefully I can get out to the outdoor range next week or maybe even Sunday and really nail down the issue.
What little I have played with an RDS, I have observed the co-witness only works when lining up the front sight through the rear aperture. Being parallax free, the eye will see the dot of the RDS on the target regardless of head position.
As the iron sights are not parallax free, the eye will only see the front sight on target when the head is positioned to look through the rear aperture.
If the RDS is mounted so it's sight plane is lower, higher or to the side of that of the iron sights, a true co-witness will not be achieved when both are correctly sighted
Belmont31R
12-11-10, 06:54
Zero them seperately. The RDS and irons lining up only works if everything is in 100% alignment. Putting the dot on the FSP is only to get you on paper (most of the time).
An RDS and irons are two sight systems (already mentioned). They are not supposed to be used together.
If the dot and irons line up after zeroing both on paper then great. If not then they don't.
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Since an Aimpoint is parallax-free, with 1/3 Co-Witness, both sights will align perfectly. It wouldn't be called a "Co-Witness" if they didn't co-witness each other.
Can't compare the view through an Aimpoint to that of a scope. If you can see the red dot, no matter where it is in the Aimpoint, that is where the bullet is going.
Don't know how it works, just know that it does??
If I had to guess, I'd say the OP has a rear sight problem (loose, slop, etc.) or the he doesn't shoot very well through peep sights. But that is just a guess until we get more info.
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I think what An Undocumented Worker is trying to say is that the red dot will not sit on the front sight post with a lower 1/3 mount, if looking through BOTH front and rear irons. That type of mount is designed to allow the use of the irons in the lower 1/3 of the RDS, so the OP should not expect the red dot to be positioned on top of the front sight post while using a lower 1/3 mount and lining up front and rear irons.
As already stated with an absolute co-witness mount this MAY occur, but both systems should be zeroed separately.
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From what I have seen looking through several different rifles with several different Aimpoint T-1's on either ADM or LT 1/3 Co-Witness mounts, with the irons sighted-in separately from the T-1 being sighted-in, the red dot will be on top of the front sight post when using the iron sights. That is why it is called "co-witness". 1/3 Co-Witness will line up in the bottom 1/3 of the T-1 and Absolute Co-Witness will line up in the center of the T-1. This is the beauty of parallax-free.
I just looked through one of my Mid-Lengths with my T-1 on it. No matter how I move my eye, I can't make the red dot move to the bottom of the Post. If I make the red dot appear at the very bottom of the window, it is only about halfway to the bottom of the Post. This field of view is with my T-1 mounted most forward on the receiver rails, if it is mounted closer to the shooter, the FOV would increase. We don't know where the OP has his RDS mounted.
If the irons are canted, I would think the red dot and the irons would only line up at an exact range. If the person is not using the peep sight correctly, then it is hard to say what is going on.
Just guessing, I'd say the OP is not using the iron sights correctly. When he is co-witnessing, the RDS is forcing him to have a different sight picture. And from what I've seen, it is not uncommon for people to use peep sights incorrectly.
But maybe it's just me....
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Shot a match today and the dot seemed to keep 'zero'...I use that loosely since I'm pretty sure my 25 yard ghetto zero wasn't all that. Missed a lot of longer shots that should've been hits. Was still close though, not off by much.
On my other rifle, and with EVERY other rifle I've handled with a 1/3 CW mounted RDS the dot will sit right on the FSP. Maybe not perfectly, but close enough. My dot is literally 1/8" off the top of the FSB, where the post screws in. Just doesn't make sense.
Anyways as stated I'll have to make some time next weekend and spend the day sighting both in 100% and checking to see where things are at and reevaluating.
An Undocumented Worker
12-12-10, 10:05
Here is a good article that covers just about anything you need to know about using red dot style weapon sights, in addition to cowitnessing etc.
http://ultimak.com/UnderstandingE-Sights.htm
OK, as a slight update to this thread...I sold the ML3 and replaced it with a Micro in DD mount (1/3 CW). Everything else is the same. Still busy as hell, so the dot is zeroed but the irons may not be...I can't remember if I had them dialed in or not, it's been forever since I used them...hopefully I can get to them soon.
Anyways, still seem to have the same problem with the Micro. When sighting through the irons (DD 1.5 rear, stock FSB) the dot is easily cutting the FSP in half, and it's already almost completely sunk into the base. Probably only another turn, MAYBE two left before it bottoms out. Even if I could get it low enough to cowitness with the dot, it'd be just barely sticking out of the base.
So clearly something isn't right...is it possible that my rear sight has something wrong with it? It looks OK and I don't see any damaged parts or anything...obviously the best answer is to go sight it in but that may not be for another few weeks. Just looking to see if anyone else has experienced this issue and may have some input before I throw some ammo away trying to get this figured out.
So... your iron sights group well and your RDS groups well, but they do not have the same point of impact? Is that what you're saying? :confused:
OK, as a slight update to this thread...I sold the ML3 and replaced it with a Micro in DD mount (1/3 CW). Everything else is the same. Still busy as hell, so the dot is zeroed but the irons may not be...I can't remember if I had them dialed in or not, it's been forever since I used them...hopefully I can get to them soon.
Anyways, still seem to have the same problem with the Micro. When sighting through the irons (DD 1.5 rear, stock FSB) the dot is easily cutting the FSP in half, and it's already almost completely sunk into the base. Probably only another turn, MAYBE two left before it bottoms out. Even if I could get it low enough to cowitness with the dot, it'd be just barely sticking out of the base.
So clearly something isn't right...is it possible that my rear sight has something wrong with it? It looks OK and I don't see any damaged parts or anything...obviously the best answer is to go sight it in but that may not be for another few weeks. Just looking to see if anyone else has experienced this issue and may have some input before I throw some ammo away trying to get this figured out.
Looking at what I highlighted in red, are you saying, that with your irons zeroed, the FSP is run down, nearly all the way to the bottom of its travel? If so, then there is something going on with the upper/barrel alignment. What distance are you zeroing at, and is it the same distance for both the BUIS and RDS? Since you said the rear sight came off another upper, did that upper also require the FSP to be screwed significantly farther down than it should be? If not, then I would suspect there is something going on, with alignment, in either or all of the FSB, barrel, and or upper receiver.
Come to think of it, I have a BCM 14.5" LW mid upper, that with its DD A1.5 rear sight, the FSP sits pretty low in the FSB. None of my other uppers, BCM or otherwise, do that. I always suspected the upper had a slight cant, or misalignment, in the barrel/upper mating surface, or a height issue with the rear sight. A misalignment condition, if extreme enough, could pose the issue you are describing, since both sighting systems are on different planes, and somewhat misaligned to the barrel. As long as you are able to maintain a zero with both sighting systems, it may never pose a functional problem, but yes, it would be irritating, when trying to verify zero through the BUIS and RDS.
You now have me curious about my upper, that has the low FSP issue.....if it doesn't rain, and I can make it out to the range, I may have to swap around some optics and rear sights, to see what I come up with on mine.
Good luck and keep us posted.
Stock handguards, or a forearm rail?
If a forearm rail, is the optic attached to that, to the receiver rail, or straddling the gap?
MOE handguards.
The irons should be sighted in but I can't remember it's been so long since I've even used them. Even if they aren't, as stated getting them where the dot is would put the top of the post almost flush with the sight base.
I also tried moving the micro around a bit to make sure it wasn't an issue there (even though it shouldn't be anyways...). Dot stays right there no matter where it is on the rail.
Tryin to follow here. Did you say you zeroed the irons at 50 and the dot at 25?
If that is the case then the dot would be lower because the bullet would still be traveling at an upwar trajectory. Meanin zeroed at 25 will impact high at 50 so it would make sense that the irons are higher than the dot if this is how thy were zeroed.
Both are zeroed at 50...I usually get it close at 25 then move to 50.
OK, so I took a measurement...the front (muzzle) of the FSB from the top of the barrel to the bottom of the 'shelf' that the sight post screws into. It measures 2.040", which from what I remember is a decent amount higher then a standard FSB....which I believe to be around 1.950"-ish.
Does it sound like this is the issue? I mean, should I even bother trying to sight this thing in again or is it pretty much FUBAR? I measured with a caliper about a dozen times, the measurement is kosher. At this point, should I just contact BCM and go from there?
Nevermind...I'm a dumbass and forgot that I have a .625" FSB, so it'll be a bit taller from the barrel.
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