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Posted: 10/16/2009 5:20:51 AM EDT
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I just posted this over in troubleshooting too. Just didn't know where it fit best.
I have 2 AR setups. My first has a fixed front post and a Yankee Hill folding rear. It also has a Aimpoint M2 that I co-whitness through. For years I have had this thing sighted in and the rear BUIS has always been pushed off to the left a bit and I've always assumed the front post is canted. It's always shot straight and true though so I've never bothered trying to change it. I've recently got another M4 with a monolithic rail with low profile gas block. First i put another Aimpoint on this setup and sighted it in without irons on it. Sighted in just fine. I happened to have some odd irons lying around and decided to put them on for the time being. The rear is a magpul mbus (folding to the rear) and the front is a Troy HK style folding battlesight (also folding to the rear). I locked them down on the rail and started to rough zero it with the aimpoint dot as a reference. Now when the dot is sitting right on the front post like it should the rear sight is biased to the left just like on my fixed front post rifle. Since it's mounted on a monolithic rail I'm fairly positive the front sight isn't canted, but here's the strange part... I turn the front sight backwards (folding to the front) and ding, ding, ding... things all come back into alignment. I also put on a generic rail mounted fixed front post behind the troy (so now I have one rear sight and 2 front sights on the rifle) to see if the posts are in line... with both posts oriented normally on the rail they line up... if I turn one backwards again, the reversed one is off from the normally oriented one by at least a half a post to the right (sitting correctly down the center). I thought the problem may be the barrel wasn't centered in the receiver at first, and I may have believed that, except for the fact that the sights don't stay centered when flipped around on the rail. So I'm thinking this is a sight issue. So my questions are: a.) Why wouldn't the front sight be down the center line of the barrel locked onto the rail backwards or forwards - I would think it would be consistently in the center either way as much as some of these rail mounted sights cost? b.) I've heard that rail mounted sights index of the right side of the rail. Due to manufacturing tolerances, does this mean one needs to run the same brand (or at least close to the same spec) front and rear? It seems that the MBUS sights index off both the left and right of the rails. c.) Since the Troy front sight does sit off to the left compared to the center line of the bore, and assuming Troy's rear sight is also made to sit slightly left to remain centered. Could that mean that the Yankee Hill on my fixed front post M4 could also be set up this way and my front sight base really isn't canted and is dead center? d.) Should I just run it with the front sight mounted backwards? e.) Could it be that I just get stuck with wonky rifles?  Strange that the 2 rifles I have, exhibiting the same rear sight issue, have totally different setups.
I'm planning on trying out an MBUS front on it and seeing what orientation works with that front post. Thanks for everyones input. |
| Also, if I leave the sights as-is (somewhat off centered) will I notice a change in windage at various distances? The dot from the Aimpoint still sits on the front post and should stay on target, but what I'm thinking is that the irons, will be on at the distance the dot is sighted in for (50 yds) but once out to 150-200 yards the impact will be to the left because the irons are basically crossing the bullets trajectory at the distance sighted in and will move further off that path the further down range you look? Does that make any sense? |
| I think I may be having a similar issue. A few days ago I sighted in my ML3 (on an LT129) at 50yds. I then adjusted my irons so that the dot sat on the front sight post and sighted in from there at the same yardage. I have a LMT front sight that is mounted to the handguard and a Matech rear folding BUIS. I then shot with just the irons at 100yds and was doing fine, just had to aim a bit low. However when I tried the red dot at 100yds it was off to the right about 4-5 inches. It's possible I didn't do a great job of sighting in, but I thought it was good to go. I'll try it again and report back if I remember. |
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I think that pretty much all rail mounted iron sights or optics can suffer from different rail widths.
Every single picatinny rail I own has a slightly different width to it... Some are very close, some are way the heck out. In order to make the iron sights able to clamp onto a wider variety of rails, they have to give them a little left to right "give". If you dont have a perfectly mil. spec. rail (down to 1/1000th of an inch) then it can most definately cause your iron sights to be a little more to one side. Most manufacturers will account for these tolerances in both front and rear sights, so they will line up. However if your iron sights are within zeroable tolerance (AKA, you can zero them properly) then I wouldn't worry about it too much if one is a little off. If its causing trouble with zeroing your rifle, then you may want to try and get a same set of irons for your rifle. Also, dont worry too much about dot placement in relation to iron sights. As long as they both independently are zeroed, the location of the dot compared to the front and back sights isn't a big deal. Hope this helps ya, and welcome to the site! 
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