Here's what I have..
I have an Eotech 512, mounted with Magpul MBUS sights, front and rear.. The problem is that my gas block's rail on the front isn't the same height as the same as the rail on the upper..
I got the rifle sighted in tonight with the Eotech, but the front sight doesn't go high enough to get where it needs to be.. Even almost fully extended, I'm well short of the height needed to line up with the sights.
The rear doesn't have height adjustment, but only windage. All vertical adjustment is done with the front.
I haven't actually shot the rifle with the MBUS sights, but common sense tells me that the rear peep and front post should be pretty close to where the Eotech is showing it's point of aim.. Right??
My question is, should I see where the MBUS sights are shooting, or is there an adapter to raise the front sight the needed amount, in order to line up with the rear??
For some reason, in my head I figure that the Eotech's dot should match up with the MBUS's point of aim, or am I overthinking this??
From your first picture it looks like you can still see the front sight post centered in where it should be with the rear sight.
Are you trying to get an absolute co witness or a lower 1/3rd co witness?
Absolute co witness means that the eotech dot will be perfectly lined up with the BUIS front and rear sight.
Lower 1/3rd Co witness seems to be more popular, the optic field of field of view will be mounted slightly higher than the BUIS sights so that the BUIS will be in the lower 1/3 of the optic making your FOV through the primary electronic optic less cluttered.
If thats not the case and your BUIS front sight really is too low for whatever your trying to do you could always unscrew the post and swap it out for a taller extended one. They do make them, they cost around $6.
Are you trying to get an absolute co witness or a lower 1/3rd co witness?
Well, I thought I was, but in reality, all I'm after is the backup sights to be point of aim at 100 yards. Again, I think I'm over thinking it.. If the rear sight's level isnt' as high as the Eotech's view, I won't be at point of aim if I get the two together..
What I have to do is shut the Eotech off and sight the gun in with the MBUS. I don't want them to work together, and the reason I have the MBUS is if the Eotech were to fail. I need to pretend the Eotech has failed and sight the back up's alone.
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They do make them, they cost around $6
Now that you mention it, I've seen them, and that's my kind of repair cost.
I'll wander to the range again tonight and leave the Eotech off and attempt to sight the rifle in with the backup sights. As the old saying goes, eliminate the extended issues and worry about the major one.
In both the absolute co-witness and the lower 3rd co-witness you should be able to look through your BUIS and see the dot of your 1x optic sitting just on top of the front sight post. This is of course provided you have both the BUIS and optic sighted in for the same point of impact, which is what is normally done. When both the BUIS and optic are sighted in the 1x optic's reticle moves within the field of view as your eye moves in relation to the optic the dot will stay on target and at the point of aim and the shooter doesn't have to line his eye up behind the sight, this is true for optics that are parallax free such as EOTech and Aimpoint.
Sighting in the BUIS separate of the optic is okay but if the optic is already sighted in it would save time to adjust your BUIS to the same point of aim as your optic. You can then turn the optic off and varify making any corrections that you see fit but little if any sight adjustment should be nessisary if your optic is sighted in correctly. If your front sight is not at the correct height I doubt you will be able to sight it in at all because you will not be able to adjust your front sight post up hight enough. Your point of aim will be high. Your front and rear sight must be the same height from the plane of the barrel.
I hope I explained myself clearly. Bottom line, you will have to do something to get the front and rear sight the same height. A new front sight or a new gas block is needed provided you are correct and the front sight is too low, from your picture it looks that way to me.
BTW a benifit of cowitnessing your optic and BUIS is that you can verify the zero of the optic and BUIS by looking through the BUIS with the optic activiated. If all lines up you are GTG. You are correct, the way you imploy the BUIS and optic is to shoot mostly with the optic and only use the BUIS if the optic fails or is knocked off zero, one doesn't routinely shoot using both the optic and BUIS at the same time.
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the Mbus front sight is meant to sit on a railed handguard not a railed gas block. Can you mount a small section rail on your handguard?
Is this written down anywhere? Because I have to disagree with this comment. Any BUIS can and should be able to also be mounted on a railed FSB and still line up with the rear BUIS. To prove that point, usually, the rail on a FSB and the rail on your handguard are level and the same height, as well as they are level with and the same height with the rail the rear BUIS is mounted on. You look at any AR flattop, with a handguard or not, the rails are level with each other. So it shouldn't matter.
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Last edited by GlockMeister; 08-04-2009 at 01:48 PM.
Sight your optic in at 100yds. and then sight your BUIS at 100yds. Will the BUIS's line up with the reticle in/on the optic, it depends on the reticle. But most are close. But that isn't the concern. As long as they are sighted in and as far as the BUIS's go, as long as they line up with each other, and again, are sighted in, it doesn't and shouldn't matter if they all line up or not. Remember, your BUIS's are only meant to provide you with a back up, should your optic fail.
I'm not familiar with the BUIS's you have. But I do know, with the Troy industries BUIS's, as long as the rails they are mounted on, whether that rail be on a/the FSB or a/the handguard, they will line up. Generally, the rear rail is the same height and is level with and as the handguard or FSB rails.
I know people that just left the original FS/gas block on their AR, bought an EoTech or other reddot or scope, and then only bought and mounted a rear BUIS, and the rear BUIS lined up and still worked with the original stock FS/gas block. They sighted in the optic, then sighted in the BUIS's, both at 100yds., and everything is fine. If their optic fails, they have their BUIS's sighted in at 100yds. as well.
Then I also know people that replace the original AR FS/bas block with a railed FSB and mounted a flip up front BUIS, and the rail on that FSB still lined up with and was level with the rear rail and or handguard rail. And again, as long as they, and the optic was sighted in at 100yds., then they sighted in the BUIS's at 100yds., they have no problems.
Just get to the range and sight your BUIS's in at 100yds.. Then sight in your EoTech at 100yds..
__________________ "My next door neighbors two dogs have created more shovel ready jobs then Obama has." - Gary Johnson
Last edited by GlockMeister; 08-04-2009 at 01:55 PM.
"The sights design height is specifically for rail mounting, not “railed gas block” mounting.
-taken from: Magpul MBUS Back-up Sights m4gery.com
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These are exclusively back up irons. They are meant for a setup with no front sight post b/c the user is relying on optics. The railed gas block is lower than the railed handguard. They are meant as a low cost/weight item for the person who will probably never have to use them.
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Also a quad rail is higher than a front sight base mount because several companies make two different height sights specifically for each.
Maybe some are, I just know there are ones that are the same height, and that's the ones you want to get. Well if you plan on mounting anything on it that needs to be the same height anyhow. If you don't plan on it, then it doesn't matter.
But most people look for and get, if they want a handguard with a rail, they get one that is the same height.
__________________ "My next door neighbors two dogs have created more shovel ready jobs then Obama has." - Gary Johnson
There are two types of AR-15 flip up/back up iron sights, Gas block & "Rail" mount (same plane). Since both attach to a picatinny rails there is some times confusion in the terminology.
Gas block sights mount to the gas block, and Rail mount sights mount on a quad rail handguard or flat top upper with no gas block ( thus being on the same plane as the rear sight).
You will be out of the range of adjustment between the front & rears sight if you mix and match the two.
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"The sights design height is specifically for rail mounting, not “railed gas block” mounting.
-taken from: Magpul MBUS Back-up Sights m4gery.com
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These are exclusively back up irons. They are meant for a setup with no front sight post b/c the user is relying on optics. The railed gas block is lower than the railed handguard. They are meant as a low cost/weight item for the person who will probably never have to use them.
Thanks, and good, you backed up what you said with fact, the way it should be.
And I stand corrected, those are indeed meant to mount on the handgueard rail, as it states. But, I would add, I think they are/were also designed for the newer weapons with a bullpup design.
I think the ones paper has, the front one anyhow, just might need to be mounted to the handguard rail, but also with his design, it might move the front and rear BUIS too close together and he might not like it?
But if you note the one in the picture JMP posted, it is also one of those with the short barrels, and just may be a bullpup, and since it doesn't show the rest of it, it's hard to tell, but it just might be a bullpup or again, just one with a short barrel. None the less, no where to mount on the barrel or gas block. I'd also point out most bullpups or CQB rifles are no longer blowback. Therefore no gas block is necessary and therefore they can make them more compact and therefore the front BUIS has to be mounted elsewhere.
Many of the newer CQB weapons are designed much shorter for better and more maneuverability and there isn't enough barrel sticking out to even mount a FS/gas block on.
And again, thanks for backing up/verifying what you said. Some don't and won't do that. Not saying any names or pointing any fingers. lol
And again, I think with Paper's application, he needs a different front BUIS.
Why I suggested the Troy Industries. I believe Yankee Hill would also work? As well as GG&G I think it is?
__________________ "My next door neighbors two dogs have created more shovel ready jobs then Obama has." - Gary Johnson
Last edited by GlockMeister; 08-04-2009 at 02:17 PM.
Yankee Hill would be a good choice I think. That's what I would end up with if I were to replace my original front sight. From all reports, they're high quality at a more affordable price.
Yea, already been said and explained but I do know what Paper is talking about, I have a railed gas block and it does sit just slightly lower than the handguard rail and top receiver rail. I just mount the front BUIS an inch closer on the forearm rail in stead of worrying about finding a gas block mounted one.
Its a BUIS anyways considering its an SPR build with a 9x power scope on it for the primary aiming system. It works out just fine for what its supposed to do.
I wanted to add, in looking at Paper's pictures again, it's visible that his gas block is already lower then the handguard. And if he indeed did get the Magpul front BUS that needs to be mounted to the handguard, it stands to reason it's even shorter then one that isn't designed to be.
It looks as if he were to mount just a rail to the top of that handguard, then mount that front BUS as far forward as he can, it should make it level with the rear.
But again, it is obvious the gas block is below the handguard thus making the front BUS too low.
This picture. See how the curve of the handguard curves over the gas block and rail on it?
I would also add that even if he got a different BUFS, he would probably run into the same problem. I think he got/went with one of those low profile gas blocks. If that's the case, that's the problem. Just needs top get a BUFS designed for a low profile gas block that will then raise it to the correct level as the rear...
__________________ "My next door neighbors two dogs have created more shovel ready jobs then Obama has." - Gary Johnson
Last edited by GlockMeister; 08-04-2009 at 02:30 PM.
As stated, the mbus is designed to be mounted on the handguard, not a gas block.
You can see many ppl here can make recommendations for you on parts, but we need to know what you want to change now. Do you want to add a quad rail and put your sight on it, get a front sight designed to go on a gas block, or change your gas block to one that has an integrated flip front sight?
And I don't know what size your barrel is, but these fit a standard .750" barrel. This is more than likely the way I'll go if I buy a flip up front sight.
Thanks everyone!! I think I'll find one of those Midwest Industries flipup BUS for the gas block.. I want to keep my bottom rail available for a light, so I'll keep this gas block. It's what I ordered for the rifle kit back in January and it's still what I want.
I really like the Magpul sights, but it's just not the right thing for my application.
And as much as I appreciate the link to Sportsman's Guide, I'll never EVER order from them again.. They lost me as a customer years ago because they never have anything in stock, and then act like it's your fault.. I can always find the same stuff at a cheaper price, and in stock.. Sorry to get off topic, but they pissed me off a few times and I just won't ever give them any credit for anything, ever again..
Anyway, again, thanks for everyone's info.. I don't want to change to a railed handguard.. Same as mentioned, as this is what I ordered because I like the free float aluminum tube.
Problem with Backup sights/Eotech 
