Ok, so I finally got my BUIS to finish my carbine.  It's a midlength, F-marked front sight base.  I have an Eotech 552 on a Larue mount on it, and I chose a Larue fixed BUIS.    I want to zero both sights at 50 yards.

The Eotech, I got zeroed.  It makes pretty groups, and makes them where I want them.  So far so good.

The irons....   not possible to zero.    With the front sight post as far up as I could get it - sticking up past the FSB ears with the entire disc part of the front sight post peeking out of its hole - it was more than 2 feet high at 100 yards.  Closest I could get it to my desired point of impact at 50 yards was several inches high.  With the 0-200 aperature it wasn't as bad, but still a no-go (and I don't shoot as consistently with that aperature anyways).  Just to add insult to injury, the rear aperature is cranked most of the way to one side.

This is really depressing, since I've been waiting so long to finish this damn carbine.  Even worse, the groups I get with the irons are really nice...     just aren't where I want them.  If I aim for the bottom edge of my 100yd silhouette, I get a nice group in the head.   IIRC, Bushmaster or someone makes a taller front sight post, but having the sight post protrude that much from the FSB is unacceptable because it'll get bent.  

What, if anything, can be done about this?

Quoted: You need to zero the irons at either 25 yds or 50 yards. 25 yard zero will give you a 300 meter zero. 50 yard zero will give you a 200 meter zero. I choose the 50 yard zero and never have a problem. It does sound like you need the taller sight post, but you should still be able to zero with what you have.

If you read his post, you'll see that he tried to zero at 50 yards with no luck.

Thats my problem;  I want a 50 yard zero, but I cannot zero at 50, the FSP doesn't even come close to going high enough.  I'd settle for a zero matching the 300m zero on my A2, but I can't do that, either.  

A taller front sight post will leave me with the front sight post sticking about 1/4" up past the FSB ears -- that's how high up it needs to go.

I'm thinking the odds that there is anything wrong with the BUIS are about a million to one. (and yes, I have it mounted in the correct spot )

Try the ARMS #40. It should zero just fine.

What exactly does the #40 have or do that would make it work where the Larue BUIS would not?

Maybe their fixed BUIS is made for standard FSB's...???

That would be very odd.

 I ZERO MY LMT  W/ ARMS 40 / 551 and THE FIRST SHOT WAS FOOT  AND HALF LOW W/ BUIS

I HAD TO DO ALOT OF CRANKING UP THE POST TO GET IT RIGHT!

going up with the post makes the rifle shoot high?

and going down lowers the POI?

Lowering the post raises the point of impact, raising the post lowers the point of impact.  The rear sight works the other way around.

Yeah thats right!  up is going down!

I meant to say is I turn in the up direction wich goes down my bad

Maybe I can just use my irons for area targets.   Like, say, the next county.

I saw someone posted this link to the extended fsp.  Maybe it will help.

extended fsp