I have an AR zero/sight problem. Looking for the best solution.

I don't want an easy, simple, quick, etc fix. I want the best fix to solve the following problem:

When I built these guns, I used, obviously, flat-top AR receivers. I purchased lightweight barrel kits from M1S and they came with everything needed. However, I did have to separately purchase the front sight posts.

At some point, I know I probably should have done this myself but I gave an SO buddy of mine one of my ARs to take to the SO range to sight in. He brought it back to me and said he was able to sight it in at 50M but, in order to do so, he had to back the front sight post nearly all the way out. Currently the FSP is far enough up that the top of it is slightly past the ears.

The rear sight is a LaRue.

Here is a picture of (one of the) uppers in question. Both are identical.

http://upload.spankdu.com/images/bvt...ghogihcm4s.jpg

More than likely you have the wrong front sight base. The kits come with a standard A2 sight base which is paired with a carry handle upper receiver. The flat tops require a F-marked front sight base which is shorter.

Quote:

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Originally Posted by longrange18

The flat tops require a F-marked front sight base which is shorter.

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The F marked FSB's are the taller ones, which is what you need.

That's what I thought.

Are they really that much taller?

If the buddy had to back the sight post all the way out ( LOWERING the POI ), It sounds like he already has a "F" front sight ( TOO SHORT ) & he needs an A2 front sight ( TALLER ) like the barrels SHOULD come with. That will cause the POI to drop
a bit.

Quote:

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However, I did have to separately purchase the front sight posts.

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I think its one of these:
LaRue Tactical B.U.I.S. QD LT-103

Which was designed to be behind a RAIL mounted Flip up Front Sight, IIRC. So that may be where the problem lies. I'd try an A2 FSB or better yet give Larue a call. They are really great people.

If I was home I could measure my A2 upper FSB & my M4gery F marked FSB and give you the height above barrel of both. But I'm on duty tonight till 08:00 tomorrow so I cant.

EDIT: You are right. I just looked on BravoCompany.com & they say that F marked sights are taller. Sorry for the mis-info.
Lincoln7 was correct.

Ty Hoeffer
Palmyra, VA

Try the .040" taller FSP offered by Bushmaster.

A2 Front Sight Post for Flat-Top Colts
Price: $5.50
Part No: 9349056-M


http://www.bushmaster.com/products.asp?cat=21

Quote:

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Originally Posted by pth3k

If the buddy had to back the sight post all the way out ( LOWERING the POI ), It sounds like he already has a "F" front sight ( TOO SHORT ) & he needs an A2 front sight ( TALLER ) like the barrels SHOULD come with. That will cause the POI to drop
a bit......

If I was home I could measure my A2 upper FSB & my M4gery F marked FSB and give you the height above barrel of both. But I'm on duty tonight till 08:00 tomorrow so I cant.

Ty Hoeffer
Palmyra, VA

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I believe you are mistaken. The F marked sight is actually taller. This thread has photos and specs for the two FSBs.

https://www.m4carbine.net/showthread.php?t=14753

Quote:

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Originally Posted by Quib

Try the .040" taller FSP offered by Bushmaster.

A2 Front Sight Post for Flat-Top Colts
Price: $5.50
Part No: 9349056-M


http://www.bushmaster.com/products.asp?cat=21

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+1

I just got the Bushmaster tall sight post because I want to try the Santose IBZ modification on my carry handle. Since the carry handle sight goes up the front sight must too.

The Bushmaster post is 0.040" taller, which is the exact difference between the A2 and F-marked FSB.


Of course this won't help with the problem of the post being above the ears but it will make it fit tighter in the base and not slop around.

Quote:

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Originally Posted by Quentin

+1
Of course this won't help with the problem of the post being above the ears but it will make it fit tighter in the base and not slop around.

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I wouldn’t rule that out just yet.

I’ve had FSP’s adjusted during zero, that sat similarly to the OP’s description of his FSP. The FSP was sitting up so high out of the FSB, that the only thing holding the FSP secure in the FSB was the FSP detent.

• I swapped out the std FSP with the BM .040” taller FSP.
• Set my sights back to mechanical zero.
• And re-zeroed.

This resulted in the .040” FSP base sitting perfectly flush with the FSB, and the FSP remained a safe distance within the protection of the FSB ears.

but if you use the bushmaster front sight. it will be above the wings of the fsb. prone to snagging. better get the correct FSB