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

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11-06-09, 15:27
spankaveli

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
11-06-09, 15:46
longrange18

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.
11-06-09, 16:49
Lincoln7

Quote:

Originally Posted by longrange18

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

The F marked FSB's are the taller ones, which is what you need.
11-06-09, 16:50
spankaveli

That's what I thought.

Are they really that much taller?
11-06-09, 16:53
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.

Quote:

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

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
11-06-09, 18:09
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
11-06-09, 18:11
fdxpilot

Quote:

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

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
11-07-09, 03:19
Quentin

Quote:

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

+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.
11-07-09, 06:36
Quib
Quote:

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.

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.
11-07-09, 08:20
lanceriley

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
11-07-09, 09:36
spankaveli

Quote:

Originally Posted by lanceriley

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

That is my main concern. I know I can get a taller FSP but that is a "quick & easy" solution but maybe not the best because, as you said, I don't want the FSP taller than the ears.
11-07-09, 21:40
spankaveli

I will try to get some pics to show how the fsp sits in the fsb currently.
11-08-09, 00:13
Molon

Quote:

Originally Posted by lanceriley

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

There is no difference in the overall height of an "F" marked front sight base from a standard front sight base. The only difference is in the "shelf" that the post screws into.

Also, you can't just swap out a standard front sight base for an "F" marked front sight base. The front sight base and the barrel need to be drilled and reamed together in a fixture.

http://www.box.net/shared/static/uei1uar8kw.jpg

http://www.box.net/shared/static/2d520zm04w.jpg
11-08-09, 00:15
Belmont31R

I would get a correct F FSB, and consider it a lesson learned.

Bravo Co sells them, and then you need to get it installed. I think ADCO would be able to do it. http://www.adcofirearms.com/shopserv...op_.cfm?code=5
11-16-09, 15:38
spankaveli

update: shot both guns today and both have the same problem and two other people shot one of them as well.

so my options seem to be:

dump teh uppers and start over
change the front sight base to an f-marked
change the front sight base to a railed gas block and get rail mount si.ghts...

You guys know anyone that makes a midlength pEncil barrel upper or just the barrel? Also, what's the easiest flattop slickside upper to acquire?
11-16-09, 16:40
bigfelipe

Quote:

Originally Posted by spankaveli

change the front sight base to a railed gas block and get rail mount si.ghts...

That's probably your easiest fix. Get a quality one piece block and make sure to have it pinned or at least heavily dimpled and loctited if using set screws.
11-16-09, 16:50
C4IGrant

Quote:

Originally Posted by spankaveli

That is my main concern. I know I can get a taller FSP but that is a "quick & easy" solution but maybe not the best because, as you said, I don't want the FSP taller than the ears.

Yes, but your other option is to fill the holes and re-drill for a new FSB. That is a rather expensive option on a VERY cheap barrel.

C4
11-16-09, 16:57
C4IGrant

Quote:

Originally Posted by spankaveli

That's what I thought.

Are they really that much taller?

Yes, they really are.

This is what you need: http://www.gandrtactical.com/cgi-bin...n&key=TALL-FSP

We are seeing your problem more and more. The question I have is, why doesn't anyone realize that you cannot use an A2 FSB with a flattop?????????????

C4
11-16-09, 18:35
spankaveli

Quote:

Originally Posted by C4IGrant

Yes, they really are.

This is what you need: http://www.gandrtactical.com/cgi-bin...n&key=TALL-FSP

We are seeing your problem more and more. The question I have is, why doesn't anyone realize that you cannot use an A2 FSB with a flattop?????????????

C4

When I ordered the barrel kit from M1S there was no distinction on the which FSB would be on it.
11-16-09, 18:53
Quentin

Sorry to hear about this problem, spankaveli.

Since you may go with a gas block FSB, I'm pasting in a picture of my ArmaLite gas block with their fixed front sight. It has the Bushmaster 0.040" taller post because I did the IBZ/RIBZ mod. The base of the post is flush with the upper base of the FSB and my carry handle elevation knob bottoms out 10 clicks below 6/3 (Only needed 6 but the tall sight was deep in the FSB well and I like it flush.) The taller post doesn't come anywhere near the ears of the FSB but I guess it's a whole 'nother ball game with a standard A2 FSB.

http://i829.photobucket.com/albums/z...adjustment.jpg
11-16-09, 19:14
C4IGrant

Quote:

Originally Posted by spankaveli

When I ordered the barrel kit from M1S there was no distinction on the which FSB would be on it.

Uhm, it says "forged A2" in the ad.

C4
11-17-09, 10:30
C4IGrant

http://www.model1sales.com/images/bar16lw.gif

;)

C4
11-17-09, 11:08
spankaveli

Or maybe I didn't remeber there being a specification because I incorrectly thought the a2 was the correct one. :(

I guess it sucks being a noob sometimes. I know now, however.

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