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| AR Technical Discussion Dive into the details and specifications |
11-06-2009, 04:27 PM
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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.
Last edited by spankaveli; 11-06-2009 at 04:34 PM
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11-06-2009, 04:46 PM
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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.
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11-06-2009, 05:49 PM
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Quote:
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.
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11-06-2009, 05:50 PM
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That's what I thought.
Are they really that much taller?
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11-06-2009, 05:53 PM
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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
Last edited by pth3k; 11-06-2009 at 10:07 PM
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11-06-2009, 07:09 PM
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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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11-06-2009, 07:11 PM
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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
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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.
http://www.m4carbine.net/showthread.php?t=14753
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11-07-2009, 04:19 AM
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Quote:
Originally Posted by Quib
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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.
Last edited by Quentin; 11-07-2009 at 04:23 AM
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11-07-2009, 07:36 AM
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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.
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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.
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11-07-2009, 09:20 AM
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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
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11-07-2009, 10:36 AM
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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
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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.
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11-07-2009, 10:40 PM
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I will try to get some pics to show how the fsp sits in the fsb currently.
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11-08-2009, 01:13 AM
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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
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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.
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11-08-2009, 01:15 AM
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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
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11-16-2009, 04:38 PM
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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?
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11-16-2009, 05:40 PM
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Quote:
Originally Posted by spankaveli
change the front sight base to a railed gas block and get rail mount si.ghts...
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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.
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11-16-2009, 05:50 PM
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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.
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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
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11-16-2009, 05:57 PM
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Quote:
Originally Posted by spankaveli
That's what I thought.
Are they really that much taller?
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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
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11-16-2009, 07:35 PM
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Quote:
Originally Posted by C4IGrant
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When I ordered the barrel kit from M1S there was no distinction on the which FSB would be on it.
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11-16-2009, 07:53 PM
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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.
Last edited by Quentin; 11-16-2009 at 07:55 PM
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