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Old August 9, 2013, 07:39 PM   #1
iraiam
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AR-15 sight mistake (build)

I built a match capable AR-15 out of an old, like new, early 90's Colt Match HBAR.

I made a mistake somewhere though, I had to screw the front sight post down into the gas block quite a ways to battle zero the irons.

So far the FSB, front sight post, and carry handle, all measure the same as my other rifles which are supposedly "milspec", this leads me to believe that my upper receiver is not of the same specification.

here's a list of some of the particular parts I used:

PSA flat top upper, I can't seem to find another one, this was purchased quite a while before I did the actual project. I got it because it was a flat top but did not have the M4 feed ramps, so I could use my old barrel. Casting marks are F with a plain square.

BCM F stamped FSB
RRA NM carry handle

Any ideas where I made my mistake? It's not that big of a deal, I am just going to shorten the font sight post or get a shorter one, aside from this small issue, it's definitely a sub-MOA rifle now.
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Old August 9, 2013, 07:49 PM   #2
sailskidrive
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At what distance were you zeroing the rifle? 34M? 50 yards?
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Old August 9, 2013, 07:58 PM   #3
iraiam
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I did the standard 25 meter battle zero target at 25 meters, with two different carry handles.

I then shot it at 100 and 200 yards without adjusting anything and got the expected results.
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Old August 9, 2013, 08:10 PM   #4
sailskidrive
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There are replacement front sights for the non "F" marked FSB that are a different height so they work properly with the standard height carry handle.

I wonder if you have one of those...
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Old August 9, 2013, 08:53 PM   #5
iraiam
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I think you got it, according to my measurements, This will fix it. My current sight post is 0.260" tall, it's screwed down far enough that I measured 0.260 -0.056" = 0.204".

the .200" post from midway should be pretty much a drop in fix.

Thanks,
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Old August 9, 2013, 08:55 PM   #6
sailskidrive
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Kewl... good luck w/ the build.
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Old August 10, 2013, 12:06 AM   #7
Quentin2
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I think the problem is the RRA carry handle, I believe they are like ArmaLite, Bushmaster, DPMS and others that have short rear sights to go along with their non-F marked FSB. Milspec carry handles like Colt, BCM, PSA and others have taller sights (you can see the difference if you compare the carry handles).

You can fix this for free by reindexing the carry handle about 15 clicks below their setting for 6/3. Google RIBZ or IBZ for more on how to reindex.

Anyway, fixing the rear sight is the way to go, not putting on a short front post (which won't work with a flip up rear BUIS if you go that route later, which many of us do).
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