Sight Geometry
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Old 06-20-2010, 06:59 AM   #1
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Default Sight Geometry

Okay gentlemen. After the prerequisite search, I was unable to find an answer to my problem. So, I will ask directly. When I built my AR I put a folding front sight on a railed gasblock. When sighting in the iron sights I had a problem. I was unable to screw the front sight post in (down) far enough to pull my shots up onto the bulls eye at 25 yards. I had to use the elevation wheel on the removable carry handle rear sight to get enough elevation to place the shots on in the bulls eye. If it had been 1 or 1 ½ inches then no big deal. But I’m talking about 3 inches at 25 yards. Right now the elevation wheel on the rear sight is on the 4. I have a DPMS gas block rail. I went cheap on the front sight and bought an NCSTAR folding front sight.
My questions to you are these.
1. Are all gas block rails the same height and should that height match the height of the rail on the upper? ( this one is a quarter inch lower than the rail on the upper and I imagine that the rounds would have been in the ground if the gas block rail matched the upper)
2. Can I file down the front sight post to get the last little bit of adjustment?
3. Is it better to buy a “matching set” of BUIS?
4. Should BUIS be mounted on the same plane?

Thank you in advance for any help you can provide.


Vince

(EDIT) Thanks for looking but I did another search and found that someone had the same problem back in 2009. I now know what to do.

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Last edited by EW1066; 06-20-2010 at 07:15 AM. Reason: found the answer
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Old 06-21-2010, 04:52 AM   #2
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So what did you discover?

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Old 06-21-2010, 05:08 AM   #3
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I discovered that some people who get a folding front sight from UTG , which is the same as the one from NcSTAR, have problems with the front sight post being too tall to work (unmodified) with a detachable carry handle. The solution is to get out the dremmel tool and (modify) take it down. I am going to take mine down 1/16 th of an inch. Which will bring the base of the front sight post back up to flush with the bottom of the sight base and hopefully the rear elevation adjustment back to where it should be (at 25 yards "Z") we’ll see. At this point it’s all about trial and error.


Vince

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Old 06-21-2010, 01:45 PM   #4
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Ah, I see. I couldnt' wrap my head around the problem at first because it's backwards from what you expect when putting a rail front sight on a railed gas block instead.

I do have a theory now though...

I suspect the problem may be that your UTG front sight is designed for a UTG carry handle (of course). But carry handles come in two heights, milspec height like Colt and others and commercial like Bushmaster. Milspec carry handles are 0.04" taller and are designed for the taller F-marked front sight base. Bushmaster and DPMS and even ArmaLite use the older A2 FSB and a shorter commercial height carry handle. If your carry handle allows you to reindex the elevation wheel (with a 1/16" allen wrench) you can raise the rear sight (yet still be on 6/3) which of course "lowers" the front sight without any filing or modification. Of course that's what you were doing by clicking the elevation wheel up to 4 - but you need it at that height yet still say 6/3 and fortunately reindexing can do that.

ETA: Clarification, etc.

What brand is your carry handle and can you provide a picture of your carry handle sights (set to 6/3)? So we can see if it's milspec or commercial...

Here'a a link on how to reindex a carry handle...
http://www.ar15.com/forums/topic.html?b=3&f=18&t=328143

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Old 06-21-2010, 08:41 PM   #5
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I "re-indexed" the front sight by taking off roughly 1/16 th of an inch last night. I won't get back to the range for a week or so but I will re-index the rear sight so I can utilize the RIBZ. Thanks for the link, it was very informative.

Vince

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Old 06-21-2010, 10:04 PM   #6
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Hey Vince, do you know what brand carry handle you have? If you bought it new then you know for sure but of course second hand it's hard to tell since they're rarely marked except for the forge marks. Even if you don't know the brand I'm curious if the "shelf" that the rear sight sits on is the commercial or milspec type.

In the first post of the RIBZ thread all the pictures except the last one are of an A2 or commercial height carry handle while the last one is milspec height. Once you see the difference it's quite obvious - the commercial sight definitely sits lower. Curious which one you have, probably the commercial.

I blame Colt for this mess since they were there during the transition from integrated to removable carry handles/flat top uppers in the military. Don't know why in the world they didn't continue to use the A2 FSB for flat tops and just use the A2/commecial height carry handle. (Which is what Bushmaster/DPMS/ARmaLite did.) By having two FSB heights and carry handle heights they created a mess.

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Old 06-22-2010, 04:21 AM   #7
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Hey Quentin,

My carry handle is made by Command Arms. Let me say that the casting of the handle itself is acceptable. The parts that are used for the sight however are ,at best, less than desirable. My initial "knee jerk" reaction was that there was something wrong with the rear sight. I can honestly say that I have not ruled that out at this point. I am considering buying the rear sight assembly parts from Brownells and rebuilding this one. My only other concern is that rear sight is soooooo far over to the left. I think I may have a fundamental alignment problem. But that will be a different thread.

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Old 06-22-2010, 05:10 AM   #8
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Yeah unfortunately it doesn't sound like a top notch carry handle. The best ones have known forge marks/icons like a cardinal head for Cardinal Forge, a keyhole for Cerro Forge or splintered A for Anchor Harvey. There are other good ones but I see those the most.

Make sure yours is worth rebuilding and I wonder if for the cost of the Brownell parts you might not find a pretty good deal on eBay for one of those above used but complete - unfortunately you have to watch out for all the ones also on eBay by Mako, Leapers, UTG, CAA and others made in China. I'm thinking of listing an extra Cerro myself just haven't got around to it since they don't sell for much, usually $40-50. You also might come across a good used one at a gun show or Craigslist locally but that's hit or miss. New name brand carry handles usually go for $80-120 which is pretty spendy.

If you're saying the windage is cranked way left that's not great but if you're just saying that the whole rear sight pulls to the left by spring action then that's pretty common. I'd try to compare yours with a quality unit and see if it's worth rebuilding or replacing.

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Old 06-22-2010, 05:42 AM   #9
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This was a BUDGET build....you get what you pay for. Lesson learned.

The windage is 1/3rd of the way over from the left. To me that's waaaayyyy to far to the left. But then again this is a chinese made handle. They may have designed it that way to make us have to futz around with our sights when they invade... That of course will delay us just long enough that they can detonate a pulsed emp bomb that will activate a special fast decaying isotope in the metal of all AR parts made in china. Causing all of those parts to turn to dust in our hands......sinister isn't it?

I need sleep......

Vince

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Old 06-22-2010, 03:02 PM   #10
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Yeah you get what you pay for. Maybe it's time to think about #3 above in your OP. A quality rifle should have decent sights. If you buy a matching set then you don't have to worry about mismatches. I am kinda surprised that the Chinese sights don't match up front and rear, I expected that they're all built the same way. Often you see the same part by many names like I listed in a previous post.

I normally use iron sights so went the opposite from you and got quality irons then play around with a cheap Barska red dot and an existing scope borrowed from my Marlin 336C. Hate to put cheap crap on my all US made AR but wanted to see what a cheap red dot can do before spending real money.

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