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Old March 15, 2009, 09:10 PM   #1
Jman
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1st AR-15 - Help Please?

I just bought my first AR-15. It's a S&W M&P 15 Flat Top. I purchased the rear handle type sight and the standard A2 front sight separate. Both fit well on the picatinny rail. In short, rear sight all the way up, front sight all the way down, it shoots 12" low at 25 yards. I compared it to a Double Star flat top and learned that the top of the rail (gas block) for the front sight is 1/4" +/- higher than the Double Star. That will explain the low elevation on the shot. My question is, what kind of picatinny rail front sight will suit my application? I wanted the A2 style/look if possible. Any suggestions or references appreciated.
Thanks in advance.
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Old March 15, 2009, 09:14 PM   #2
ar15chase
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I dont know if I understand this. Do you have the elevation set all the way up at 25 yrds?
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Old March 15, 2009, 09:19 PM   #3
Jman
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Yes. Rear sight elevated as high as it will adjust, front pin turned down as low as it will go. Still shooting way high. The front rail/gas block on this rifle is taller than the gas block on other flat tops. Well, as compared to the Double Star for sure. You can see a picture of this rifle on S&W's site and see that the elevation of the rear rail is the same as the front rail. I don't think this is typical with other brands? Hope this helps.
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Old March 15, 2009, 09:22 PM   #4
ar15chase
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"Still shooting way high" It is shooting high because your elevation is all the way up. But you said in your 1st post it was shooting way too low. So my question is does it shoot too high or too low?
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Old March 15, 2009, 09:22 PM   #5
armedtotheteeth
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uhm, why are you shooting 25 yards< and , if it dont work with the open sights, get a scope.
Ars are way bettern open sights anyhow, specially the S+W models.
IF that is out of the question you can get a dremel tool after that pesky front sight and take off the offending 1/4 inch
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Old March 15, 2009, 09:24 PM   #6
45_Shooter
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If the rifle is shooting high relative to point of aim then the rear sight should be lowered and the front sight should be raised, not the other way around.
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Old March 15, 2009, 09:26 PM   #7
TheManHimself
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What manufacture of detachable carry handle do you have?


Some manufacturers use the wrong (A2) FSB on their fixed-sight flattop rifles as a cost-cutting measure. Since the A2 FSB is shorter than the F-marked FSB for flattop rifles, they make their detachable carry handle sights lower to compensate. The result is that when you use the out-of-spec carry handle on a rifle with the correct height front sight (Or vise versa), it becomes impossible to zero.


If that's not the case, take a look at your railed gas block. Is the rail on it level with the rail on top of the receiver? If so, you need a front sight designed to attach to top-railed forearms, such as the fixed LMT front sight or the Troy folding front sight. If not, you probably need the front sight made by the manufacturer of the gas block, since there's no set standard on the railed gas blocks.

Last edited by TheManHimself; March 15, 2009 at 09:32 PM.
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Old March 15, 2009, 09:59 PM   #8
Jman
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I'm sorry for the confusion guys. On my second post I meant to say that it is still shooting way low. It's always low.

To answer the questions below, I bought the Double Star rear detachable handle sight, equipped with two different sized peeps. For the front sight I bought the A2 UTG detachable. I was advised that some of the Chinese manufactured rear handle sights might have adjustment issues and went with the Double Star instead. Paid a little more for it but it appears to be well built/performing.

I can't take a 1/4" off of the front pin. There's not enough pin to do that and I would rather return it for something that will work instead of altering it.

I first tried 25 yards just to see where I would be on paper without wasting a lot of ammo trying it at further distances. I have also shot it at 100 yards (less than 2" group with cheap ammo) but again, very low.

Yes, my rear rail and my front rail are level.

Yes, I could scope it. But isn't that supposed to be one of the attractions to the flat top?.......option to do either?
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Old March 15, 2009, 10:19 PM   #9
Catfishman
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I don't know how to fix your problem but shooting a new rifle at 25 yards was a very smart thing to do. There are many AR-15 nuts, I mean experts, out there. Try to find one at a gun store or range and I'll bet they can diagnose and possibly fix your problem quickly. Gun guns are usually very eager to help.
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Old March 15, 2009, 10:38 PM   #10
armedtotheteeth
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it does seem you have missmatch betwixt your front sight and rear sight. my advice is to get a six pack of coors and think it over for awhile. Thats what I do in these situations. I have lots of situations
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Old March 15, 2009, 11:03 PM   #11
ronl
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If I read this right your receiver rail and gas block are the same height. If this is so you will need to get a front sight for a rail not a gas block mount. The rail mount sight is shorter than a standard gas block one. I do not think there is a tower type sight that will meet your needs. I encountered this on my Robinson Armament XCR and had to do some searching to find the correct front sight. I went with YHM. Just be sure it is a rail mount sight and not a standard gas block one as they are different heights.
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Old March 16, 2009, 12:12 AM   #12
Three44s
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Do you have a caliper?

I would measure the relative heights of the rear and front sights when removed from the rifle. (I am refering to the bottom of each sight to the top of the post on the front and roughly the center of the peep on the rear)

Perhaps someone here can give you a measurement of theirs ....... the difference if any is what you're concerned with ..... not the precise height.

Best of luck getting this rectified.

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Old March 16, 2009, 06:07 AM   #13
Jman
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Gentlemen, I appreciate your help/advice regarding my situation. I intend to contact S&W today and get their take/suggestion on it. Thanks again for the input.
Jman
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Old March 16, 2009, 10:05 AM   #14
Technosavant
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Quote:
If I read this right your receiver rail and gas block are the same height. If this is so you will need to get a front sight for a rail not a gas block mount.
This.

If you have the M&P 15T, which is what it sounds like, the rail for the front sight is NOT located on the gas block (as it in on the M&P 15OR), but on the railed forearm itself. That puts it quite a bit higher than it would be had it been on the gas block.

Not a problem, but you need to remember that many detachable front sight bases are set up at a height for a gas block rail, not a forearm rail. You need a front sight base that's specifically sized for a forearm/handguard rail.

Note the height difference between these.
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Old March 16, 2009, 10:07 AM   #15
Technosavant
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Quote:
Some manufacturers use the wrong (A2) FSB on their fixed-sight flattop rifles as a cost-cutting measure. Since the A2 FSB is shorter than the F-marked FSB for flattop rifles, they make their detachable carry handle sights lower to compensate. The result is that when you use the out-of-spec carry handle on a rifle with the correct height front sight (Or vise versa), it becomes impossible to zero.
The F-marked front sight base was developed for carbines. If you use a non-F marked FSB on a carbine, it isn't impossible to zero, but the front sight post (assuming they include one that's tall enough) will stick up past the protective ears of the FSB.
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Old March 16, 2009, 11:13 AM   #16
TheManHimself
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Quote:
The F-marked front sight base was developed for carbines. If you use a non-F marked FSB on a carbine, it isn't impossible to zero, but the front sight post (assuming they include one that's tall enough) will stick up past the protective ears of the FSB.
Read what I wrote more closely. I know you can zero a standard rear sight with the wrong front sight, but when you have the right front sight and one of the out-of-spec rear sights, you run into issues similar to the OP.

Regarding the OP, since it seems you need a front sight made for a receiver-height rail, LMT makes a rail-mount front sight that looks like the standard fixed triangle FSB.
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Old March 16, 2009, 01:32 PM   #17
Jman
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Guys, I hate to expose my ingornance (again) so blatantly on here but I see no way around it. I apologize for not knowing all the correct terminology.

I have spoken with Smith and Wesson this morning and here is what they have to say. My rifle is a "same or single plane" rifle. Both rails being level with each other and therefore I need a "hand guard" or "same plane" front sight. They didn't offer any suggestions on who supplied this type of sight.

I see from some of your comments above that you have already determined my error on using the work "gas block". Again, sorry for the confusion. I'm being told that a "LMT front sight" will do the job. Here's a picture of one that will allegedly work (second one from the top) http://www.bravocompanyusa.com/AR15-...ights-s/57.htm

I prefer the look of the triangle type or style. Does anyone know any other brands of this design that will work?
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Old March 16, 2009, 03:03 PM   #18
Technosavant
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Jman, that LMT will work. The problem is that there really aren't many triangle-style front sights for handguard mounts. Reason being, fully railed guns like the one you have are usually bought by people intending to mount optics like an EOTech or Aimpoint. The irons that get mounted are for nothing more than a backup, and in that case flip-down ones are often preferred. To tell the truth, you could have saved a rather lot of money by going with a rifle that wasn't all railed up. But yes, you can rectify that by getting that LMT sight, but I would recommend considering some kind of red dot or holographic optic.

TheMan, you're right, I missed that "out of spec" carry handle bit.
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Old March 16, 2009, 04:00 PM   #19
Jman
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Thanks Techno,

I will in fact have optics on it eventually. While my eyes are still good enough I just like the open sight shooting. This wasn't the exact style gun I wanted but everything in this area was sold out and back ordered forever. I found this one a few miles from my town on the shelf. It wasn't priced too bad so I elected to go with it.

I really appreciate all the advice and help.
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Old March 16, 2009, 10:57 PM   #20
ronl
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If I were you I'd go ahead and spring for the flip type sights now; especially it you are going to put optics on it later. Yankee Hill also makes a flip type sight that is for single rail mount that is excellent.
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Old March 17, 2009, 01:37 PM   #21
Evyl Robot
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Of course, you could replace the whole gas block with a more traditional, barrel-mounted A-Frame, if you like. Or, you could use one of the collapsible A-Frames and have the best of both worlds... It's funny - Mine is a Double Star flat-top, and my wife's recently purchased gun is a S&W M&P15OR. I had actually noticed the height difference in the front block for the first time about two weeks ago.

It appears that you have a couple of choices, though. Get the front sight intended for hand guard mounting, or replace the front block - which really isn't all that big a deal in my experience. I would certainly do some homework before diving into any such operation - or let a pro do it. When I replaced my front block, I was honestly pretty nervous about it. In the end, it was really easy though. That being said, I don't think that all of these things secure to the barrel in the same way. I could be mistaken, as I don't know the guns really well yet.
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Old March 17, 2009, 05:56 PM   #22
longranger
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I have Troy Diamond Head folding rear and folding front on my AR with a detachable scope rings.I mounted them shot the rifle made a elevation adjustment and it will hold the 9-10 ring @ 100yds all day.
The Troy Diamond Head is the best rear AR sight I have used
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