View Full Version : AR Sighting Problem

July 19, 2012, 08:57 AM
I think I've got myself a problem with my brand new AR-15 Carbine build.

I bought this thing a piece at a time and started puting it together, falling in love with it a little more every step, (like you do). And once I get my entire upper assembled, I think I see a problem. I have installed a Low Profile Railed Gas Block, and it is uneven with my Upper Receiver Rail.

http://www.freeimagehosting.net/t/wgr3k.jpg (http://www.freeimagehosting.net/wgr3k)

I have a 4x32 Illuminated Optic that I have mounted on a QD mount on a 1" riser on my upper receiver. My plan was to install flip up front sights on my receiver and gas block to use when the optic was too powerful. With this unlevel setup, I don't think it will sight in correctly (shooting way high?).

My question is, does anyone make a weaver front sight that solves this problem? Or does someone know of somewhere I can find an F-marked FSB to use instead of my current gas block? I assume a swap at this point won't be very difficult.

Thanks for looking and I hope someone can help.


Crow Hunter
July 19, 2012, 10:03 AM
I can't see the pic.

Only a red x.

July 19, 2012, 10:54 AM
Thank you, I think I've fixed the image now to better explain what I'm talking about.


July 19, 2012, 10:55 AM
If the problem is that the plane of the railed gas block is lower than the rail at the top of the reciever, you have three options.

1. Buy a front sight that is designed for a railed gas block (easiest option).

2. Buy a railed riser for the railed gas block to place a rail at the same plane as the rail on top of the reciever (fairly inexpensive part).

3. Install a same-plane gas block (most difficult option).

Bartholomew Roberts
July 19, 2012, 11:04 AM
Typically, the gas block is on a different place than the upper receiver/rail. This is one of the reasons why there are several different heights of front sights.

Another option is to use a low profile gas block that fits under the rail and an extended rail that covers the gas block.

July 22, 2012, 12:26 AM
Be careful buying a front sight designed for the lower gas block rail. Most front sights are designed for the standard rail height.

Fishbed77 has some good suggestions though I've never seen anyone use a spacer to raise the gas block rail it could be the easiest way. Don't know what height spacer you'd need but something like 1/4".

ArmaLite has a front sight that's designed for the low gas block rail but it doesn't quite bring the post up to milspec height, you need to buy a .04" taller front post - $6 from Bushmaster. (I know this because it happened to me. ArmaLite front and rear sights are A2 height, not flat top height - F marked.) Also this sight isn't a flip up it's fixed, however it won't be seen in a 4x or greater scope. Offhand I don't know of a proper flip up for the low gas block rail.

July 22, 2012, 11:31 AM
Most any of the sights are designed to fit the lower gas block and you shouldn't have any problem with sight alignment. It is harder to find the rail mounted sights. I put flip sights on my XCR, which is one solid rail, and it took me a while to find one. Luckily, I found one at a local gun shop. I went with Yankee Hill. Co-witnessed perfectly with my EOtech.

July 22, 2012, 12:39 PM
Your best option (not always the easiest) is to just buy a railed gas block that will be on the same plane as the receiver. This shouldn't be that difficult to find if you are using a standard flattop upper and a standard barrel.

This one here should work...http://www.brownells.com/.aspx/pid=26844/Product/AR-15-M16-PICATINNY-RAIL-GAS-BLOCK