urgent gas block question AR15


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dubya450
January 28, 2013, 10:26 PM
I just ordered a Doublestar gas block with top picitiny rail for my BM Carbon 15 Superlight ORC. I read after i ordered it that there are two diffferent types of gas blocks (as far as how they mount to the barrel). I want to cancel the order before it ships if its the wrong type. Doesanyone know if this gas block will work with the BM Superlight (.625 barrel). Or which would work? You're a lifesaver if you can help!

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Z-Michigan
January 28, 2013, 10:30 PM
There are two common sizes, based on the barrel diameter at the gas port. 0.750" is the more common by far, while 0.625" is found on the lightest (skinniest) profile barrels. The gas block should be advertised with the size it's intended for. If it's silent, most likely it's for a 0.750" barrel diameter.

The other common division is attachment method - set screw, clamping, or pinned. The first two are much easier to install on a new barrel (assuming you don't have a production shop or barrel gunsmithing shop) and are most common if you buy a gas block by itself. Generally either one will work on almost any barrel, but the clamping is stronger and the set screw works much better on barrels that have a dimple or flat to catch one of the set screws.

dubya450
January 29, 2013, 12:21 AM
Okay, thanks for the quick reply. My barrel and the gas block i just ordered are .625 so i got that right and ibeleive it uses a set screw. Im not sure if the barrel has dimples or not as im waiting for it to arrive from Alaska. I asked the same question on another well known forum and was told I'm basically a dummy for wanting to use a gas block with a rail because it'll heat up too much and the sight wont stay on (flip up Magpul BUIS sights). Is this true or was he exaggerating? Im pretty new to AR's but can fo anything i want with an AK or bolt gun so sorry if these are frequently answered questions.

Cory

JustinJ
January 29, 2013, 10:05 AM
A polymer magpul sight should not be mounted directly to a gas block as the heat will warp or melt it.

Z-Michigan
January 29, 2013, 10:56 AM
The Magpul MBUS are plastic, not heat-resistant and specifically warn against mounting on a gas block. HOWEVER, that warning is based on people who get their rifles hot from shooting quickly in training classes, competition, etc. If that's you, don't use MBUS on a gas block. If you're going to shoot slowly like someone might shoot a bolt-action, it will probably be just fine.

However, there's another issue with mounting any sight on a gas block - most railed gas blocks don't have the rail at receiver height (some do), and most detachable front sights are sized for the receiver height rail (assuming a quadrail type forearm, not a gas block). So you may want to see what height the rail is on your gas block to see if this setup will even work.

dubya450
January 29, 2013, 09:58 PM
I tried to cancel the order and the azzhole hung up on me! This is an actual store in Texas too (can't remember the name). I do have a bunch of different sized Magpul rails i can attach on my MOE handguard but would that be goofy? Its a carbine length habdguard and the sight radius whould be pretty short im guessing.

Sam Cade
January 29, 2013, 10:12 PM
I do have a bunch of different sized Magpul rails i can attach on my MOE handguard but would that be goofy?

Plastic hand-guards are unsuitable for mounting sights. Not only would they be unstable they would be of an unknown height.

Z-Michigan
January 29, 2013, 10:42 PM
A sight on a plastic handguard isn't going anywhere good.

Sounds like you are starting your AR spare build parts bin. They have a tendency to become complete rifles over time.

dubya450
January 29, 2013, 10:48 PM
Grrrrreat! So i should buy a classic style gas block with the sight built in? I honestly wouldn't think that mounting the light magpul sight on the plastic MOE would be that bad but I've never done it so really wouldn't know. What's the possible issue? Just not staying tight?busting off easily? This rifle isn't going to be a "battle rifle", i have my Saiga for that, but more of a plinker and backup rifle.

dubya450
January 29, 2013, 11:51 PM
I found a Troy flip up front sight, either normal or HK style, for around $100. I could mount one of them on my new gas block and it should be fine since its metal right? Think it'd be okay matching up the MOE rear and Troy front?

Sam Cade
January 30, 2013, 12:46 AM
What's the possible issue?

Any pressure on the forearm will cause the front sight to shift.

In short, the rifle would never have a constant repeatable zero.

JustinJ
January 30, 2013, 10:44 AM
I found a Troy flip up front sight, either normal or HK style, for around $100. I could mount one of them on my new gas block and it should be fine since its metal right? Think it'd be okay matching up the MOE rear and Troy front?

Yes, metal sights are fine to mount on gas blocks. However, you must ensure that the height of the gas block rail allows for the front sight to sit on the same plane as the rear sight. You could test this with a long level, laser, etc.

MrCleanOK
January 30, 2013, 11:23 AM
You should have done some research before you started buying parts. It would have saved you a lot of headache and anxiety.

Rail-type gas blocks come in two heights. The STANDARD is lower than the plane of the rail on the upper receiver. The other, taller railed gas block is the same height as the upper receiver and as this is non-standard it will more than likely be advertised as "receiver height" or similar.

The majority of flip-up front sights are meant to be installed on a railed hand guard that is on the same plane as the upper receiver. Therefore, STANDARD front sights work with NON-STANDARD gas blocks, and vice-versa. See this link (http://www.midwestindustriesinc.com/index.cfm?fuseaction=category.display&category_ID=363) for a good example in the difference between a standard front sight and a gas block front sight. See how the base for the gas block sight is taller?

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