Gas Blocks & rail height

[44Dave]

My barrel finally arrived today and I had a chance to test fit everything.  It's all great, except my gas block.  I bought an AR15 rail height gas block, but it seems to not be tall enough.  Are there DPMS height specific gas blocks out there and I just missed that?  I'm using Black Rain Ord and a Samson Evolution handguard - they mate up perfectly.  The gas block isn't tall enough.  Is this how it's supposed to be or do I need to be looking for a better gas block?

 

 

 

[98Z5V]

JD Machine.

[blue109]

guessing the BRO is DPMS old height?

[98Z5V]

 

 

http://www.jdmachinetech.com/product/308UPRP/308RGB.html

[44Dave]

Well, it looks like I have an extra .936" rail height gas block that I may need to build an AR15 around.  I'd hate to see it go to waste after all. 

[survivalshop]

Didn't know that with gas block mounted sights , the gas block rail had to match the hand guard rail height or is that just what you want . Rear BUIS sights used to come on two heights , one for mounting on the hand guard rail & one for gas block mount , no ?

[44Dave]

Gas blocks come in 2 sizes, gas block regular height and rail height.  You have to match the front sight with the correct gas block to acheive correct line of sight with the rear.  The front sights that fit on the gas block regular height blocks are taller than the front sights that are made to go on the rail height gas blocks.  You can go either/or but you have to keep to the same system. 

 

Anyway, I did buy a rail height block but the AR15 rail must be lower than a DPMS 7.62 rail, so my front sight will be too short.

[blue109]

Old standard DPMS height is taller than AR15. The newer low height DPMS pattern is the same. It's a confusing mess.

[44Dave]

 

 

http://www.jdmachinetech.com/product/308UPRP/308RGB.html

 

Looks like those use a set screw instead of the clamp type.  Worth while to put a little dimple on the barrel?

[shibiwan]

Looks like those use a set screw instead of the clamp type.  Worth while to put a little dimple on the barrel?

 

Yes, I personally prefer the setscrew gas blocks over the clamp on ones.

 

P.S. Why arent you mounting the sight on the float rail instead? Simple fix; problem solved; cost $0.

Edited February 12, 2014 by shibiwan

[44Dave]

I was brought up to not put any sights on the rail.  I'm a firm beliver in sling usage - I know the rail would probably not have any deflection on it, but I can't stand the thought of moving the sights out of line.  Better safe than sorry. 

[44Dave]

That said, I'm sure gonna throw a LER scope on there to see how well it does.  Damn I love scout rifles.

[blue109]

I "scouted" a mosin. Fun little beater. I like the concept.

[survivalshop]

I was brought up to not put any sights on the rail.  I'm a firm beliver in sling usage - I know the rail would probably not have any deflection on it, but I can't stand the thought of moving the sights out of line.  Better safe than sorry. 

Not sure if that would be a problem . I have many different FF hand guards & you would have to put some major pressure on it to deflect them, to disrupt a Iron sight picture . I see your point though.

[98Z5V]

 Worth while to put a little dimple on the barrel?

Yep, well worth it.  Only one - put the dimple on the barrel directly below the gas port in the barrel.  No need for another one.

 

Use the gasblock for a template, to put the dimple in the right spot.  <thumbsup>

[survivalshop]

Also make sure the gas block is indexed/aligned on the barrels gas port .

[44Dave]

I'm gonna use Robo's 2 business card trick. 

[Robocop1051]

You really only need that trick for piston systems. The hole in your gas block is .125" and the hole in your barrel is likely around .09". Not that it'll hurt anything to be lined up more. Just saving you from extra headache

[survivalshop]

I'm gonna use Robo's 2 business card trick. 

That all depends on the Barrel , some are made to have a gap for the Hand guard front support & some Barrels are made to seat right up against the gas block stop on the Barrel . Your present Gas Block seems in the photo, to butt right against the Shoulder on the Barrel.

 

With a drill bit close to the size of you gas poet , measure the distance from the barrels gas block shoulder or stop 

 

With another drill bit or anything you can get into the Gas Blocks gas port ( remembering the Gas Blocks port will be a little larger in Dia. ) Measure the distance from the ports edge to the Gas Blocks outer flange (flange face that seats on the Barrels gas block stop Flange, I'm using the depth feature of my Digital Caliper  ) This measure meant will give you close to where the Gas black fits . Since there is a Difference in Port sizes , you have to use some math to see where it needs to be .

[44Dave]

OK, thanks; I'll use actual measurements to determine. 

The photo I posted above is a bit misleading; I just shoved it on there to test fit everything else so it should not be considered an acurate representation of correct mounting.