Gas Block Alignment

[Keep The Change]

So I'm trying finish up my first Grendel Build Target gun.
It's a 24" Brux barrel that has dimple on the bottom of the barrel for indexing the gas block so that you are aligned with the gas port.

I'm using a LaRue 0.875" gas block that is the set screw type of block. It has 2 set screws.

Well, the set screw hole doesn't line up perfectly with the index dimple. The gas block is pushed up against the profile step of the barrel and the set screw hole is too far forward.

The gas block needs to go back a little further in order for the hole and dimple to line up. I'm guessing it is 1/4 of the hole diam off or maybe a little less.

A portion of the bottom of the set screw might make it into the dimple and keep it from moving much. But I'm just a little hesitant to move forward this way. The port in the gas block is bigger than the barrel port it looks like and I don't think the gas port of the barrel is covered up.

My thought is to turn down the aft side of the gas block to allow it to slide back on the barrel a little more.

If I bought a clamp style, I would think the same issue would occur. Thoughts?

Also, do you guys think the set screw type of gas blocks affect accuracy at long distances. I've read this from the Robert Whitley article. I'm having a hard time to believe that on a .875" diam barrel.

[LRRPF52]

Just bed it with Red Loc tite and don't use much torque at all on the set screws.

You can also drill your own dimple if you want, but I just bed the blocks and don't worry about it after the Red sets.

Measure your barrel shoulder to center of the gas port.

Measure your gas block edge to center of gas block port. See if there is a difference. Gas block ports are usually .125", whereas your port should be no larger than .0935".

[cory]

I haven't tried it yet, but this looks interesting.

http://www.uniquetek.com/site/696296/product/T1511

[Beerswimmer]

I haven't tried it yet, but this looks interesting.

http://www.uniquetek.com/site/696296/product/T1511

That's pretty neat!

[LRRPF52]

Here is my process. I'm not saying it's the best or only way, but it has worked very well for me for years. When I put a gun together, they just work out of the gate.

I start by measuring the distance from the barrel shoulder to the center of the gas port. I actually use both inside and outside jaws of the caliper to take readings. This shows the inside jaws, which worked better here because the shoulder is very shallow compared to the gas block journal on this bull barrel.




Then I measure the edge from the rear of the block to the center of the internal gas port inside the block, which you can't really use the jaws on unless you assume that the set screw hole is aligned with the port hole. They usually are, but this is another method.






I then slip the block onto the barrel and test for pneumatic circuit being free with canned air, by plugging the chamber and introducing air into the muzzle. You should hear air flowing from the gas tube hole in the rear of the block. If you have a through and through tube hole, like some of the low profile blocks have, you need to have the tube installed and listen/feel for air coming through the back of the tube inside the upper.

[A5BLASTER]

Thanks for that explanation and pics LRRPF52 very informative.

[Keep The Change]

Thanks LRRPF52, I did take the calipers to the gas block and gas port this past weekend. That is why I think I'm ok as far as not having any of the port covered.
I didn't really think of bedding the block. That's a good idea.

Does that come off fairly easy after heating it up?

[Keep The Change]

I think I might do what you suggest LRRPF52. That will save me from machining the gas block a little.
Now how do you keep from getting Loctite in the port of the barrel or block?

[LRRPF52]

I think I might do what you suggest LRRPF52. That will save me from machining the gas block a little.
Now how do you keep from getting Loctite in the port of the barrel or block?

I make a thin ring that leaves a gap where the gas port section is, so that when I back the block onto the journal, no Loc tite will be pushed into the port. You can also place a little behind the gas port so that the block will have a complete seal in the rear.

Apply logic as to what it needs to look like when you're done, use reverse planning, and develop a simple sequence.

[Keep The Change]

That's a good solution. The one thing I was worried about was being able to keep a seal after many rounds with the dimple and set screw not being lined up completely. But the bedding of the gas block solves this and I can relax a bit on the tightening of the set screws. Thanks again for the advice.

[Cornbread]

In my experience building ar's the gas block is usually off the shoulder of the barrel a tad. Measure from the shoulder to the hole and the shoulder to the center of the dimple.

[Keep The Change]

Well I went and measured things a little more closely. It looks like I'm .020 off with a portion of the barrel port being covered up.

I get .230 from shoulder of barrel to back edge of gas port.
I get around .245 to .250 from back edge of block to edge of port in gas block.
Thus part of port will be covered up.

I think I'll need to have the block trimmed .020.

I have an SLR clamp style block laying around and it measures .230 from edge to edge of gas port. Just like the barrel. I think LaRue may have screwed up.

[just_john]

In the original AR design, the front handguard mount was between the gas block and the barrel step. The thickness of the sheetmetal mount is about .024. All of the assemblies that I have done, the port on the gas block was positioned such that the block either butted up against the step or to allow for that handguard mount. Prior to assembly ( same as LR55 ) I measure that spacing on both the block and barrel and if that spacing allowed for the handguard mount, I use a set of feeler guages used for measureing automotive valve clearances. Most sets have a tab that is at or very close to .024.

[Keep The Change]

So I got on the phone with LaRue and asked them to give me some measurements from the Gas Block I ordered from them. It appears they sent me the AR10 version of the gas block because the measurements they gave me don't match what I have.

[LRRPF52]

Well I went and measured things a little more closely. It looks like I'm .020 off with a portion of the barrel port being covered up.

I get .230 from shoulder of barrel to back edge of gas port.
I get around .245 to .250 from back edge of block to edge of port in gas block.
Thus part of port will be covered up.

I think I'll need to have the block trimmed .020.

I have an SLR clamp style block laying around and it measures .230 from edge to edge of gas port. Just like the barrel. I think LaRue may have screwed up.

Something is seriously wrong if you're getting .230" to the rear edge of the port from the barrel shoulder. That's the shortest I've ever heard of and have never seen a barrel like that over untold numbers of barrels.

Re-check your measurements. What barrel is that?