Gas Block Problem

Andielouroo

Andielouroo

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May 10, 2017
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#1
Put together a new .308 gas gun a few weeks ago and I'm having a hell of a time getting the gas system running. Thought I'd see if anyone here has faced a similar issue. The components are as follows:

LaRue 16" Midlength Barrel
Superlative Arms Adjustable Gas Block
Melonited Mid-Length Gas Tube

I've assembled a lot of gas guns over the years and have never had this much trouble with a gas system. I installed everything and test fired only to seemingly get no gas to the bolt. So, I disassembled and the reassembled with the same result. I'm wondering if it is an issue with the gas block or gas tube. I have a new gas tube coming to see if that fixes things. I used the spaghetti trick to make sure the gas hole is lined up with the gas block.

This is the first time I've tried a SA adjustable block, so I'm not sure if there is a defect with the gas block. I set it up per the instructions, but I don't know if I might also be doing something wrong. I guess I can replace the gas block with a standard one as well to make sure it's not a barrel or bolt issue.

Anyone have suggestions or other obvious things I might be missing?
 
#2
Andielouroo said:
Put together a new .308 gas gun a few weeks ago and I'm having a hell of a time getting the gas system running. Thought I'd see if anyone here has faced a similar issue. The components are as follows:

LaRue 16" Midlength Barrel
Superlative Arms Adjustable Gas Block
Melonited Mid-Length Gas Tube

I've assembled a lot of gas guns over the years and have never had this much trouble with a gas system. I installed everything and test fired only to seemingly get no gas to the bolt. So, I disassembled and the reassembled with the same result. I'm wondering if it is an issue with the gas block or gas tube. I have a new gas tube coming to see if that fixes things. I used the spaghetti trick to make sure the gas hole is lined up with the gas block.

This is the first time I've tried a SA adjustable block, so I'm not sure if there is a defect with the gas block. I set it up per the instructions, but I don't know if I might also be doing something wrong. I guess I can replace the gas block with a standard one as well to make sure it's not a barrel or bolt issue.

Anyone have suggestions or other obvious things I might be missing?
Click to expand...
First step I would take is to measure the gas-port size for your .308 gas gun using calipers. My 18" .308 required a drill-out of the gas port because it wasn't getting enough gas to push the BCG back to cycle the rifle. Pre-drill out gas port size picture attached for clarity...

Once you have the measurement, post here and you'll get plenty of 411 from the learned members here! :)
 

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#6
Ok, pics attached. To answer questions:

Gas hole in barrel fits a .050 allen wrench, but not a .0625. A quick search makes me think the port needs to be enlarged to work. The first few links I clicked said it should be .0625 minimum.

I haven’t tried a different gas block yet, but I have one on the way in case other trouble shooting fails.

Definitely sure I got the spaghetti out. I can see light in the barrel through the gas hole.

IMG_5420.jpeg
IMG_5419.jpeg
IMG_5418.jpeg
 
#13
DeathBeforeDismount said:
Must have gotten one of the barrels mark made on a Friday. A sub .0625 sized gas port? Thats some QC for ya.

Return the barrel as defective for a refund and buy a craddock.
Click to expand...
A hex wrench isn’t exactly a precision measuring instrument. And as they are measured from flat to flat, the size of the wrench isn’t giving a true numerical value for the port diameter.
 
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#17
Just a few checks I do when I see this problem. First, never diagnose by parts replacement. Always truly diagnose the problem. A lot of times this will create bad habits by fixing the problem then the end user just throws away the 'bad' part, and never truly finds out the ACTUAL problem. So now their guns fixed, but they go forward never really understanding what happened in the first place, and how to avoid it in the future.

I run a rod just smaller than the gas tube inside diameter all the way through to verify the gas tube is of the same I.D. all the way. I then check the free play of the BCG by hand for any binding or increased friction through it's entire travel. Then I check the freeplay of the buffer with spring though the buffer tube to check for any binding. I then go on to the hammer spring. This one is a one in a million check, but like they say, 'it's broke until you check it.' Sometimes they're a higher rate than normal. Just like the gas port hole- one in a TRILLION chance they're too small but you still have to check them, and not with calipers. MOST of the time they're too large.

You can also check the interface of the bolt in the chamber. If for some odd reason theres burrs or what have you, that will put enough friction in there to start delaying or interrupting things. I then check for gas leaks. Shoot a mag through it with a can on, or a few mags without (dirty ammo works better) and see if there's exccess carbon shooting out of the gas tube/gas block junction, and the gas block to gas block journal junction. (remember, on bleed off mode on that particular block, there should be a small jet of carbon shooting out of that specific port) Of course you can also kind of tell by how tight or loose that gas block slides onto the journal as well.
Next is the gas tube to gas key seal. How is that fit? How is the gas key to bolt carrier fit? Any loose screws or excess carbon shooting out of places it shouldnt be? Can you run a length of something stiff down the gas key to see if that passage way is clear? How is the gas ring seal? Is your bolt easy to move through the gas ring run?

For me, I'd check all of that before drilling that hole, because if you do have any leaks, even minor ones, and combine that with say, a rough bolt carrier through way, and/or a heavy hammer spring, you may end up drilling that gas port to a size that may not be necessary. Later on down the line if you happen to just find out by accident your hammer springs too hard or theres too much friction somewhere where there shouldnt be, now your gas port hole's just a tad too big. Check the cheap, easy to fix stuff first before replacing anything or drilling.
 
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#21
fedupflyer said:
Yup, I believe it is about 18 click outuntil the gas block is in the "normal" gas position.
Click to expand...

Definitely made sure I had the block set correctly.
hlee said:
A hex wrench isn’t exactly a precision measuring instrument. And as they are measured from flat to flat, the size of the wrench isn’t giving a true numerical value for the port diameter.
Click to expand...
I realize that it’s not the most precise measurement, but I think it clearly shows that it is smaller than .0625. Whether that matters or not, I don’t know. LaRue finally emailed me back and said that’s in spec. All the reading I’ve done says that less than .0625 is too small, so who knows. 🤷‍♂️

I asked LaRue what the exact correct size is and told them I’ll go to McMaster and get pin gauges to check. That was yesterday and no response yet.
 
#26
Couple things I see right off the bat is that gas tube is sticking pretty far out. Make sure your bolt carrier isn't sticking on it. And also make sure that end of the gas tube isn't peened over or have any burrs around the opening. The second is the cerekote/paint on the gas block journal up by the shoulder. That is not allowing the gas block to get a good seal against the barrel. As posted by everyone above, start looking over the fit and function of everything else from the bolt to the buffer as well. I would guess you have a combo of small issues rather than one main cause. Even with the port being barely in tolerance, it would still cycle the bolt far enough to extract a fired case, may/may not pick up the next round, but it should at least extract unless there are other issues.
 
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#30
USK308 said:
Your gas tube appears (to me) to be too long. The tube should sit right in the middle for optimal performance. Will it work there...yeah, but not optimally (IMO).
Click to expand...

mtrmn said:
Yes, that tube is too long. You'll never know if that's really part of the problem until you get it corrected. Usually they're too short. Either way, it messes with the timing.
Click to expand...

I have a new gas tube on order, but I think I’ll cut this one down a bit and see if that helps.

I’m also going to strip everything down to check for burrs, fit, function, etc tomorrow.
 
#34
Andielouroo said:
Maybe, but it might already be jacked from being too long. I’ll get some better pics when I play Dr. Dremmel.
Click to expand...
Tube cutters and deburr tools are your friends when cutting tubing. You might not get a square end when using dremel and you'll still have to deburr it:

Proto tube cutter

L.E. Wilson burring tool

I wouldn't skimp on the tube cutter quality. Proto or Ridgid for the best bang for buck. I use that Proto I linked at work when replacing very small diameter copper lube oil lines on the machines. Leaves very little burr and doesn't crush the tubing if not trying to take heavy cuts, unlike the Harbor Fright cutter I use on aluminum cutting oil lines. I've used the Proto for harder stuff like conduit as well. I won't even consider using the HF cutter on any type of steel tubing.

If you insist on the dremel, just as long as the burr on the od is not heavy, the outside diameter can be deburred with sand paper and the inside can be deburred with the point of a drill.
 
#36
XTCBoltAction said:
It does. Looks like several dings/dents and the 'bulb' seems almost gone. But, maybe just the pic.
Click to expand...
It's just mil-spec and certain companies that double flare the end of the tubing. A lot of civvy aftermarket gas tubes don't have that double flared end. They just run straight tubing. But yeah, there's definitely some rubbing/wear going on between the gas key and tube.
 
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