Just put together my first SBR upper and I'm having trouble getting it to run.  I think it is under-gassed, but I wanted to see what you guys thought.  

Specs:  12.5" Ballistic Advantage barrel on an Innovative arms WAR upper receiver.  Direct impingement carbine gas system with YHM lo-pro gas block.  

First Outing:  Took it to the range to sight in un-suppressed and it was a single shot rifle.  I was using anH2 buffer and a M16 carrier.  Most shots, the shell casings would not even extract from the chamber, much less eject and load a new cartridge.  Sometimes the casings did eject, but a new cartridge was never successfully loaded.  The gas setting on the WAR receiver was in the open (un-suppressed setting).   Fired a total of about 25 shots.  Ammo was XM193.

At home:  Put a pipe cleaner through the gas key to confirm no obstructions.  Ran pipe cleaner through the switch mechanism on the WAR upper and confirmed it was not blocked.  Sprayed compressed air through the gas tube (from the receiver end) and could feel air movement at the muzzle.  

Second Outing:  Switched the H2 buffer for a carbine buffer.  By now, the stamp for the suppressor had come in, so I took it along.  Un-suppressed, same result as first outing.  Suppressed, worked like a champ with the WAR upper set at the open setting.  At the closed setting, it functioned fine, but did not always hold the bolt open on the last round.  Fired about 50 more rounds.  I was only loading 3 bullets in each magazine to see if it was holding the bolt open.    

Third outing:  Tried the SBR lower with a 16" upper to make sure it wasn't the lower--16" upper ran fine.  Put the AR-15 carrier from the 16" upper in the 12.5" upper and un-suppressed it still didn't work (well, it successfully cycled once in about 10 shots).  Suppressed, however, it functioned perfectly with the WAR upper set at the closed (suppressed) setting and held to bolt open every time.  Ran about 50 more rounds through it, still XM193.

I'm now at the point where I think I may need to open up the gas port in the barrel.  The only other option I can think of is a low-mass carrier (Seekins or JP).  If the low mass carrier would work, I think it is a better option than drilling out the gas port, but I'd hate to spend the money and it still not solve the problem.  It also occurred to me that if I shoot it a little more, it may work itself out.  

What you guys think?

UPDATE--Over the weekend, I took everything apart (which involved having to buy a propane torch to get the hi-temp thread lock to let go of the gas block screws).  

Anyway, it appears the gas port in the barrel and the gas block line up great, so no problem there.  

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Not sure if you can tell from the photo, but there is a ring around the port in the barrel where the hot gas has marked the larger hole from the gas block on to the top of the barrel, and the hole is well centered.  

Gas port in the barrel measures .063" (using digital calipers).  I've ordered wire gauge drill bit numbers 49, 50 and 51 to incrementally open the gas port up a little.  It looks like the existing hole isn't large enough.  

We'll see.....

Since this is a new build, I would shoot it for a few hundred rounds suppressed and then try unsuppressed again before making any further changes...

If the bolt is new, make sure you are running it wet.  My last build was very tight.  My 10.5 Bushmaster ran fine with no mods in 5.56.  It was already broken in before it got shorter.

My .300AAC 8.5" with pistol gas system had a very tight bolt.  I recognized this right away and slathered it with CLP.  Supersonics ran fine.  Subs needed a powder change and now run fine.  I know, not too relevant, other than the lube.

Check gas block alignment with gas port. If you are using a free float handguard, you did leave a space between the shoulder and the block, right?

I am using a free float handguard, but I'm not sure what you mean by a space between the shoulder and the block.  The barrel has a shoulder that the gas block sits against, if that's what you mean.  I've never heard of leaving a space there.

ETA:  Just did a google search and realize now what you mean.  I've never left a space before and it's never been an issue, but that doesn't mean it's not this time.  I'll take it apart this weekend and check.

Let us know if that fixes the issue. I seem to remember someone else on here having the same issue and that solved it.

The thread has already left the SBR forum and hit the archives, so I can't link it.

First thing I check for with any upper build is alignment issues if there is a sign it is under gassed.  That fixed every one of them.  On one build I opened up the entrance of the gasblock because the alignment holes and gas port on the barrel were off a few degrees.    

try different ammo?

See edit in original post--for some reason, an edit didn't pop it back up to the top.

Open up the gas port a little bit. That will do the trick. Still think you might want to try different ammo.

Edit. Did some research your port hole seems to be small. Ported for a 14.5. I'd try to see how .07 does. See link here

amphibian, thank you very much for taking the time to post that--it's nice to hear from someone with experience that I'm on the right track.  

Also, thanks for the tip regarding running full mags--it makes sense that the extra resistance can make a difference.  

Once the bits arrive I'll update again.

UPDATE:  I drilled out the gas port with a No. 51 bit (.0670).  Took it to the range at lunch and the problem appears to be solved!

Fired 3 rounds unsuppressed with the WAR upper set in the open position and the rifle cycled perfect and locked the bolt open on the empty magazine.  

I then inserted a full magazine and fired about 10 more rounds unsuppressed with no malfunctions.

Finally, I put the suppressor on, set the upper to the suppressed setting, and finished off the mag with no problems.  

I'll shoot it some more tomorrow to make sure, but everything looks good!