To start, I have researched for numerous hours and just too many variables to not make another post specific to my gun.

Rifle Info:
-18" BA SPR rifle length gas. (Port size .092)
-Aero upper.
-YHM set screw gas block that needed some force to press on the .750" profile, so minimal/normal gas "leak" behind block on barrel/tube.
-BCM and AIM nitride bolts used/tested.
-Anderson gas tube (I.D. .114)
-Less than 1,000 rounds through it.

When I first built it I had a Failzero NiB bolt with no issues. The coating was chipping in areas so I sent it back and got the above mentioned BCG's. Initially this happened when I put an H2 buffer in to help reduce bolt bounce (fully shouldering at that time as I wasn't in the troubleshooting phase). So when testing now, I am not shouldering the gun. Has also failed with regular carbine and spikes ST-T2 buffers...very surprised in this when using a light carbine buffer with a rifle gas system ??.

Typical short stroke symptoms. Sometimes BCG does not lock back on a single loaded mag...multiple different mags tested. other times the expended case will get lodged between the bolt face at an upward angle and the CH while the next round in mag gets 1/2 fed. Failures have occurred with the typical light test loads (PMC bronze and Tula) have also tried Freedom munitions and wolf gold. I believe wolf gold (loaded a little hotter) has not had any failures. Other mixed 69/77 grain stuff also failed occasionally.

I've checked all the normal areas: As mentioned above: ammo, mags, BCG's, BCG gas keys, buffers, gas block seated and tight and aligned perfectly. Gun is cleaned and lubed properly.

So my only thoughts are: Gas tube ID is on the small side at .114 vs .125 as proper spec? And gas port hole is on the small side as well at .091 vs .093-.098 or even .10.

What say you on those 2 things possibly being the issue? Drilling the port is obviously my last resort, but I think it's coming down to that now...besides ordering a different tube and hoping it's a bit wider.

Thanks in advance!

Also, I did notice the gas tube had quite abit of carbon on it just in front of upper receiver, however I'm pretty sure that's fairly normal as well as excess gas is going to travel through the "cloverleaf" once the gas key clears it.

First, H2 is too heavy for rifle gas and 18 barrel.  Assuming that is corrected, you still might have issues.

BA has a history now of chamber issues, several rough chambers that needed to be replaced or smoothed out... so that's a possibility.  Unlikely in your case, as it was performing well.

Bottom line?  .092 gas port is just on the small side for an 18" rifle gas, and if anything else adds friction or isnt fully broken in, you risk issues.

I'd drill that port out to .098.

If the gas tube is in question, replace it.  It's cheap, and easy to do while you drill the gas port.

It is? BCM recommends an H2 or even an H3 with a rifle length gas. H buffer still caused bolt bounce, H2 alleviated it. But short stroked with carbine, H, and H2...what led me to assuming the port size was just too small.

I'll have to research BA chamber issues. Never heard of them having problems. Thanks for the heads up!

Will replace tube for $12 and likely drill port gradually to .098 max.

In a 20" I wouldn't argue that.  But in an 18" there is very little dwell time.   that, and BCM historically WAY overgasses their rifle barrels.

The gas port seems too small.

The gas port on my 18 inch rifle gas gun is .104 or something like that, it runs great on anything with an H buffer.

What does this mean?

If "seated and tight" means the gas block against the handguard ring retainer shoulder on the barrel, generally that is wrong. There are some particular barrels and gas blocks that are non-standard where against the shoulder is appropriate, but usually there should be roughly .040" of space.

Have you removed the gas block by chance? If the block is aligned correctly there should be a circle of discoloration/fouling with the gas port in the center. If the gas port is partially obstructed or even at the edge of the gas block's hole it can cause short stroking.

Yeah I've come to the conclusion that is it after all the testing I've done. Saved drilling it out as my last resort. Have a #40 (.098") and #41 (.096") drill bit ordered, would like to keep it on the conservative side, but if neither work, will go to a #39 (.0995"). Even with an H buffer though I was still getting bolt bounce. So I'm still confused on that a little.

Yeah, everything has been checked. No handguard retainer ring, it's free floated. Seated as in pushed against the shoulder on barrel. Perfectly aligned carbon ring over gas port hole.

Explain what you mean by "bolt bounce".  Actual bolt bounce is incredibly uncommon so you might be meaning something different here.

Slow motion video. Once the bolt travels forward you can see when it reaches the full forward position, then moves rearward slightly (bounces), then returns for full forward. Shooting only semi auto, so I know it's not a huge deal. But just when experimenting with the short stroke issue, It was very noticeable with a carbine and H buffer, but and H2 alleviates any "bouncing" and returns the bolt forward with more authority. The bolt bounce is in no way causing me any failures. Just want this gun to run like a top.

That is why we use buffers with reciprocating weights, to have a secondary inertia "deadblow" to the bolt, when the bolt locks, if there is any bounce, the buffer weights impact a split second later and take care of this issue.  We generally run heavier mass buffers for slower unlock time, not to "cure" bolt bounce.  I'd focus on real issues.

Pull that H2 buffer out and put a carbine buffer back in. 18" barrels with rifle gas are very soft shooting, i cant imagine that your getting bolt bounce. lube it up and go break it in.

Since the tube is in question, replace it.

I fought with my Colt for a very long time, finally replaced the tube and it runs now.

You mentioned light ammo and not shouldering the gun, either of those can make many otherwise ok guns short stroke.

Doing both together is asking it to short stroke.

If you need it to run low powered ammo while not shouldered, don't know why you would, then you'll probably need a bigger gas hole...but it'll be overgassed.