Ive read the FAQs, sample post as well, and am asking some assistance, here's a part summary first, and the issue after:

Upper: VLTOR MUR-1A
Barrel: Proof Rifle+1 Gas system, 6mm arc chamber, 18" barrel length, model 128688
Handguard: CMT HDM Arca 17"
Gasblock: Superlative arms Clamp model .750 gas journal
Gas tube: Proof supplied with barrel, rifle+1 length
BCG: CMC Triggers 6 arc enhanced, bolt headspacing verified to barrel with go/no go gauges, model 81635

Lower:
Aero X15 stripped. Assembled approx. 2017 with PSA LPK (no FCG)
Trigger: Geissele SSA-E
Buffer System: full VLTOR RE-A5 (H2 weight - default parts what they supplied at the original time when it was launched), spring also standard vltor milspec rifle), VLTOR A5 receiver extension
Stock: Magpul PRS Lite

Original lower has approx. 4,000-5,000 rounds through it, possibly a few more. Had a white oak assembled SPR Upper mated to it.

I built the 6mm arc upper in the description back in December to use on this lower. This is about the 10th AR Ive assembled in total. The lower used here, I had hands-on help assembling back in 2017 from a cerakote specialist and FFL, as it was my first one.

Initial testing showed a few issues:

1: BCG failed to lock back on bolt catch, bolt would sit locked against the magazine follower
2: accuracy was pretty bad, approx. 1.5" moa with hornady ELD Match at 100 yards.

Contacted proof, they advised me to send the barrel back to them. They noted unusual gas port erosion, and "a mix of small issues" that they said together would explain the accuracy issues, and replaced the barrel (and gas tube) under warranty.

Reinstalled new barrel to the VLTOR upper (used steel shim, and green loctite, as barrel extension to upper fit was loose). reinstalled all above parts as well.

Back to testing, followed their break in process (clean/shoot in intervals)...BCG issue presenting itself again.

So far, to test, Ive tried a different BCG entirely (Milspec 6 arc from optics planet "Trybe"), tried a JP Enhanced bolt in the CMC carrier, Tried CMC bolt in mil spec carrier, and tried all 3 bolts in a known working carrier (original from white oak).

Also tried ASC grendel and 6mm arc mags, duramag 6mm grendel steel mags, PMAG, lancer mag, etc, all in total tried at least 5 different magazines.

All result in Bolt locking on magazine follower. Went through testing the full range of gas block adjustment, only results I got were getting it to eject a round out very limp, or fail to eject, or fail to even lock back.

I have a few different VLTOR weights on order (H0, H1, H3, H4) to try different buffers, but working through the possibilities, only thing I can think of so far is defective gas block (maybe explains the gas port erosion on the first barrel? maybe not), or possibly out of spec older bolt catch that may need replacing by now. Didnt get a chance to try it on a different lower yet to rule that out, as it just came up as a possibility from reading threads in here last night.

I also measured the gas port in the barrel, and it is to the proof spec, (0.096") as well, as this was a known issue with them shipping out under-sized gas port 6mm arc barrels earlier on.

Also, photo of the issue:



There's at least 200 rounds through this BCG and barrel, and another 100 on the first barrel (and same BCG) before it was sent back to Proof for replacement.

Ammo used so far is Hornady Black 105 grain, # 81604 and Hornady ELD Match 108 grain # 81608,  boxes have come from different sources too, some bought locally, some online, so not one specific lot. Have only gotten the bolt to fully lock back on the 108 grain once or twice, but no repetition or consistency.

Also, if loading a mag with multiple rounds, it will cycle, bolt goes forward, loads next round, etc, this operation is normal. The failure only happens on last round, or if loading 1 round to a mag.

Also, gas port and gas block alignment was verified both times on install, I ran a fiber optic light through the gas tube, and then a bore scope through the muzzle end, to verify alignment.

It sounds like you're on the right track as far as changing the buffer. The AGB may not be needed as it may be optimally gassed at wide open. I'd step down to an H0 buffer and try to achieve lock back. I'd then tweak gas from there. That would be me but, I prefer to adjust gas over swapping buffers. My guess, though, is that you'll likely find no need to do either, presuming it runs with the H0.

The worst mistake I ever made building a rifle was on my .260 Remington.  I had a rifle length gassed DPMS 24" I Bought when they closed and it ran lovely, never missed a beat with any powder.  But my accuracy (1.5" minimum / 5 at 100) was not what I wanted no matter what powder or bullet or seating depth...  I switched to a +2 length gas X-Caliber barrel, also 24 inches, and that barrel would not run at all  It would never lock back on empty and I enlarged the gas port as much as I dare.  It is very accurate but it will not function properly.  I had to dick around with several slower burning powders, which did not give me the velocity of the other powders that would not function, and finally got it working properly with the bullets but I did lose about 150 fps because of the powders it took to make it function correctly.  Yes, I had tried adjusting gas port size and buffer weights; it was not going to function no matter what.

Point is, I listed to the hive here about going to longer gas port length and that royally mucked me up.  I suspect that might be your problem.  Who said you needed a +1 gas port?  And is your load PROVEN by others to work in a +1 length gas system?  And I don't mean the barrel maker, X-Caliber said the +2 on mine would work just fine, it didn't.  An adjustable gas block does nothing unless you open up your gas port in the barrel, which will void any warranty offered.

 I open up gas ports all the time because I mainly play with wildcats, but I cannot advise you to do that.  If your rifle is not running with the gas port you have, an adjustable gas block will do nothing.  Before spending a lot of money chasing buffer weights and spring weights, unless you are 100% certain your powder and load is proven to work with your bullets and your gas length by others, I'd try a slower burning powder or three.  You only have to load up three rounds of each powder at start load and test them out, one at a time and see if it locks the bolt back properly on all three rounds.  DO NOT LUBE up your carrier and bolt.  Shoot them relatively dry.  Do this with a standard buffer and spring, none of this HB and XP crap.  Since you are getting your bolt to lock back on the follower, you are very close, so you may only have to go to a slightly slower powder.

Before doing anything though, can you lock the bolt back properly by hand?  Are you sure your buffer is not bottoming out and preventing your bolt from going all the way back or your spring is bottoming out from being too long?  A long stock screw or spring can really muck you up so check those before chasing other times.  

 Get it to work first with standard stuff, then make your tweaks.  If you try several powders, progressively slower, and it will not function, and if and only if you feel comfortable drilling your gas port, open it up .002-.003 at a time (one number size) using a load KNOWN to work by others.  If you go to large, THEN you can use an adjustable gas block to tone it down some. A lot of people here seem scared to open up gas ports and I have never understood that.  You cannot ruin a barrel doing so since you can always us an adjustable gas block if you go too large.  And being over gassed is not the end of the world as many have you think.  When drilling, you just have to be smarter than your drill bit and make sure you don't drill too deep and gouge the other side of your bore.  Block the barrel off with something.  I use a brass rod shoved down the barrel to stop over drilling, or eye-ball it through the barrel so you see when the bit comes through the top side, but a brass rod is more safe.  That's really the only way you can get in trouble, and use a drill press and plenty of cutting oil.  Free handing it can result in a broken bit and blocked gas port, and they are bitch to remove sometimes.

I have a number of Proof barrels and they’re all gassed very lightly.  I didn’t see, but how is the gas block tuned?  I ditched adjustable gas on all my Proof barrels (to include two 6 ARC barrels) even when suppressed due to this.

If you’re not suppressing it, I’d throw an H0 in there.  Failing to fully lock back like that looks like an undergassing issue.

Im running factory ammo only (Hornady ELD Match and Hornady Black), and Rifle +1 is the only configuration that Proof sells the 18" barrel in.
Beyond that, yes cycling it manually can lock the bolt back. Im leaving drilling out the port as a last resort.

The "H2" is the standard weight buffer for VLTOR as well, its a rifle length system to begin with.

I have the H0 (along with the other weights) on order, should be here friday to test. Ive heard these gas blocks can leak. Ill probably end up trying both, even considered trying to do a "smoke test" based on someone else's recommend of running compressed air through it.

If it runs with an H0, then hopefully that H0 causes no issues with the other upper I run on this, as that thing is a tack-driver, and has run the H2 without issue.

Testing for leaks isn't a bad idea. It sounds like you're almost locking back but not quite there. If the H0 gets you there, it's minimally more effort to swap buffers when you deal uppers if you need to. I'm almost never in favor of changing the reciprocating mass to correct for gas but this really sounds like the cartridge characteristics, barrel, and buffer weight call for lowering that mass a little. It really sounds like gas is on the edge of functional.

As an update, I was not able to get the aero adjustable gas block to function reliably with the rifle either. When I removed the handguard I saw a lot of carbon in front of the gas block, pretty sure it was a sign of gas leaking?

Following that, I installed a standard gas block (non adjustable), from my parts bin, old ballistic advantage one that came with a hanson barrel.

Moving forward, the CMC Triggers 6 arc enhanced BCG still would not function reliably, despite cycling through the buffer weights (H0, H1, H2, H3, H4).

I switched that BCG out, and stuck in a cheap one I got from optics planet (trybe brand - milspec) and it ran reliably for the first time for about 10 rounds, then the extract broke off on the bolt.

I had my JP bolt with me, and a standard mil spec bcg (white oak), which ran reliably about 80% of the time when moving to the H1 buffer (standard is H2). By that point I ran out of ammo...but making some progress.

I was told  that the JP 1 piece gas ring can cause some issues as well, so when I got home I pulled my field repair kit, removed the one piece gas ring, and installed standard 3 piece gas rings on the JP bolt.

Tomorrow I'm likely going to try the CMC bolt in a standard carrier, and the JP bolt in a standard carrier, see if there's any difference in reliable operation.

Im also beginning to wonder if opening up the gas port on the barrel itself is the next step..

I did the white oak test for the bolt catch, and the lower passed.

I did at one point swap lowers as well, just to rule that out as well.

I did get to running a bore scope down here again, and seeing quite a bit of gas port erosion considering its only seen about 200 rounds down this barrel..



I contacted superlative arms about it and was given some interesting feedback from them to try.

They are convinced it should run on "open" if it runs with a regular gas block.

Told me to ditch the vltor system (or try a sprinco reduced power spring). Also recommended I try a standard carbine buffer weight and spring (that proof's rifle +1 is meant to run on that)..first time Ive ever heard that.

So, I've no experience with  Vltor systems or +1 gas.  I do run a lot of Superlative Arms GBs. Here's my voodoo, yours certainly will vary as it sounds like you're on the lean side of gas and I never am.

I run standard springs on standard weight buffers, whether rifle or carbine, and adjust gas to match. It doesn't make sense to increase the reciprocating mass AND reduce gas. Spring rate shouldn't be screwed with until consistent tock back is achieved.  Then you're really fine tuning operation. I've never had to swap springs in my builds but helped a couple of buddies that were outrunning their magazines and assorted other issues.

If you are using all of your gas, the next step is to reduce the reciprocating mass. While I don't know where SA gets their info from, what they say regarding reduced power spring is interesting. I would certainly say a starting point would be nothing more than a standard power spring.

I was surprised myself, as from what Ive seen, VLTOR's buffer system, by and large, has resolved a lot of the "what strength spring + what weight buffer" do I need to run reliably type questions..

VLTOR's standard rifle milspec spring, and H2 (standard weight) buffer runs my 18" SPR rifle gas setup, my 16" proof intermediate gas setup, and my 14.5" criterion core mid length gas setup, all without hiccup.
I believe 2 of those 3 also have superlative arms gas blocks too..with minimal tinkering from "open" to get it ejecting correctly and running smoothly.

Finally got this figured out...

So far the parts count goes:
CMC Enhanced BCG that wont run no matter what.
Trybe BCG (extractor broke after 200 rounds)
JP Enterprises Bolt (gas rings were leaking)
Superlative arms adjustable gas block (wont run no matter what setting)
Seekins precision adjustable gas block (gas adjustment screw blew out)
Aero precision adjustable gas block (leaking gas)

Got it finally running by replacing the one piece gas rings on the JP Bolt with spare mil spec gas rings I got from my CMMG field repair kit, and putting a standard gas block on the barrel  from my parts bin(not adjustable).

The above setup runs now with both the VLTOR A5 system (standard h2 buffer), and a parts bin setup I put on another lower, that was a basic carbine buffer, and standard buffer weight.

I cant believe how much ammo I wasted in this so far figuring it out.

Glad you're getting it sorted. It sounds like there was a dark cloud following that build for a while!

Thank you, its definitely been pretty crappy all around. Ive never had anything even remotely similar to this, and it was odd to have the parts companies involved more or less helpless too.

Now Im waiting on some more ammo (ran out in the process of the above) so I can finally chrono it, zero the scope, and shoot some damn groups.

I will be picking up spare gas rings now..to replace my spare set, another standard gas block to have a spare on hand (if this situation ever comes up) and I still need another bolt carrier since the one Im using with the JP Bolt I pulled out of my other upper.

Pick up a whole buttload of gas rings.  They’re cheap and usually let go without much warning when they decide to quit on you.

As you’ve found out, standard gas block, H0 - H2 buffer, and I’ve found a regular quality CL FA carrier work perfectly. Proof appears to have done a lot of testing and research on their “CAMGAS” program and it truly does work.

Are sprinco worth the price? or any mil spec gas rings?

I've never used their gas rings but I've upgraded my AR 10 and all of my Toolcraft AR 15 ejector and extractor springs with Sprinco. The weak ones showed consistent improvement and it certainly gave me confidence in all of them. Sprinco is known quality. As for their gas rings, I can't vouch for them but if I were in need I would certainly not feel like I were gambling.