Need Help With Cycling Issue - Proof Barrel

[AleksanderSuave]

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 also measured the gas port in the barrel, and it is to the proof spec, (0.096") as well.

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.



A few things Ive noticed so far:

After I removed the superlative arms gas block..I tried an aero gas block (visibly leaked air as well), which did not help. After that I went back to a standard gas block.

Then I started trying different buffer weights. H1 will occasionally eject correctly but still not fully reliable. the CMC Enhanced BCG wound not reliably lock back at all, the mil spec carrier with proof bolt would about 50% of the time, and the Trybe bcg and bolt did about 80% of the time, until the extractor claw broke.

So far Ive emailed my question to proof, CMC, and superlative arms about the above build.

Proof's website recommends the JP bolt, adjustable gas blocks, and full mass carriers and buffers. I emailed them to verify and they told me to use a fixed gas block, and "mil spec everything", instead. So my plan is try to an older carbine standard buffer weight as well.

Superlative arms recommended against the vltor system, and said I should try a sprinco reduced power spring instead.

I was also told by others running the JP bolt to try swapping the 1 piece gas ring to traditional 3 piece gas rings, so I went ahead and did that to test next.

Any other recommendations or ideas?