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?

[grayfox]

You've done a lot of troubleshooting, so that's a plus.
I am not a fan of rifle length buttstocks, springs, etc. So the A5 I believe is one of those?
I had a lot of problems, twice IIRC, with rifle lengths, one of which is the 6Arc, 1 was grendel I think. I transferred that rifle length butt stock to my uintah, which is a bolt action and doesn't care about which butt stock I'm using, and no spring. Once I also tried to "shorten" a rifle length with a plastic follower behind the carbine spring in a rifle stock... no good either. For me. So I use all carbine stocks.
Yes, go to a standard carbine setup, with carbine spring, normal buffer (you will probably go to an H1 later, but for now, use normal 2.8-ish oz), carbine butt stock. For cheek risers I like the Luth-AR MBA's or the PRS lite works also, carbine style. Both are solid.
I would focus on the 105 Hdy black, as those have shot well pretty much all the time, better than 108's for a lot of guys. But either one will tell you something.

This next is purely an IMO, but for the 6 Arc and 18", I am not a fan of the +1 gas lengths, but that's what proof is using so go with it, but lighten up on the buffer/stock/spring action in the rear and see what you get. Not trying to get any debates going on gas lengths, it's what you have and proof has been able to make it work, just throwing in my 2 cents for the next barrel/upper/etc you go for. Arc is not a high-pressure (52ksi) so doesn't major on lots of gas flow/pressure, and rifle length for a 556, 18" is plenty dwell time. So go "back to carbine" for now, use a lower you have on hand maybe...

[300BLK]

If the gas block is aligned with the port, the answer is not enough gas volume/pressure for the BCG/buffer/spring.

You CAN run a carbine length spring and buffer in a rifle length buffer tube with a spacer to make up for the difference in buffer tube length; I do one one of my lowers.

A rifle buffer usually weighs 5oz vs a carbine buffer at 3oz, but there are others. If you can't make it work with a standard gas block (or Superlative 18 clicks out from fully closed) and carbine buffer and spring, there simply isn't enough gas.

[AleksanderSuave]

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.

[Old Bob]

Aleksander ... You said your original buffer was a H2 buffer. Was that a standard Carbine type buffer? I have a VLTOR A5 receiver extension on my 6.5 Grendel AR with a Magpul UBR Gen2 stock. The instructions I had for using the A5 extension said to use the longer VLTOR buffer & a standard rifle spring. The VLTOR buffer is about .75" longer than a standard Carbine buffer. My set up seems to be working OK. Haven't had any issues with bolt lock-back with factory ammo. It has an adjustable gas block that is set to fully open to get things working OK. I'll probably put a regular gas block on it eventually since the adjustable block seems to be of no real improvement.

I'd follow grayfox advice & go to a Carbine extension with a standard gas block. Start with a standard Carbine spring & a H1 buffer. You can replace parts as needed to improve function.

EDIT: Ooops didn't see your last post before I hit the send button. Glad things are working OK now.

[grayfox]

Glad it's working, bummer when different parts are broken... drives you bonkers.
sixfive outfitters ( a sponsor of this forum and good guys) has spare extractors and other parts, as well as bigger stuff. On extractors, you want to be careful to use 6.5 grendel/6 Arc extractors, the 556/762x39 extractors are the wrong extractor-groove depth/size. Also keep all your stuff synched up to a 0.136" bolt face depth -- I'm hoping that the proof barrel is set that way - kind of sure but just saying the 0.136" bolt face depth is the defacto standard for 6.5 grendel and 6 Arc. the 556 and some early 65 grendel's use(d) a 0.125" bolt face depth, so those parts can lead to mis-fitting stuff, bad head spacing etc.

[AleksanderSuave]

Aleksander ... You said your original buffer was a H2 buffer. Was that a standard Carbine type buffer? I have a VLTOR A5 receiver extension on my 6.5 Grendel AR with a Magpul UBR Gen2 stock. The instructions I had for using the A5 extension said to use the longer VLTOR buffer & a standard rifle spring. The VLTOR buffer is about .75" longer than a standard Carbine buffer. My set up seems to be working OK. Haven't had any issues with bolt lock-back with factory ammo. It has an adjustable gas block that is set to fully open to get things working OK. I'll probably put a regular gas block on it eventually since the adjustable block seems to be of no real improvement.

I'd follow grayfox advice & go to a Carbine extension with a standard gas block. Start with a standard Carbine spring & a H1 buffer. You can replace parts as needed to improve function.

EDIT: Ooops didn't see your last post before I hit the send button. Glad things are working OK now.

no problem, just wanted to clarify for anyone else reading who this may help. The original buffer was the correct vltor h2.

I run them on my 14.5 mid length, my 16” intermediate, and my 18” spr rifle length.

I do agree this caliber does seem to very low pressure. I’m still perplexed as to why none of the adjustable gas blocks ran even on open setting..

[donno]

A couple of things I have found since building an ARC. Forget what you've learned from building 223/556 rifles. 6 ARC seems to take a lot of gas. A lot more than I thought. I was having "short stroking" issues. Fixed that and now have "bolt bounce" causing the bolt to hit and back up a thousand or two. I think the fix will be a heavier buffer and flat spring and also a full mass BCG.

[Old Bob]

donno ... There was some discussion on "bolt/carrier bounce" on the Grendel forums. IIRC, increasing the buffer weight was only part of the cure. It was suggested you need to change to a heavier recoil spring as well along with a standard M16 carrier. You could try one item at a time to see what stops the bounce.