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Need some troubleshooting help: Bad Gas Block or Out of Spec Bolt Catch

AleksanderSuave

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Jul 26, 2020
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Detroit, MI
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:

O2AzGX1.jpeg


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.
CKsRUsS.jpeg
 
Any weird wear on the mags? Or on the bcg? I don’t think I could get a bolt to hang up there on my AR15s if I tried. It’s almost like the mag is riding too high in the lower.

I have one “AR10” that gets its own mags, because the mag catch is just a bit higher in that gun than others. An unmodified empty mag won’t seat on a closed bolt. And the bolt will stop on the feed lips of an unmodified mag. I have filed a bit from the upper edge of the magazine catch hole on a couple of mags, to allow them to seat a bit lower. I won’t be able to monopod off the mags, as I’m sure that will induce a feed issue, but the gun runs otherwise.
 
If you manually cycle the bolt with an empty mag will it lock back like it should?
 
as far as unusual wear, Im not entirely sure, to be honest. This is the first Upper Ive ran steel mags on. Everything else I own is Pmags. I can get some photos though.

The paint is a little worn on the feed lips on the mags, but thats about it.

As far as manually cycling the bolt, yes, I can get it to lock back as it should. If I pull the charging handle really slow and lightly, I can get it stuck on the follower, but I highly doubt that normal operation is anywhere near that light when its cycling a round.
 
Ok, hers something that I just thought about. You say that ejection just sort of dribbles out. What if you load a round, eject the mag, then fire? My suspicion is that the drag on bcg, caused by the mag is causing the issue. Either way, this would be a simple test to rule that out. If ejection is similar==> gas. If ejection is not similar==> mag interference…
 
I was asking to ensure the bolt catch was functioning correctly. A weak spring under the catch could contribute to that issue. I would assume the rifle is short stroking. I run the A5 on my 6 ARC with a 20" Proof and same VLTOR upper. I ran is prior with an A2 and rifle buffer. It locks back correctly. Both buffers are in that 5.2-5.3 ounce range.

There could be an issue with mag height in that lower. Maybe the bolt is dragging a little on it and causing the catch to actually grab the carrier instead of the bolt face. Do you have another assembled lower you can swap the upper to? Try that and see if it gives you issues.
 
I was asking to ensure the bolt catch was functioning correctly. A weak spring under the catch could contribute to that issue. I would assume the rifle is short stroking. I run the A5 on my 6 ARC with a 20" Proof and same VLTOR upper. I ran is prior with an A2 and rifle buffer. It locks back correctly. Both buffers are in that 5.2-5.3 ounce range.

There could be an issue with mag height in that lower. Maybe the bolt is dragging a little on it and causing the catch to actually grab the carrier instead of the bolt face. Do you have another assembled lower you can swap the upper to? Try that and see if it gives you issues.

Im going to run downstairs and try it manually cycling it on another lower. Not sure if that will answer it in full but I will at least see if it can get "stuck" the same way if dragging real slow.
 
Magazine wear:
RpAbX6J.jpg

iPYKXMO.jpg


looks consistent across all 3. Some saw more use than others.

As far as manually running it on another lower, I was able to get it to catch on a newer lower (aero enhanced).

I did notice that the mag sits at a slightly different height in the newer enhanced design:

New lower:
LCaReKx.jpg



Original lower (Aero XM 15):
JuRJSo3.jpg


The left feed lip specifically seems to be a little lower in the newer enhanced lower, my photo may not show it well as one was taken at an angle..but not sure if that would make significant difference.

I also tried two different bcgs and the issue was replicable with both bcgs and both lowers.
 
That magazine wear is normal. Just checked most of my mags. It’s from the mag catch as you insert it up into the mag well.

Your symptoms are common to short-stroking of the gas system. Not enough gas.

Is there a way to defeat the bleed-off feature of that gas block?
 
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That magazine wear is normal. Just checked most of my mags. It’s from the mag catch as you insert it up into the mag well.

Your symptoms are common to short-stroking of the gas system. Not enough gas.

Is there a way to defeat the bleed-off feature of that gas block?
There’s an “open” setting but that didn’t seem to help either.

I grabbed another gas block to try. Aero adjustable. Not the nicest but should work to test with to see if there’s a difference.
 
Not sure if this was mentioned, and it might not solve the issue, but unless you're using tall buttons, your mag catch isn't screwed in far enough.
Im sure these are basic LPK buttons, nothing special or tall, but I tighten them to "just enough" to hold a pmag securely and drop it when I hit the button. Shoot enough with gloves that it sticking out a bit further is preferred.
 
Any weird wear on the mags? Or on the bcg? I don’t think I could get a bolt to hang up there on my AR15s if I tried. It’s almost like the mag is riding too high in the lower.

I have one “AR10” that gets its own mags, because the mag catch is just a bit higher in that gun than others. An unmodified empty mag won’t seat on a closed bolt. And the bolt will stop on the feed lips of an unmodified mag. I have filed a bit from the upper edge of the magazine catch hole on a couple of mags, to allow them to seat a bit lower. I won’t be able to monopod off the mags, as I’m sure that will induce a feed issue, but the gun runs otherwise.
Mine will always hang up just like that if it's short stroking little.
 
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Im sure these are basic LPK buttons, nothing special or tall, but I tighten them to "just enough" to hold a pmag securely and drop it when I hit the button. Shoot enough with gloves that it sticking out a bit further is preferred.
"Just enough"? Well, OK then!!! Maybe buy the tall buttons and do it right next time, they're not expensive.
 
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There’s an “open” setting but that didn’t seem to help either.

I grabbed another gas block to try. Aero adjustable. Not the nicest but should work to test with to see if there’s a difference.
My brother's rifle wouldn't run all the way open on his Aero gas block. I put a diffrent gas block on it finally and it ran great after that.
 
That magazine wear is normal. Just checked most of my mags. It’s from the mag catch as you insert it up into the mag well.

Your symptoms are common to short-stroking of the gas system. Not enough gas.

Is there a way to defeat the bleed-off feature of that gas block?
Yes you center the adjuster. Then turning it in makes a restrictive gas block and turning it out makes it bleed off gas block.

It looks to me like something might be blocking the gas hole in the bore scope pick.
 
Yes you center the adjuster. Then turning it in makes a restrictive gas block and turning it out makes it bleed off gas block.

It looks to me like something might be blocking the gas hole in the bore scope pick.

Thats the illuminated fiber optic piece I fed through it during bore-scoping. its a poorly focused photo but it glows blue and helps to make sure the gas block is aligned over the gas port.
 
My brother's rifle wouldn't run all the way open on his Aero gas block. I put a diffrent gas block on it finally and it ran great after that.

Good to know..I also have an older ballistic advantage gas block here too. Non-adjustable. May put that on another gas tube and bring it with me, and swap it out at the range if that happens.
 
I run the Aero adjustable on mine. No issues for me. I was surprised that it fit fairly tight to my barrel. It seems they tend to run a little on the larger end of internal diameter. That means a poor gas seal in most cases and could be the cause of some problems.
 
I run the Aero adjustable on mine. No issues for me. I was surprised that it fit fairly tight to my barrel. It seems they tend to run a little on the larger end of internal diameter. That means a poor gas seal in most cases and could be the cause of some problems.
That’s interesting because on the tube roll pin slot, it felt like they ran on the smaller side.

I picked up some 5/64 diameter pins locally when I didn’t want to pay $10 to ship “milspec” pins, and I believe 5/64 is just a hair smaller than normal gas tube roll pins, but it still was a tougher fit than I normally encounter when pinning the gas tube.
 
I haven't had issues with the gas tube roll pins or the hole for the pins. I was talking about the actual internal diameter of the gas block for the barrel journal. So the barrel has a .750 journal and the block might be .753+ I had one that I measured .756 on the block and the barrel was .749.
 
I built a 6arc upper, ended up with a H3 buffer. Solved my cyclic issue and started functioning 100%.

YMMV
 
If it's barrley running on the heaviest bullet. Extra buffer weight and extra spring power are not the answer. You are loosing somewhere. Not getting enough gas somewhere or there is something tight or dirty impeding movement.
 
Get yourself a set of these

I watched a video for these, Im surprised no one is 3d printing it yet. seems easy enough to make. Ive heard of guys doing it with a piece of uncooked spaghetti too..regardless, I dont think port alignment is my issue.
 
If you have 5/16" - 3/8" from the face of the bolt to the bolt catch, you are experiencing an overgas situation. The bolt is cycling so fast it's over running the bolt catch, mimicking other suspected known issues.

Two ways to slow it down, remove gas or add weight in the buffer. I choose weight since I don't want to remove the one thing the rifle needs to function, gas.

Loved or hated, I learned a lot from Slash with regards to using heavy buffers to make a rifle run correctly. I had a 358 Winchester that had been back to the manufacturer twice and still was sporadic. I removed the adjustable gas block and replaced the buffer with one of his XH Buffers and Spring, rifle ran perfect after that. No more damaged brass, no more failure to lock back. I have since started with a standard spring and H3 buffer and worked up from there, haven't had to go down in weight once.

I am relating my personal experience with multiple rifles in both small and large frame, there are far more knowledgeable members here.
 
If you have 5/16" - 3/8" from the face of the bolt to the bolt catch, you are experiencing an overgas situation. The bolt is cycling so fast it's over running the bolt catch, mimicking other suspected known issues.

Two ways to slow it down, remove gas or add weight in the buffer. I choose weight since I don't want to remove the one thing the rifle needs to function, gas.

Loved or hated, I learned a lot from Slash with regards to using heavy buffers to make a rifle run correctly. I had a 358 Winchester that had been back to the manufacturer twice and still was sporadic. I removed the adjustable gas block and replaced the buffer with one of his XH Buffers and Spring, rifle ran perfect after that. No more damaged brass, no more failure to lock back. I have since started with a standard spring and H3 buffer and worked up from there, haven't had to go down in weight once.

I am relating my personal experience with multiple rifles in both small and large frame, there are far more knowledgeable members here.
I never knew Slash took a break from playing guitar to build rifles :D

I'll definitely try adding weight, and report back.
 
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Set your SA gas block to 18 clicks out from full closed. Does it lock back? If not it's probably under gassed. Proos is not known for using over sized gas ports and your parts would still be sufficient even if it were.
 
Set your SA gas block to 18 clicks out from full closed. Does it lock back? If not it's probably under gassed. Proos is not known for using over sized gas ports and your parts would still be sufficient even if it were.
Correct. It does not lock back on open (18 clicks).
 
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. Im also wondering if its possible that something with the upper may be out of spec, as its about the only part I havent tried a replacement of.
 
In all this experimentation have you measured the gas port? Sounds like it could use a little drilling out to me...JMO
 
In all this experimentation have you measured the gas port? Sounds like it could use a little drilling out to me...JMO
Yup, its as it supposed to be, about 0.096.

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..

FveFHES.jpeg


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.
 
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"..or Out of Spec Bolt Catch"

That's ridiculous, who ever heard of an out of spec bolt catch! That's exactly what I would have said and probably was saying. Years ago I spent hours at the range with a friend trying to trouble shoot his rifle. (Honestly I don't remember exactly what it was / wasn't doing exactly anymore, it's been too many years.) But we'd painstakenly swapped one part at a time from one rifle to another until we were down to things "It just couldn't be". We eventually swapped the catch (that looked perfectly fine and seemed to function just fine, moved freely) and the problem finally went away. I'm not saying that's your problem, just saying even though it may look and feel fine, it still can be wrong.
 
"..or Out of Spec Bolt Catch"

That's ridiculous, who ever heard of an out of spec bolt catch! That's exactly what I would have said and probably was saying. Years ago I spent hours at the range with a friend trying to trouble shoot his rifle. (Honestly I don't remember exactly what it was / wasn't doing exactly anymore, it's been too many years.) But we'd painstakenly swapped one part at a time from one rifle to another until we were down to things "It just couldn't be". We eventually swapped the catch (that looked perfectly fine and seemed to function just fine, moved freely) and the problem finally went away. I'm not saying that's your problem, just saying even though it may look and feel fine, it still can be wrong.
I tried a different lower to rule that out.
 
Carbon around the gas block is a sign of leakage but can be "normal" and typically will seal up after some rounds. If the BCG isn't efficient it can cause some issues since you aren't getting a good gas seal there since the bolt and carrier function like a piston.

I would try and measure the gas port in your barrel and see where you are at. Pin gauges or worst case numbered drills if you have some. This will give you the best measurement. If the port is undersized contact Proof and see what they say. Or you can drill it out yourself if you have the ability and a good numbered drill set.

I have a 12.5 midlength that would short stroke with a Microbest chrome BCG but ran flawless with a BCM or Geissele group. Same result you are seeing. The bolt catch would catch the carrier and not the bolt face. Since this was something I was building I knew I would have to play with gas port size and buffer weight. Once I got it to run with a known good BCG (gas port and buffer corrected) I tried that chrome BCG and it was the same issues all over again. Ended up leaving the BCM group in there.
 
Carbon around the gas block is a sign of leakage but can be "normal" and typically will seal up after some rounds. If the BCG isn't efficient it can cause some issues since you aren't getting a good gas seal there since the bolt and carrier function like a piston.

I would try and measure the gas port in your barrel and see where you are at. Pin gauges or worst case numbered drills if you have some. This will give you the best measurement. If the port is undersized contact Proof and see what they say. Or you can drill it out yourself if you have the ability and a good numbered drill set.

I have a 12.5 midlength that would short stroke with a Microbest chrome BCG but ran flawless with a BCM or Geissele group. Same result you are seeing. The bolt catch would catch the carrier and not the bolt face. Since this was something I was building I knew I would have to play with gas port size and buffer weight. Once I got it to run with a known good BCG (gas port and buffer corrected) I tried that chrome BCG and it was the same issues all over again. Ended up leaving the BCM group in there.
Ive measured the gas port, it is at 0.096" to proof's spec.

Proof responded today and said "With camgas barrels we recommend running milspec everything"
 
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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.
 
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This thread has been a master class in “why it’s cheaper and less frustrating to buy a factory assembled rifle…”
 
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This thread has been a master class in “why it’s cheaper and less frustrating to buy a factory assembled rifle…”
that’s hilariously untrue.

I’ve seen so many gas guns and bolt guns have issues..fresh from the factory too. My first rifle purchase was a factory assembled rifle..that went back for issues twice, and eventually ended with a refund.

Half the time something gets posted on here with a rifle having issues people spend half the comments trying to say how it’s great and the shooter is the error.

Learning to diagnose and fix your own issues is pretty important, to say the least.
 
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