Home Forums Hunting & Shooting AR and Tactical Rifles Lets circle back to your 6 ARC build

How is that working out for you?

I ask this, as mine is "complete" but I've been struggling with a few issues, and truthfully, I'm not versed in AR's enough to diagnose them.

I know I could have purchased a complete rifle, but I've wanted to build one for a while now, and this seemed like a perfect opportunity to jump in. So I did, with both feet. The barrel came first, and I did not skimp much. Proof Research (stainless, not the carbon wrapped version - that WAS too spendy!) 18 inch, rifle +1 gas port, 7.5 twist. VG6 Gamma brake. Since I was going almost whole hog, I opted to go with a Seekins billet upper and lower, and parts kit, as well as their adjustable gas block. Trigger is a Trigger Tech Adaptable. Looking for a reputable bolt carrier group during this Scamdemic, I was able to locate a CMMG unit to procure. Radian Raptor charging handle. For the butt end of the rifle, I opted to go with the Magpul PRS adjustable stock, along with an Aero- Precision H2 buffer (carbine length). Ergo Grip. I made a choice for an (I think) interesting handguard, opting for a WWII themed unit from SD Tactical. If I can afford to have it all cerakoted, it ought to look pretty awesome. Haven't settled on an optic yet, but pulled the Vortex off my 5.56 for the time being.

So here are my issues - The gun goes bang just fine. Ejection is in the 2-4 o'clock zone (border line over gassed?). Bolt does stay open on a single round/last round scenario, but does not feed another round into the chamber with multiple cartridges in the mag (Stoner 6.5 Grendel mags). So, effectively, so far I have a really nice looking, expensive single shot. Looking at the hold open on single rounds, it looks to me like the bolt is catching on the bolt body (?) (rather than the locking lug face) with the locking lugs positioned over the magazine follower. What does that signify? Too heavy of a buffer? Under gassed? Try as I might, I can't make that situation occur with my 5.56. Too much spring tension on the bolt release?

Thinking that the rifle wasn't getting enough gas to properly open the bolt, I opened the gas block most of the way with no change. Which leads me to think that it is getting enough gas, but the bolt buffer is too heavy? I've swapped it out for the carbine unit out of the 5.56, but haven't been able to test that yet. Or Should I be running a rifle length buffer tube with the heavier buffer? My neanderthal logic says that if I'm running a rifle length gas tube I shouldn't be running a carbine buffer? Like I said, I'm not well versed in the ins/outs of this platform......

Can someone generously walk me out of this minefield I've trapped myself in? Please?

Kaiser Norton

Kaiser, you sound like a smart guy. Pretty mechanically minded. That is good. First off, you should be perfectly fine with a carbine length buffer tube and H/H1 (carbine length buffer) or similar buffer weight, the H2 may be a bit too heavy for your application. I generally run the Spike's Tactical ST2 (4.3 oz) buffer in all of my rifle length gas system rifles that use the carbine length buffer tubes. They actually run smoother than my rifle length buffer tubed rifles. Slightly less recoil and perfect function. So, I'd probably scratch that off your list. Just for reference, I will tell you about my most recent experience with my "fighting rifle", the great mighty BCM (bullshceidt, I've had more issues with this rifle than any other, still no failures of any kind with my Northtech Defense, Noveske, and Spike's Tactical). It had many failures to feed on Friday. That was the first time I took that rifle out with its new barrel. I fired 220 rounds down range with it:

(From Friday: 220 rounds with new BCM and 80 rounds with Spike's Tactical: 300 rounds total^^. Don't worry, I poked the primer out of that one )

I can say with 100% certainty that it was a load issue. Or powder choice issue. Wishing I had more AR Comp powder on hand. I'm using a powder that I am not familiar with, but had 2 pounds of it and gave it a try. As I worked up the powder charge, the problem went away. While ejection stayed pretty much in the same place (3-4 o'clock), so sometimes ejection patterns are misleading. My bolt also locked back, but not all the way. At first though, with the lighter loads, the bolt was not locking back on the last shot. In your case I'm thinking it's also a load issue or if you are running factory loads, your buffer may be a hair too heavy. I had the same issue with a rifle length buffer tubed (in a Gen II PRS) rifle chambered for the 6WOA. The problems went away after going to a captured spring system. No further issues with that 6WOA, which is based off the 6.8 SPC. I thought it may have been a mag issue at first, but it wasn't. After switching to that style of buffer system, no more problems. I use the Armaspec stealth recoil system, which is similar to the JP enterprises captured spring system, but about half the cost. Also, something else you didn't mention is if you are running your bolt dry or "wet". A small amount of lube on the bolt goes a long way. If you don't mind, share what load you are using in your rifle as well. Your rifle sounds like a beauty. Post some pics of it when you can. Sounds like something I'd conjure up. I love the PRS stocked rifles.

I’m gonna start off by saying I have never used a PRS stock and didn’t know they worked with a carbine tube.

But they are optimized for a rifle tube minus the A2 spacer and I’ve never seen a smoother system than the rifle tube and buffer with the longer softer spring. If I could, I would use it every chance I got.

Rifle gas plus 1 on an 18” barrel has me concerned. If I’m reading that right, that puts the port 1” past rifle gas? That certainly doesn’t leave much dwell time to build pressure to operate the system. But I know nothing about the 6 ARC either. I’d call the barrel mfg and ask them what they suggest.

I will throw this out there, my first jump into a Noveske barrel was an 18” SPR with their intermediate gas system (think mid length +1) and I just knew I would only need a carbine buffer. Gun fired and locked back on an empty mag but would never feed a second round from the magazine. I called them and they said the bolt is outrunning the mag spring, try an H buffer. I did and problem was solved. The mag spring was weak and wasn’t pushing the next round up at the right time.

Just something to think about cause diagnosing a problem on the internet is usually a futile concept.

Does everything feel smooth when you manually pull the bolt rearward? Can you confirm that it has the full range of movement to operate properly?

Hand operation is very smooth - every bit as smooth as my 5.56. Full range of movement by hand and, I assume, under recoil.

TWR - The PRS is a gen 3, compatible with both a carbine or rifle length buffer tube. Only difference is an added spacer for the extra length of the rifle tube. I used the carbine length tube as that was all that was available when I was sourcing parts. My 5.56 has the A2 stock/tube setup - if I need to I can swap things around.

Thanks, gents.

Kaiser Norton

First of all, let me say that I don't have a 6 ARC, nor a PRS stock..............

But the statement above describes typical short stroking since the bolt is not going far enough back for the bolt catch to engage the front of the bolt as it should.

So then the solution is to find the cause & there could be several & probably a combination of contributors..............but in the end, it means that for your parameters of gas pressure, spring rate & buffer weight & ammo, you simply do not have enough pressure to push the bolt back far enough to allow the bolt catch to lock the front of the bolt.

And pressure is pressure, whether it comes from a 556 or a 6 ARC.

I also very much agree with TRW's assessment on a +1 rifle gas system as being pretty suspect & marginal, with regard to not only dwell, but pressure as well.

You've taken a rifle gas system, which is already low on pressure & have reduced that pressure even more, & reduced the dwell time. As I said, pressure is pressure, no matter what round it comes from.

I've encountered the situation that you have many times through the normal tuning of a rifle with an AGB as I work through getting the gas tuned.............there is usually a point where you go from no lockback to locking back (marginally) but not on the bolt face to finally getting enough gas to fully lock back the bolt.

I would suggest opening the gas block fully open with your original setup & see what happens & if the bolt will lock fully back. If it will, then fine tune the gas to where it will just reliably lock back. Whether you can get there with your spring & buffer & ammo combination will then determine if you need to change spring or buffer or ammo or all of them.

Ejection pattern is also dependent on the extractor & to some degree, ejector spring pressure, it is not just a function of gas alone.

I almost always like to start with the softest spring & the lightest buffer that I think that I can get away with using for a given barrel / gas configuration. ( Most rifle length 556's really don't need an AGB unless one is shooting a really extreme range from light to heavy ammo; I doubt that your barrel needs one either)

Since the 6 ARC is fairly new, in the end, it's also possible that there may still be some unknowns & uncertainty about gas port sizing as well, so if all the above fails, that could still be an issue.

BSA also makes a good point about ammo................make sure that your test loads have enough pressure to be a "full power" load & I'd advise using factory (Hornady) ammo in that round as a baseline starting point as it should be a reliably known pressure round.

As for magazine spring pressure, I've found Lancer's to be about the stiffest, but I don't think that's an issue here at least until you can get the bolt to lock back correctly.

The guy here that probably has the most experience with non 556 based rounds is probably Yondering, so maybe he will offer some comments as, IMO, he knows & understands the system very very well.

YMMV

MM

Good info MM.

With that out of the way, what are you feeding it and have you run it over a chrony?

I guess I should have mentioned this earlier, I'm running Hornady's Black 105gr load (so far). I have about 5 boxes left, and another 5-6 boxes of the 108gr ELD load. I'd really hoped to get the rifle dialed in with the cheaper load. I've not run it over the chrono, as I have a Magnetospeed and I haven't even thought about trying to mount that to this barrel.

Again, thanks!

Kaiser Norton

Problems with ammo seem less likely going factory. As others have stated, the only option left is a gas problem. I would check gas port alignment and work my way back from there.

Alright, slapped everything back together. I had a Superlative adjustable gas block with the bleed off feature show up in the mail yesterday, so I replaced the Seekins unit with that. Set it (as near as I can tell - it is supposed to have positive clicks to the adjustment. They seem rather inconsistent to me) to their recommended setting to achieve bleed off. First few rounds I was single loading, to check bolt stop function. Here is what I am seeing:


Bolt is locked back, but by the body of the bolt not the locking lugs, which protrude over the magazine. Ejection was solidly in the 4 o'clock position for all three rounds. I opened up the gas block about two full turns (between rounds) with no apparent effect. In frustration, and with my hands freezing in the cold here, I put in a magazine with 3 rounds in it. Rifle fired, self-loaded, and ejected all three! I opened the gas block a couple more clicks before coming home. Bolt locked open same as before though. This short stroking (is that the proper term for what is happening?) only happens when the rifle is fired. Manually operating the bolt results in it locking back with the lugs behind the bolt stop. Thoughts/recommendations?

Kitchen counter pics, per BSA's request:














Definitely need some color on this thing!

Kaiser Norton

The answer is in my previous post, but I'll try again.

You do not have enough pressure sustained over a long enough period of time, from the round firing to push the mass of the bolt & the buffer with the spring weight that you are using, (assuming no obstruction or other extraneous resistance) back far enough to engage the bolt catch lock.

Very cold weather exacerbates that issue too.

You can try a non-adjustable gas block to be sure you are getting full gas flow, probably won't fix it.

You can reduce mass & reduce spring weight.

If none of that works, all that is left is increasing gas port size in the barrel.

If you reduce mass & spring weight & it still doesn't fully lock the bolt back, then I suggest that you call Proof Research.

By going to +1 gas system, you've made the situation worse on that 18" barrel as the reduced dwell time drops the pressure too quickly, & I suggest discussing it with them.

You would have been better off leaving it at standard rifle length or even going down to intermediate length, especially since you are using an AGB, with regards to reliability.

FWIW, every 18" barrel w/rifle gas 5.56 rifle that I own has a carbine buffer & a standard weight (Sprinco white) spring. They will all function on modest 223 ammo or full house 5.56 pressure ammo. On the one with an AGB, it is wide open, meaning it could actually use a tad more gas..............but it works.

MM

Just to be clear for those that don’t fully understand.

A fired 5.56 NATO round explodes at 62,000 PSI, the confined space forces the bullet and gasses out the barrel. As they travel further out the pressure decreases due to the bullet moving and the confined space is growing.

At carbine gas length, the gasses are tapped at 26k. Mid length the pressure has dropped to 17K. Rifle length it has dropped to 13.5K.

The tapped gas is forced back to the gas key moving the BCG back.

The other thing is dwell time. On a 16” barrel the bullet passes the gas port at 7.5” roughly, the time it takes the bullet to clear the barrel is dwell time. The GI guns with a 14.5” barrel but same gas system has a 1.5” less distance meaning less dwell time.

Take an 18” barrel and a rifle gas at 12.5” but less pressure and things run at an easier pace requiring different resistance needed in the form of buffer and spring. Add an inch to the gas port and pressure drops even more as does the time gas is forced down the gas tube or dwell time.

So you can open the gas port up, lighten up the spring and buffer or increase cartridge pressure to get enough power to move the BCG back to where it needs to go.

A carbine buffer weighs 2.9 oz. H weighs 3.8, H2 weighs 4.6, 9mm buffer weighs 5.5 and an H3 weighs 5.6 oz.

The rifle buffer weighs 5.2 oz but is longer and requires a spring 11.75”-13.5” at a lighter pressure rate than the shorter carbine spring that is 10 1/16-11.25”.

Call the barrel maker before you do anything else and see what they recommend.

Nice rifle. Did you try the lighter buffer yet? I believe in a previous post, you said you were going to give that a try.

In the attached link, the 2nd graph near the bottom illustrates gas system length vs pressure vs dwell time just as I have discussed & TWR went into detail to explain above.

It might help you to better understand what happens............if you add another orange tick mark down the chart to represent a rifle +1" gas system, you'll see that both pressure & dwell time drop. Maybe to the level of not fully cycling the gun.

MM

AR Gas Systems Info

Any updates?

Well, to date I have heard back from Hornady, who's only advice was to contact the parts suppliers for their observations. Superlative says they haven't experimented with the 6ARC yet. Proof asked what components I was using, ammunition being used, and was the gas block installed properly, and then decided they want me to send them the rifle.

First three shots on paper went into 3/4 of an inch.....with an improvised rest and an 8x scope.

Kaiser Norton

I've seen some other info posted about this on other sites regarding Proof 18" barrels with +1 gas systems.

I'm gonna bet that they will increase the size of the gas port in the barrel & send it back.

MM

All the parts for my daughters 6 ARC build will be here this coming week.

So, I did an experiment today. I opened up the Superlative gas block another full turn before doing anything. But I took all three lowers that I have, along with. I fired a total of 20 rounds through the three combinations, all loaded singly to test bolt hold open. Of the six rounds that went through the Seekins upper (Magpul PRS stock, carbine buffer/spring assy), the bolt locked open properly once. With the Stag lower (Luther adj. stock, H2 buffer/spring), the bolt locked back properly 6 out of 7 shots. With the DPMS lower (A2 stock/buffer/spring combo) the bolt locked back properly all 7 times.

While not exactly an exacting test, empirically, it makes me think that either the Seekins Bolt catch isn't set up properly, or something is binding in the buffer tube

Kaiser Norton