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6MM ARC Help !!

Benhonda1

Benhonda1

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Minuteman
Jan 22, 2012
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Fellow Hide Members,

I have built a 6mm ARC And running into a little problem. My build is a 20in proof with a radian Mod 1 builders kit, Aero Precision Gas block adjustable.

1. When the bolt is back in the locked position, and I push bolt release it chambers into battery correctly.

2. I fire a round (no noticeable damage on brass) and bolt cycles but does not pick up next round, the mag is a duramag 10 rd by c products, (6.5 Grendel)
Ejection pattern is at the 3:00-4:30 mark.

3. Gas system is at full value, all way open.

Any Ideas/ experience with this problem and how to get this to run more than one round without manual intervention would be greatly appreciated!

Thanks
 
hlee said:
Load one round. Shoot it. Does it lock on an empty mag? If so, does the bolt-stop catch the bolt or the carrier
hlee said:
Load one round. Shoot it. Does it lock on an empty mag? If so, does the bolt-stop catch the bolt or the carrier?
Click to expand...
Click to expand...
I will try that, what are we looking for that test and what will it tell?

Could the bolt be going to fast? Someone mentioned that to me?
 
Benhonda1 said:
I will try that, what are we looking for that test and what will it tell?

Could the bolt be going to fast? Someone mentioned that to me?
Click to expand...
That test is to see if the bolt is short stroking or not, if you have bolt lock back on an empty mag after you fire the round, then you aren't short stroking.
 
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Benhonda1 said:
The gas system is fully open, ideas to fix it
Click to expand...
Check gas block alignment with a borescope
Check if the gas block is sealed
Check gas tube alignment
If everything checks out then try these one by one in order
Lighter buffer
Reduced power buffer spring
And the last ditch effort is to check the gas port on the barrel to see if it was drilled undersized and you can open it up with the next larger drill bit. But don't do that unless you don't care about voiding any barrel warranty.
 
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Also make sure you have the proper length gas tube. Another thing is with those aero adjustable blocks, some of them don't run up tight against the gas block journal shoulder and you need a small gap between the shoulder and gas block to get the ports lined up without obstructing the flow.
Best way to see is with a borescope.

I just went through this whole process on my own 6 arc build.
 
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Might also be worth trying a different magazine for the heck of it. 6ARC seems to be a little finicky with that.

It's been a while, but I remember a thread over on ARFcom where a guy was having feeding issues with 6ARC out of grendel mags. Around that same time, the mag manufacturer (I believe it was Duramag) released a 6ARC-specific mag. When he compared them, the 6ARC mag presented the cartridge much higher and had a deeper cut in the front panel. That solved his feeding problem.

1000006821.jpg
 
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I have a 22" rifle +2 port barrel from LC. I have to say mine is under-pressured. It does not lock back on the bolt catch. Check GB alignment as stated. On mine I think my gas port is just too small. The GB self aligns being the barrel has a GB screw pilot dimple.

Anyone know of have info on gas port hole size for the 6mm ARC?
 
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Also, you could be over gassed. It's possible that your bolt is returning faster than the mag can get the next round in place.

Did you follow the directions on the gas block about the correct starting position and work your way up? You can outrun the magazine.
 
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One can run 18 rds per second or 1085 rds per minute without any malfunctioning from the AR 15 platform...the mag will keep up.
A shot timer, for gamers, was used to improve AR functioning speeds past normal full auto speeds.
The bolt speeds are very fast, infact hammer follow through can become a problem, as the trigger can't reset fast enough.
Getting the right amount of gas to the light bolt carrier was the key.
All solved with titanium bolt carrier, rifle length gas, on a 16.5" barrel, originally 20" rifle length Colt barrel, cut down and fluted HBar barrel, with the gas port drilled out to a giant .122" in dia..keep drilling until it locks back.
Fun for 300 blackout, also.
Everything else is carbine mil spec.
 
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Took a scope to it, looks aligned and I can clearly see the screw go in and back out.
 

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Hecouldgoalltheway said:
Also, you could be over gassed. It's possible that your bolt is returning faster than the mag can get the next round in place.

Did you follow the directions on the gas block about the correct starting position and work your way up? You can outrun the magazine.
Click to expand...
I had this exact problem! BCG was returning so fast that the bolt's lug would catch the next round half way on the brass, damaging the brass and failing to feed.

Proof 20" carbon, , CMC 6 ARC enhanced bcg, Armaspec H2 buffer and Superlative Gas block. I originally had a carbine buffer and buffer spring.
 
Last edited:
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What buffer and spring? What bcg? With the proof barrels I have a full mass bcg and standard buffer and spring cycles perfectly.
 
AznTactical said:
I had this exact problem! BCG was returning so fast that the bolt's lug would catch the next round half way on the brass, damaging the brass and failing to feed.

Proof 20" carbon, , CMC 6 ARC enhanced bcg, Armaspec H2 buffer and Superlative Gas block. I originally had a carbine buffer and buffer spring.
Click to expand...
I've had it too, but that's impossible according to @45-90, even though I've seen it 3 times myself..
 
I am running a arm spec heavy buffer slient capture and a Santan bolt carrier and a JP bolt, what about magazines potentially? and gas value is full
 
I’m not a newb to the platform but I’m not an expert, by any means. If it was me, I’d check the buffer, spring, and tube to make sure everything is the right length and then I would start over with the gas block turned off and a single round in the mag. I don’t think more gas is your solution.
 
Benhonda1 said:
I am running an arm spec heavy buffer slient capture and a Santan bolt carrier and a JP bolt, what about magazines potentially? and gas value is full
Click to expand...
Do the one round test. Post the results. That the gas block is aligned and fully open does not say that the rifle is getting enough gas to cycle. And, it could be getting so much gas that it is outrunning the mag. We currently have virtually nothing to go on, which is why you’ve gotten a bunch of questions, more guesses, and no solutions. Regardless, overgassed or undergassed is the first question. But, given the symptoms, it could be either. The test I suggested will check for under-gassing…
 
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hlee said:
Do the one round test. Post the results. That the gas block is aligned and fully open does not say that the rifle is getting enough gas to cycle. And, it could be getting so much gas that it is outrunning the mag. We currently have virtually nothing to go on, which is why you’ve gotten a bunch of questions, more guesses, and no solutions. Regardless, overgassed or undergassed is the first question. But, given the symptoms, it could be either. The test I suggested will check for under-gassing…
Click to expand...
Will do
 
Ok hide warriors, took the gun to the range, ran 1 round in magazine would not chamber next round. Took gas value to all way open nothing.... closes it wouldn't eject tried middle of road still won't feed next round, also tried acs mags and duramag
 
Benhonda1 said:
Ok hide warriors, took the gun to the range, ran 1 round in magazine would not chamber next round. Took gas value to all way open nothing.... closes it wouldn't eject tried middle of road still won't feed next round, also tried acs mags and duramag
Click to expand...
I'm having a hard time deciphering this.

You should have loaded one round into the magazine, inserted magazine into the rifle, chambered that one round, fired that one round. Your result should be bolt locked back on an empty mag.

Sounds like you didn't do this?

Can you pin the gas port hole to get a measurement?
Can you try a lighter spring setup?
Is it rifle length gas? +1? +2?

IMO it seems like you're under gassed for your setup, but it doesn't sound like you actually performed the test correctly to see if it would lock back.
 
godofthunder said:
I'm having a hard time deciphering this.

You should have loaded one round into the magazine, inserted magazine into the rifle, chambered that one round, fired that one round. Your result should be bolt locked back on an empty mag.

Sounds like you didn't do this?

Can you pin the gas port hole to get a measurement?
Can you try a lighter spring setup?
Is it rifle length gas? +1? +2?

IMO it seems like you're under gassed for your setup, but it doesn't sound like you actually performed the test correctly to see if it would lock back.
Click to expand...
 
Benhonda1 said:
No would not pick up next round, and when shooting one the bolt hold open would not stay open
Click to expand...

I'd agree with @godofthunder that the next step would be swapping in a lighter buffer/spring and see if you can get lock back with the same one-round test.

One other idea, I've heard of a few folks having issues with JP's one-piece gas ring, if that's what you have on your bolt. They seem to be ground slightly under-size in order to reduce friction. JP claims they seal just as well as standard rings, but I seem to recall a few users on here reporting gassing issues with them. Switching to a standard 3-piece setup could be another inexpensive tweak worth testing.

Outside of that, it might be time to look into re-sizing the gas port.
 
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Clint H said:
One other idea, I've heard of a few folks having issues with JP's one-piece gas ring, if that's what you have on your bolt. They seem to be ground slightly under-size in order to reduce friction. JP claims they seal just as well as standard rings, but I seem to recall a few users on here reporting gassing issues with them.
Click to expand...
That describes myself and quite a few acquaintances. Not sure why every JP part I've used has been top notch except the gas rings.
 
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Ernest 5.56 said:
I think a lot of people have out of spec bolt carriers

People also think standing the bolt carrier on end with low friction rings installed is a good diagnostic test.
Click to expand...
I'm sure some people do, but in my case and some of the people I know, we were using JP carriers. If a JP carrier won't work with JP rings, but that carrier works with any other rings, I'm just gonna toss the rings and not over think it beyond that.
The instructions also mention that the BCG stand up test isn't applicable to the one piece rings.
 
In process of building new AR15 6MM ARC. Looking complete for $2000 or less. Wanting suggestions on receivers, BCG and barrel! Building for long range target and hunting! TIA!
 
Rottyfan1 said:
In process of building new AR15 6MM ARC. Looking complete for $2000 or less. Wanting suggestions on receivers, BCG and barrel! Building for long range target and hunting! TIA!
Click to expand...

Personally, I'd just get this. If you look around, you can find them for even cheaper (~$1500), and it's pretty hard to build something with similar quality for that price.

www.eurooptic.com

Seekins Precision DMR 6mm ARC 18" 1:8" 5/8-24 TPI Bbl Black Rifle 0011300117-F-BLK

Seekins Precision DMR 6mm ARC 18" 1:8" 5/8-24 TPI Bbl Black Rifle 0011300117-F-BLK For Sale
www.eurooptic.com www.eurooptic.com
 
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Had the same issue on a +2 barrel. Gas block 100% open, would not lock back on empty mag. Drilled gas port per manufactures recommendations. Now works after adjusting with ejection @ 3 o'clock & lock back on last round.
 
Clint H said:
Personally, I'd just get this. If you look around, you can find them for even cheaper (~$1500), and it's pretty hard to build something with similar quality for that price.

www.eurooptic.com

Seekins Precision DMR 6mm ARC 18" 1:8" 5/8-24 TPI Bbl Black Rifle 0011300117-F-BLK

Seekins Precision DMR 6mm ARC 18" 1:8" 5/8-24 TPI Bbl Black Rifle 0011300117-F-BLK For Sale
www.eurooptic.com www.eurooptic.com
Click to expand...
Correct me if I’m wrong but it’s my understanding that the 1:8 twist limits the rifle to a lighter bullet range of 105 or less and I’d prefer to be able and shoot 108gn to 110gn or heavier. I’ll be hand loading so I’m sure I could play around with the 1:8 twist but just don’t want to be limited.
 
Rottyfan1 said:
Correct me if I’m wrong but it’s my understanding that the 1:8 twist limits the rifle to a lighter bullet range of 105 or less and I’d prefer to be able and shoot 108gn to 110gn or heavier. I’ll be hand loading so I’m sure I could play around with the 1:8 twist but just don’t want to be limited.
Click to expand...
Not to mention I just enjoy putting these rifles together and watching them come to life!
 
Rottyfan1 said:
Correct me if I’m wrong but it’s my understanding that the 1:8 twist limits the rifle to a lighter bullet range of 105 or less and I’d prefer to be able and shoot 108gn to 110gn or heavier. I’ll be hand loading so I’m sure I could play around with the 1:8 twist but just don’t want to be limited.
Click to expand...
I've got a 1:7.7 that does 115gr bullets fine. Not sure exactly where the cutoff is for the .3" slower twist of a 1:8.
 
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Rottyfan1 said:
Correct me if I’m wrong but it’s my understanding that the 1:8 twist limits the rifle to a lighter bullet range of 105 or less and I’d prefer to be able and shoot 108gn to 110gn or heavier. I’ll be hand loading so I’m sure I could play around with the 1:8 twist but just don’t want to be limited.
Click to expand...

I think Hornady designed it around a 1:7.5 twist. Not quite sure why Seekins chose to go with the 1:8.

I've read 103-108gr is kinda the sweet spot for 6ARC in gas gun because the 110-115gr get a little long for max COAL in standard mags. I've only played around with 105gr, so not sure the truth of that.

You can play around with the Berger twist rate Calc if you're interested. I just did a quick comparison of 108gr at 1:8 and 1:7.5 and they both fell in the stable range.

bergerbullets.com

Twist Rate Stability Calculator

Our newly updated Twist Rate Stability Calculator not only tells you what your SG is, but it also lets you know if you are reaching your optimized BC.
bergerbullets.com bergerbullets.com

Here's 108gr @ 1:8
Screenshot_20240309_085142_Chrome.jpg



And here's 108gr @ 1:7.5
Screenshot_20240309_085203_Chrome.jpg
 
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Clint H said:
I think Hornady designed it around a 1:7.5 twist. Not quite sure why Seekins chose to go with the 1:8.

I've read 103-108gr is kinda the sweet spot for 6ARC in gas gun because the 110-115gr get a little long for max COAL in standard mags. I've only played around with 105gr, so not sure the truth of that.

You can play around with the Berger twist rate Calc if you're interested. I just did a quick comparison of 108gr at 1:8 and 1:7.5 and they both fell in the stable range.

bergerbullets.com

Twist Rate Stability Calculator

Our newly updated Twist Rate Stability Calculator not only tells you what your SG is, but it also lets you know if you are reaching your optimized BC.
bergerbullets.com bergerbullets.com

Here's 108gr @ 1:8
View attachment 8368543


And here's 108gr @ 1:7.5
View attachment 8368544
Click to expand...
Yeah I’ve recently been introduced to this calculator. I think for the most part it comes down to the barrel and velocity. I haven’t purchased the barrel yet but was looking into hart and x caliber. Also seen a few others out there but haven’t researched them much just yet. My thinking is build the rifle around the barrel! BCG and optics being next thing to consider!
 
Rottyfan1 said:
Yeah I’ve recently been introduced to this calculator. I think for the most part it comes down to the barrel and velocity. I haven’t purchased the barrel yet but was looking into hart and x caliber. Also seen a few others out there but haven’t researched them much just yet. My thinking is build the rifle around the barrel! BCG and optics being next thing to consider!
Click to expand...

I don't have much experience with either of those barrel manufacturers, but as a general building philosophy, I agree on tailoring around the barrel.

I've been patiently waiting to try out one of these bolts, but they've been "coming soon" for almost a year.

Young Manufacturing - 6mm ARC Chrome (HMB) Bolt # 1-50-12-307

# 1-50-12-307 - 6mm ARC Chrome (HMB) Bolt
www.youngmanufacturing.net www.youngmanufacturing.net

Otherwise, RCA or JP have always treated me right (aside from the aforementioned JP gas rings). Wouldn't hesitate getting one of them again for the BCG.
 
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Rottyfan1 said:
In process of building new AR15 6MM ARC. Looking complete for $2000 or less. Wanting suggestions on receivers, BCG and barrel! Building for long range target and hunting! TIA!
Click to expand...
JP bolt and carrier they are just that nice, BCM MK2 upper, Vltor, Iron City receivers. A upper with extra material around barrel extension and a thermal fit is what you want. I like my Iron City with those features and a little larger ejection port because the 6ARC is bigger plus it’s as tight upper/lower fitment as my JP billet for a lot cheaper. Talk to Mike at http://www.dtechuppers.com/ar-15-upper-receivers.html Yotesniper on here has one of his uppers that drives tacks and runs perfect. My parts are getting sent to him soon for him to finish up with a Shilen select barrel. Compass Lake does excellent work you can send parts to them as well the Douglas and Criterion barrels I’ve gotten from them have been very good. Later if you love the round throw a Krieger or Bartlein in it. Especially if you’re looking for accuracy Shilen, Douglas or Criterion should be your minimum quality. Post pics, your experience and combo when it’s running summer is coming!
Cheers
 
Westcoast308 said:
JP bolt and carrier they are just that nice, BCM MK2 upper, Vltor, Iron City receivers. A upper with extra material around barrel extension and a thermal fit is what you want. I like my Iron City with those features and a little larger ejection port because the 6ARC is bigger plus it’s as tight upper/lower fitment as my JP billet for a lot cheaper. Talk to Mike at http://www.dtechuppers.com/ar-15-upper-receivers.html Yotesniper on here has one of his uppers that drives tacks and runs perfect. My parts are getting sent to him soon for him to finish up with a Shilen select barrel. Compass Lake does excellent work you can send parts to them as well the Douglas and Criterion barrels I’ve gotten from them have been very good. Later if you love the round throw a Krieger or Bartlein in it. Especially if you’re looking for accuracy Shilen, Douglas or Criterion should be your minimum quality. Post pics, your experience and combo when it’s running summer is coming!
Cheers
Click to expand...
Thanks for the pointers. I will check out theses parts you mentioned. Currently I’ve been pondering the Aero upper and lowers with their BCG and either a Hart or X caliber for the barrel but no definite decision just yet. Definitely going to consider all options. Since joining this forum, I have learned of a ton of different manufactures and quality so I am putting my build on slow Mo to be sure I make the best decisions in terms of quality and price. I have built a few other AR’s in the past. But just use the normal run-of-the-mill equipment this time I am wanting to build a top quality rifle, so definitely not going to jump into anything to quick.
 
Rottyfan1 said:
Thanks for the pointers. I will check out theses parts you mentioned. Currently I’ve been pondering the Aero upper and lowers with their BCG and either a Hart or X caliber for the barrel but no definite decision just yet. Definitely going to consider all options. Since joining this forum, I have learned of a ton of different manufactures and quality so I am putting my build on slow Mo to be sure I make the best decisions in terms of quality and price. I have built a few other AR’s in the past. But just use the normal run-of-the-mill equipment this time I am wanting to build a top quality rifle, so definitely not going to jump into anything to quick.
Click to expand...
Also, could you elaborate on the extra material? On barrel, extension and thermal fit I am not familiar with what that does, and how it affects performance.
 
Westcoast308 said:
JP bolt and carrier they are just that nice, BCM MK2 upper, Vltor, Iron City receivers. A upper with extra material around barrel extension and a thermal fit is what you want. I like my Iron City with those features and a little larger ejection port because the 6ARC is bigger plus it’s as tight upper/lower fitment as my JP billet for a lot cheaper. Talk to Mike at http://www.dtechuppers.com/ar-15-upper-receivers.html Yotesniper on here has one of his uppers that drives tacks and runs perfect. My parts are getting sent to him soon for him to finish up with a Shilen select barrel. Compass Lake does excellent work you can send parts to them as well the Douglas and Criterion barrels I’ve gotten from them have been very good. Later if you love the round throw a Krieger or Bartlein in it. Especially if you’re looking for accuracy Shilen, Douglas or Criterion should be your minimum quality. Post pics, your experience and combo when it’s running summer is coming!
Cheers
Click to expand...
What twist are you going with from dtech ? Them 4 groove ,that I really like, are only available in an 8 twist....has me wondering about 105 - 108s at 6arc speed,if they'll stabilize ok.
I have a 22 Nosler from them that as you said,drive tacks. Highly recommend dtech ,if one doesn't want to build an upper themself.
 
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WasNH4X said:
What twist are you going with from dtech ? Them 4 groove ,that I really like, are only available in an 8 twist....has me wondering about 105 - 108s at 6arc speed,if they'll stabilize ok.
I have a 22 Nosler from them that as you said,drive tacks. Highly recommend dtech ,if one doesn't want to build an upper themself.
Click to expand...
5 groove 7 twist is what I’m thinking
 
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