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6mm ARC headspace

I am shooting 6arc in an ar platform. Craddock Precision 22” bartlein barrel with a +2 gas system, shooting suppressed. Factory Hornady ammo, 105gn- 2640fps and 108gn- 2550fps. Slower than I had hoped for. Both rounds show soot on the cases, and in the mags. Shooting suppressed= dirty gun. I don’t believe there is a fix for this in a semi auto.
(flame suit on.)
 
I am shooting 6arc in an ar platform. Craddock Precision 22” bartlein barrel with a +2 gas system, shooting suppressed. Factory Hornady ammo, 105gn- 2640fps and 108gn- 2550fps. Slower than I had hoped for. Both rounds show soot on the cases, and in the mags. Shooting suppressed= dirty gun. I don’t believe there is a fix for this in a semi auto.
(flame suit on.)
With a suppressor I'm not surprised about the soot.

With a 22" AR that velocity sounds about right, test barrels are 24", that should give a + 50 FPS. Depending on freebore and other factors it's not far off base.
 
Having listened to the smarter people on the board, I did test WITH the gas completely off:
(Drum roll...) Zero soot. looks like typical bolt gun brass.
Fired cases extracted as easily as unfired rounds, using only the charging handle.
My CFE and Lever loads, and Hornady Factory. All the same as far as zero soot.
Who would have thought? Still exactly the same amount of case stretch as reported earlier.
Did not have time to chronograph with the gas system shut off, but:

Here are results (with gas system operating) from Hornady Factory ammo in the
Odin DMR 18" with +2" gas tube, TBAC 6.5 Ultra 7, 34 deg. F, 5 shots each
103 ELD-X 2579/50es/19sd
105 BLACK 2558/54es/18sd
108 ELD-M 2536/45es/15sd

Surprised at how close the 108 gr in an 18"compares to the 22" barrel listed above.
 
I have early made Odin 18" 6mm ARC barrel, but no suppressor, as I live in a shit hole state. I had to open the gas port to #36 from size #38, doing that has really let it run any ammo w/o issue, and its tamed by an adjustable gas block. The cases were no more sooty than other ARs I run.

Went thru the same mental gymnastics the OP went thru on headspace. The factory Hornady ELD-M and Black BTHP were stretching 11 to 14 thou. after first firing. As someone else mentioned earlier, if the factory cases are toward MINIMUM SAAMI specs, and a CHAMBER is on the longer side of SAAMI specs, it can really stretch the cases out from their initial factory size.

My PTG CHAMBER Go gauge in Wilson CASE gauge is right at min. CHAMBER spec, the NO-Go gauge is +.005 over the Go. Most factory Hornady ammo is -.003 to -.005 below SAAMI max CASE spec. My bolt/odin closes on both PTG Go and No-Go gauges, and adding single layer of Scotch tape to No-Go, will close on it with a really hard thumb press, but a field gauge will not. I'm guessing the chamber is 7 to 8 thou longer than SAAMI min chamber.

PTG actually has some good info, about factory AR chamber headspace, that can be found here.
https://pacifictoolandgauge.com/content/19-ptg-headspace-gauge-info
"I have a lot of customers calling me about their hand guns or gas guns closing on the No-Go, such as an AR-15 closing on a standard No-Go gauge. Remember most gas gun manufacturers run their headspace a little longer for rapid firing and to have the ability to perform with a dirty chamber.

If a precision gunsmith chambers a bolt action barrel or an AR for you, he will most likely ream until the bolt just closes on the Go, but a +.005 No-Go should work as a No-Go. Remember, most manufacturers chamber their AR-15’s and other gas guns to +.002 for Go from SAAMI Min to +.007 from SAAMI Min for their production No-Go. I always check my gas guns with a military 11 piece incremental set of gauges to find out the true depths of the headspace. "
 
Having listened to the smarter people on the board, I did test WITH the gas completely off:
(Drum roll...) Zero soot. looks like typical bolt gun brass.
Fired cases extracted as easily as unfired rounds, using only the charging handle.
My CFE and Lever loads, and Hornady Factory. All the same as far as zero soot.
Who would have thought? Still exactly the same amount of case stretch as reported earlier.
Did not have time to chronograph with the gas system shut off, but:

Here are results (with gas system operating) from Hornady Factory ammo in the
Odin DMR 18" with +2" gas tube, TBAC 6.5 Ultra 7, 34 deg. F, 5 shots each
103 ELD-X 2579/50es/19sd
105 BLACK 2558/54es/18sd
108 ELD-M 2536/45es/15sd

Surprised at how close the 108 gr in an 18"compares to the 22" barrel listed above.
Just thinking out loud, does the gas system fit up right?
 
Just thinking out loud, does the gas system fit up right?
When I did the no-gas test, I used an inexpensive fixed block, turned 90 degrees to totally block the port.
That $12 bargain block sealed tighter than the block Odin shipped with their barrel.
I was quite disappointed in the Odin supplied "tuneable" gas block. It's ID is .7505"
The barrel gas journal is .7495" You can feel the slight wiggle in the parts before tightening the setscrews.
I get more soot leaking out around the gas block front than I have had with most of the cheap blocks.
Odin does make a clamp on fully adjustable gas block that would probably solve that, but at $99, I'd be inclined to go to the version from SLR.

My next project is how to delay bolt opening while using a rifle +2 gas tube on an 18" barrel.
 
I have early made Odin 18" 6mm ARC barrel, but no suppressor, as I live in a shit hole state. I had to open the gas port to #36 from size #38, doing that has really let it run any ammo w/o issue, and its tamed by an adjustable gas block. The cases were no more sooty than other ARs I run.

Went thru the same mental gymnastics the OP went thru on headspace. The factory Hornady ELD-M and Black BTHP were stretching 11 to 14 thou. after first firing. As someone else mentioned earlier, if the factory cases are toward MINIMUM SAAMI specs, and a CHAMBER is on the longer side of SAAMI specs, it can really stretch the cases out from their initial factory size.

My PTG CHAMBER Go gauge in Wilson CASE gauge is right at min. CHAMBER spec, the NO-Go gauge is +.005 over the Go. Most factory Hornady ammo is -.003 to -.005 below SAAMI max CASE spec. My bolt/odin closes on both PTG Go and No-Go gauges, and adding single layer of Scotch tape to No-Go, will close on it with a really hard thumb press, but a field gauge will not. I'm guessing the chamber is 7 to 8 thou longer than SAAMI min chamber.

PTG actually has some good info, about factory AR chamber headspace, that can be found here.
https://pacifictoolandgauge.com/content/19-ptg-headspace-gauge-info
"I have a lot of customers calling me about their hand guns or gas guns closing on the No-Go, such as an AR-15 closing on a standard No-Go gauge. Remember most gas gun manufacturers run their headspace a little longer for rapid firing and to have the ability to perform with a dirty chamber.

If a precision gunsmith chambers a bolt action barrel or an AR for you, he will most likely ream until the bolt just closes on the Go, but a +.005 No-Go should work as a No-Go. Remember, most manufacturers chamber their AR-15’s and other gas guns to +.002 for Go from SAAMI Min to +.007 from SAAMI Min for their production No-Go. I always check my gas guns with a military 11 piece incremental set of gauges to find out the true depths of the headspace. "
Very useful information.
Thanks for posting.
 
When I first got my 6ARC, I bought some O.F. brass from a company that purchases it from companies test firing completed rifles. I buy from them because it is truly once fired, not range pick up brass. At least 25% of the brass had a visible stretch bulge so bad they could not be reloaded. Many of the other cases had the halo about 1/4" up the case from the rim, but not so bad. I sent the pictures to Hornady, and they said the brass was toast from being fired in an improperly headspaced rifle. I found more O.F. brass later, and I noticed more of the telltale "ring around the collar". Is that what you are seeing?

My fired brass looks good, but I have not measured it. I am still firing factory rounds just to get brass for reloads.
 
Having listened to the smarter people on the board, I did test WITH the gas completely off:
(Drum roll...) Zero soot. looks like typical bolt gun brass.
Fired cases extracted as easily as unfired rounds, using only the charging handle.
My CFE and Lever loads, and Hornady Factory. All the same as far as zero soot.
Who would have thought? Still exactly the same amount of case stretch as reported earlier.
Did not have time to chronograph with the gas system shut off, but:

Here are results (with gas system operating) from Hornady Factory ammo in the
Odin DMR 18" with +2" gas tube, TBAC 6.5 Ultra 7, 34 deg. F, 5 shots each
103 ELD-X 2579/50es/19sd
105 BLACK 2558/54es/18sd
108 ELD-M 2536/45es/15sd

Surprised at how close the 108 gr in an 18"compares to the 22" barrel listed above.

Perhaps I overlooked if this was mentioned, Are you using a silencer when firing this rifle ? If I understand correctly, the sooty cases are par for the course when firing any AR type rifle when utilizing the moderator.
 
1. No collar rings. Brass has held up remarkably well.
2. TBAC = Thunderbeast Arms Corp. 6.5 (Caliber) Ultra 7 (silencer).
3. All of my .223/5/56's, and a 6.5 Grendel all with the Ultra 7 suppressor, never had any noticeable soot.
All of these have the gas system tuned for suppressed use. 11-1/2, 16, 18, and 20 inch barrels
I thought the 6 ARC was tuned , too, but I clearly was wrong about that.

Thanks again to everyone for all your help.
Now I need to find a way to delay the 6 ARC bolt opening, with a rifle length spring and buffer.

3/26 UPDATE: Range today; Without a suppressor, slowly opened the gas until it started to function
with the standard rifle spring and buffer, stopped when the bolt just locked open.
Surprise: No soot. All is well. I must have misjudged when initially setting the tuneable gas block.
Will do more tests and report anything worthwhile.
 
Last edited:
1. No collar rings. Brass has held up remarkably well.
2. TBAC = Thunderbeast Arms Corp. 6.5 (Caliber) Ultra 7 (silencer).
3. All of my .223/5/56's, and a 6.5 Grendel all with the Ultra 7 suppressor, never had any noticeable soot.
All of these have the gas system tuned for suppressed use. 11-1/2, 16, 18, and 20 inch barrels
I thought the 6 ARC was tuned , too, but I clearly was wrong about that.

Thanks again to everyone for all your help.
Now I need to find a way to delay the 6 ARC bolt opening, with a rifle length spring and buffer.
I don't believe this guy was using a suppressor, and it seems to go against convention, but he has rifle length extension, and was able to turn down the gas a ton, worth a read...sorry if links to other forums aren't permitted, then remove it and I'll PM
 
I don't believe this guy was using a suppressor, and it seems to go against convention, but he has rifle length extension, and was able to turn down the gas a ton, worth a read...sorry if links to other forums aren't permitted, then remove it and I'll PM
Thanks for the link. 49 pages of 6 ARC discussion!
 

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