6mm ARC headspace

[Minshooter]

Hoping someone with a 6mm ARC "GO" gauge can compare it with factory rounds.
Most other calibers of factory ammo seem to run 2-3 thousandths under a "go" gauge.
I have the Hornady 103 ELD-x, 105 Black BTHP, and 108 ELD-Match factory ammo,
but am having some chamber issues.
Just looking for a reference point in thousandths, compared to a "Go" gauge.
Thanks much, in advance.

 

[EOD Rob]

Hoping someone with a 6mm ARC "GO" gauge can compare it with factory rounds.
Most other calibers of factory ammo seem to run 2-3 thousandths under a "go" gauge.
I have the Hornady 103 ELD-x, 105 Black BTHP, and 108 ELD-Match factory ammo,
but am having some chamber issues.
Just looking for a reference point in thousandths, compared to a "Go" gauge.
Thanks much, in advance.

SH@%! I have exactly what you need, but in the middle of a move and everything is packed up and 10hrs away! Sorry!

 

[Minshooter]

SH@%! I have exactly what you need, but in the middle of a move and everything is packed up and 10hrs away! Sorry!

Thanks very much for replying.
If and whenever it could become possible, it will be a great help.

 

[Herzo]

I don't know if you are working with an AR? If so there are 2 bolts available with the Grendel family of cartridges,Type I & Type II. A type I bolt with a Type II chamber should headspace ~0.010" shorter than the SAAMI specification, whereas a Type II bolt with a Type I chamber could headspace ~0.010" longer. The Type I bolt is effectively the same as a 7.62 X 39 for the AR.

 

[David Milisock]

If memory serves the 6MM ARC uses a type 2 bolt.

 

[Minshooter]

Yes, the standard bolt for 6 ARC is the .136 type 2.
My main issue is an Odin barrel that has factory ammo stretching .014-.016".
Odin was very good about returning the barrel and (their) bolt for a check.
They sent it back exactly as shipped, saying that it was within spec.
Maybe I just have all the tolerances stacked in this case.
I've narrowed the gap by sizing 6.5 Grendel brass to within .005" of the fired 6 ARC brass.

 

[Herzo]

It sounds to me as though the Odin barrel was chambered to a Type I bolt.

 

[David Milisock]

It sounds to me as though the Odin barrel was chambered to a Type I bolt.

This is why I will only buy completed uppers built to SAAMI specifications. There's only 1 SAAMI specification for 6MM ARC and I don't need any half assed BS.

 

[Minshooter]

This is why I will only buy completed uppers built to SAAMI specifications. There's only 1 SAAMI specification for 6MM ARC and I don't need any half assed BS.

Their .136 bolt was packaged with their barrel. gas tube, and block, as a complete assembly by Odin.
When I sent the barrel and bolt assembly to Odin for inspection, they returned the same barrel, with
a different bolt that measured exactly the same as the original bolt. Talked to them on the phone.
They insist that this is correct, and that my assembly gauges okay, and is made for their type 2 bolt.
Maybe it is ok, if .015" stretch from factory ammo is ok.
I can work with it by sizing all brass .010" longer, but I don't have to like it.
(I know it could go tighter in a bolt gun, but this is the dirtiest cartridge I've ever worked with.
Factory and my Lever/CFE223 loads all leave the fired brass looking like it was in a charcoal grill.
So, I'm leaving at least .005" clearance in this AR).

 

[Herzo]

Their .136 bolt was packaged with their barrel. gas tube, and block, as a complete assembly by Odin.
When I sent the barrel and bolt assembly to Odin for inspection, they returned the same barrel, with
a different bolt that measured exactly the same as the original bolt. Talked to them on the phone.
They insist that this is correct, and that my assembly gauges okay, and is made for their type 2 bolt.
Maybe it is ok, if .015" stretch from factory ammo is ok.
I can work with it by sizing all brass .010" longer, but I don't have to like it.
(I know it could go tighter in a bolt gun, but this is the dirtiest cartridge I've ever worked with.
Factory and my Lever/CFE223 loads all leave the fired brass looking like it was in a charcoal grill.
So, I'm leaving at least .005" clearance in this AR).

A Type 1 bolt (7.62 x 39) should tighten the headspace. The logic that the type 2 was stronger never made any sense to me in that the only apparent difference was that the boltface was machined 0.010" deeper???? Other than that and needing a longer firing pin, they are dimensionally the same.

In looking at the SAAMI drawing for the 6mm ARC, the headspace for the chamber is given as 1.1901/1.2001" and the cartridge is 1.190/1.183". The barrel could indeed be in spec at the high limit (1.2001") and the cartridge at or near the low (1.183") giving 0.017" for the case to stretch -- this would correspond to the op`s situation.

 

[David Milisock]

Their .136 bolt was packaged with their barrel. gas tube, and block, as a complete assembly by Odin.
When I sent the barrel and bolt assembly to Odin for inspection, they returned the same barrel, with
a different bolt that measured exactly the same as the original bolt. Talked to them on the phone.
They insist that this is correct, and that my assembly gauges okay, and is made for their type 2 bolt.
Maybe it is ok, if .015" stretch from factory ammo is ok.
I can work with it by sizing all brass .010" longer, but I don't have to like it.
(I know it could go tighter in a bolt gun, but this is the dirtiest cartridge I've ever worked with.
Factory and my Lever/CFE223 loads all leave the fired brass looking like it was in a charcoal grill.
So, I'm leaving at least .005" clearance in this AR).

Your brass shouldn't be doing that. My brass is clean and fired brass should not look like soot.

 

[gunsandgunsmithing]

Your brass shouldn't be doing that. My brass is clean and fired brass should not look like soot.

Are you saying that .010 of headspace...maybe... is why you believe you have sooted brass? I strongly doubt that is the case with good brass. We don't get that fireforming .100 short brx or dasher brass. I believe you need to at least reconsider your seemingly strong opinion. That's all I have to say on this.

 

[Minshooter]

Are you saying that .010 of headspace...maybe... is why you believe you have sooted brass? I strongly doubt that is the case with good brass. We don't get that fireforming .100 short brx or dasher brass. I believe you need to at least reconsider your seemingly strong opinion. That's all I have to say on this.

Thanks for everyone's replies.
Hornady, Lapua, and Federal (all -6.5 Grendel resized to 6 ARC, but .010" longer than factory) brass
-all come out equally dirty. So does 3 types of Hornady Factory 6mm Arc ammo.

A type 1 (7.62x39 .125") bolt in this rifle will let me just push it closed (with moderate thumb pressure)
on an empty case sized to minimum in a Hornady die -which gets it about .0015" below the factory ammo,
and that bolt can be forced to close with hand pressure on the factory ammo, in a clean lubed chamber.
I suspect the factory might be actually getting sized back, but brass springback cancels any measurement.
I might run like that in a bolt gun for first firing, but that bolt would be way too tight to run in an AR.

 

[gunsandgunsmithing]

Thanks for everyone's replies.
Hornady, Lapua, and Federal (all -6.5 Grendel resized to 6 ARC, but .010" longer than factory) brass
-all come out equally dirty. So does 3 types of Hornady Factory 6mm Arc ammo.

A type 1 (7.62x39 .125") bolt in this rifle will let me just push it closed (with moderate thumb pressure)
on an empty case sized to minimum in a Hornady die -which gets it about .0015" below the factory ammo,
and that bolt can be forced to close with hand pressure on the factory ammo, in a clean lubed chamber.
I suspect the factory might be actually getting sized back, but brass springback cancels any measurement.
I might run like that in a bolt gun for first firing, but that bolt would be way too tight to run in an AR.

Could it simply be low pressure or that the bolt is opening a tad early? An adjustable gas block may help but if you're not a the top end load wise, I'd see if it still does it if you bump the load up a little.

 

[Minshooter]

Shooting various 58, 60, 80, 85, 90, 100 grain Hornady and Sierra's.
The loads are at or within .5 grain of the top on Hornady's Data, with CFE and Lever.
450, #41, and 7-1/2 primers all look normal -no flattening.
This Odin barrel is their 18" DMR, with their XL gas tube, so only about 4" of dwell time.
The Odin tuneable gas block is set to just reliably eject with a Thunderbeast 6.5 Ultra 7.
Brass is ejected at around 5 O'Clock.
As mentioned in first post, Factory loads of Hornady 103, 105, and 108 brass also come out dirty.
Trying to find someone locally who has a 6 ARC, to do some comparisons, but so far no luck.
My Odin .223 barrel (set up the same for gas & can) has clean brass and only about .003" stretch.
An unknown make (but now Odin) 6.5 Grendel, set up similar to the 6 ARC -never had any problems,
and fired brass looks about normal clean -for suppressed.

 

[gunsandgunsmithing]

Shooting various 58, 60, 80, 85, 90, 100 grain Hornady and Sierra's.
The loads are at or within .5 grain of the top on Hornady's Data, with CFE and Lever.
450, #41, and 7-1/2 primers all look normal -no flattening.
This Odin barrel is their 18" DMR, with their XL gas tube, so only about 4" of dwell time.
The Odin tuneable gas block is set to just reliably eject with a Thunderbeast 6.5 Ultra 7.
Brass is ejected at around 5 O'Clock.
As mentioned in first post, Factory loads of Hornady 103, 105, and 108 brass also come out dirty.
Trying to find someone locally who has a 6 ARC, to do some comparisons, but so far no luck.
My Odin .223 barrel (set up the same for gas & can) has clean brass and only about .003" stretch.
An unknown make (but now Odin) 6.5 Grendel, set up similar to the 6 ARC -never had any problems,
and fired brass looks about normal clean -for suppressed.

I just doubt it's a brass or sizing issue and think you'll find joy in solving it somewhere else. If you're near top end pressures, I think it'll be in timing, ie bolt opening a tad early. FWIW, I've experienced some pretty remarkable lot to lot differences in powder. You may still be low on pressure, regardless of the book data. A chrono might be of some help in determining that.

 

[Jon08089]

Hoping someone with a 6mm ARC "GO" gauge can compare it with factory rounds.
Most other calibers of factory ammo seem to run 2-3 thousandths under a "go" gauge.
I have the Hornady 103 ELD-x, 105 Black BTHP, and 108 ELD-Match factory ammo,
but am having some chamber issues.
Just looking for a reference point in thousandths, compared to a "Go" gauge.
Thanks much, in advance.

Clymer 6mm ARC Go
Using Wilson 6mm ARC Cartridge Gage and Wilson Micrometer

Go reads 0, max saami as expected

Hornady Black 105gr measure tad over -5, so little under saami minimum

Hornady Match 108gr measure right at -1, so on the high end, I would bet the 103s would be the same

 

[Minshooter]

Could it simply be low pressure or that the bolt is opening a tad early? An adjustable gas block may help but if you're not a the top end load wise, I'd see if it still does it if you bump the load up a little.

Clymer 6mm ARC Go
Using Wilson 6mm ARC Cartridge Gage and Wilson Micrometer

Go reads 0, max saami as expected

Hornady Black 105gr measure tad over -5, so little under saami minimum

Hornady Match 108gr measure right at -1, so on the high end, I would bet the 103s would be the same

Thanks very much.
I will get some measurements and post. Getting an exact .350 diameter gauge.
Already to the top of Hodgdon and many other other posted tests. Might try slightly higher.
18" bbl with only 4 inches of dwell length on a longer than rifle gas tube; gas turned down to just function.
Am going to turn the gas block completely off, and see what happens to the brass.

 

[gunsandgunsmithing]

Thanks very much.
I will get some measurements and post. Getting an exact .350 diameter gauge.
Already to the top of Hodgdon and many other other posted tests. Might try slightly higher.
18" bbl with only 4 inches of dwell length on a longer than rifle gas tube; gas turned down to just function.
Am going to turn the gas block completely off, and see what happens to the brass.

Well, that might answer some questions and it's not difficult at all. I'm a little surprised that you haven't already tried. What's the worst that can happen? You've spent way more time justifying why not to do it than it will take to find out, already.

 

[Minshooter]

Well, that might answer some questions and it's not difficult at all. I'm a little surprised that you haven't already tried. What's the worst that can happen? You've spent way more time justifying why not to do it than it will take to find out, already.

Absolutely right. Should have tried that sooner.
Just plain wanted to use it as the AR it's supposed to be, so stayed only on that track.
Will report what I find after a range trip. Will be a couple days.