6 ARC headspace

[memilanuk]

So... got an 18" 6 ARC barrel with a rifle gas system, and type II bolt a while back, and finally got around to shooting it not too long ago. It's a new cartridge for me, so I wasn't sure what to expect. I've been using the L.E. Wilson case gauges, along with their case gauge depth micrometer in other cartridges with great results... so I was a little shocked when I dropped a fired case in the gauge and it visibly stood proud of the top Ran the depth mic down, and it showed it at +11 over '0'. Plunking an unfired factory round in there showed as -3 or so. That range seems... excessive, to me.

Like most comparators, that thing is still a 'relative' measurement. I dug out the appropriate shoulder angle insert for my Sinclair headspace comparators, mounted them on the Mitutoyo calipers, and zeroed things out. Loaded round measured 1.128", fired cases measured 1.141" or so. Yikes.

Coughed up the $$$ for a set of go, no-go and *field* gauges in 6 ARC. When they finally arrived, I got the following measurements:

Loaded round: 1.128"
Go gauge: 1.130"
No-go gauge: 1.134"
Field gauges: 1.138"
Fired brass: 1.141"

Then, I went and pulled the upper apart, popped out the ejector pin from the bolt, cleaned the bolt and chamber (never seen that much brass buildup in *either* the bolt or the chamber of my .223 ARs), and found that the bolt would rotate closed on the go gauge (as it should)... but it also closed on the no-go gauge, and damn near closed on the field gauge.

I double checked the depth of the bolt face; it is a type 2 (0.136" deep) bolt.

I'm guessing this isn't supposed to be the 'normal' amount of shoulder stretch on firing factory ammo in a 6 ARC barrel?

Any ideas what might be the likely culprit here? Chamber set up for a type 1 bolt (0.125" deep) instead of a type 2? Seems like that would almost account for the difference I'm seeing here. Or more likely the chamber is just f$cked up, some other way?

The fired brass coming out measuring longer than the chamber is another one that had me going 'WTF?!?' Someone else suggested that it might point towards a gas system issue, with the bolt trying to unlock before the pressure has dropped enough for the case shoulder to release from the chamber walls.

 

[buyobuyo]

Sounds like you have it figured out. Either the camber was cut for a Type 1 bolt or cut too deep. I'd fix the headspace issue then worry about the brass stretching. Given your numbers, it sounds like it's only stretching a few thousandth over headspace.

 

[memilanuk]

I'd fix the headspace issue

Any suggestions on that? I sat on this barrel for about 6-7 months before shooting it and finding this. Not sure whether the retailer or the maker will take it back. Not sure I want to mess around with a type 1 bolt. But I don't see any other alternatives...?

 

[buyobuyo]

Any suggestions on that? I sat on this barrel for about 6-7 months before shooting it and finding this. Not sure whether the retailer or the maker will take it back. Not sure I want to mess around with a type 1 bolt. But I don't see any other alternatives...?

Who made the barrel? I'd go straight to them as it sounds like it was chambered incorrectly. Most barrel makers have a decent warranty policy.

 

[memilanuk]

Ballistic Advantage, purchased through Primary Arms.

 

[memilanuk]

Well, I'm (way) outside the return window for Primary Arms. Now waiting on an email response from Ballistic Advantage

 

[Taylorbok]

Difference between a type 1 and 2 bolt is .012"
You've got about .013 of stretch so you might get away with that but IMO that chamber was cut wrong since a Go gauge is 1.130 the bolt probably won't close on that. so if the ammo was at actual spec you are gonna have a crush fit and that's not gonna run to nice in a gas gun.

 

[LRRPF52]

Sounds almost like it’s a 6mm AR chamber, but I see a lot of shoulder movement with Grendel from virgin brass and factory ammo.

 

[WeR0206]

Well, I'm (way) outside the return window for Primary Arms. Now waiting on an email response from Ballistic Advantage

What ended up happening with this barrel return and did you ever figure out how/why your fired brass was longer than the chamber?

 

[memilanuk]

After some back and forth with their support desk, the situation is that they are adamant that the chamber is as it should be and the problem is either a) I lack the ability to measure correctly, b) my PTG headspace gauge(s) are short / out of spec, c) my Rubber City bolt is out of spec. I have a hard time swallowing any one of the three, but especially that both b) and c) would happen concurrently. Is it possible? Sure, anything is possible (other than their barrel being less than perfect, apparently)... I just think the likelihood of *both* my NO-GO gauge *and* a quality name-brand bolt being out of spec is somewhere in the realm of "not fucking very".

 

[msgriff]

The depth of the bolt face by itself doesn't tell you much when it comes to headspace. It will tell you if you have a type 1 or type 2 bolt, though. The critical dimension of the bolt concerning headspace is measured from the rear of the bolt lugs to the bolt face, and that is the relevant dimension spec'd on the M16/M4 technical drawings. It eliminates the allowable tolerance of the bolt lug length. I believe that type 1 bolts were based on that spec, I don't know for type 2. You could obviously measure the lug length and subtract bolt face depth if you knew the spec for a type 2. That said, I doubt that the RCA bolt is out of spec, but I wouldn't assume the same with some other manufacturers.

 

[memilanuk]

Trust me, we (me and BA) went over all that.

By my measurements (from the back of the lugs to the nose of the bolt, and subtracting the depth of the bolt face) it's a thou or so past the middle of the range for the spec (IIRC, it's been a minute) - but the fired cases still come out measuring too long in headspace. That's when they got into the 'our instrument is accurate to 0.0001" ' and 'your bolt must be out of spec'.

 

[flogxal]

Had a similar situation w/ OdinWorks 6.5 Grendel barrel, Manson go/no-go gauges, and Toolcraft bolt. Sent all of it to OdinWorks for them to see if it was the gauges, the bolt, or the chamber/extension. They resolved it by giving me an OdinWorks labeled BCG which has run problem free.

I suspect that said indirectly -- the Toolcraft bolt did not agree with the OdinWorks chamber/extension. They did not say the Manson gauges were off. They did not swap extensions on the barrel, or do any cutting on the chamber.

 

[supercorndogs]

PTG gauges. Tgose guys seem to get a lot of credit for making things out of spec over the last 10 years.

I have read the shoulder can keep blowing forward some in an ar15 as the bolt unlocks. What is the measurement for brass that actually fits back in chamber?

 

[memilanuk]

I have read the shoulder can keep blowing forward some in an ar15 as the bolt unlocks. What is the measurement for brass that actually fits back in chamber?

Dunno if I have that data anymore. Kinda done flogging this dead horse; not planning to pursue this any further. If I get another 6 ARC barrel, it'll be from a better quality manufacturer, and I'll probably spring for a 'matched' bolt to avoid this kind of shenanigans.

 

[flogxal]

JP bolt seems the universal choice for 6ARC. I am running one with Proof 18" SS barrel that shot great for about 350rds until Proof had a recall on the extensions. Have not shot the post-recall version with new extension.

Folks on the 6.5 Grendel forum have a group-urged maxi bolt for the Grendel, can't remember what it's called, Rexus Arms sells it I think. Have not tried one myself.