Headspace gauge

[pavementends]

Anyone found a source for go no-go gauges?
Thanks,
Pavementends

[grayfox]

I haven't looked but I would go for Manson and not for PTG. PTG caused lots of problems early on with bad Grendel gages. All of my Mansons have been good.

[Old Bob]

Manson doesn't list 6mm ARC headspace gauges in their product catalog but it hasn't been updated since March so you might shoot 'em an email:  david@mansonreamers.com  Or call 'em at 810-953-0732. I might do this myself.

PTG does have headspace gauges for 6mm ARC. They're the only ones I've been able to find. Neither Clymer or Forster list them as well.

I had a PTG 6.5 Grendel GO gauge that my BCG wouldn't close on. I got a Manson, no problem! I measured both gauges at the datum line, the PTG was .0005" longer. Is that enough to stop the bolt from closing? I guess so...

[Lemonaid]

E-Mail Manson about them, they are pretty good on answering questions.

[Old Bob]

They aren't listed on their website but Manson does have GO & NO-GO headspace gauges for 6mm ARC. That's what they told me in an email reply. You need to call & place an order if you want a set. They are a bit pricey at $140 for the set.

[pavementends]

They aren't listed on their website but Manson does have GO & NO-GO headspace gauges for 6mm ARC. That's what they told me in an email reply. You need to call & place an order if you want a set. They are a bit pricey at $140 for the set.

Thank you
pavementends

[Old Bob]

Using a Hornady die I resized 50 Alexander Arms 6.5 Grendel brass cases to 6mm ARC. I used my Hornady case comparator gauges to measure the cases at the datum line (.350" dia. at the shoulder). I measured 10 cases & they were all 1.185" at the datum line. Next I measured my Manson GO & NO-GO gauges. At the datum line, the GO gauge was 1.180" & the NO-GO was the same as my resized brass, 1.185."

SAAMI dimension specs for the 6mm ARC chamber is 1.1901" minimum to 1.2001" maximum, measured from the breech bolt face to datum line. So, what I'm wondering; when I finally get my barrel & I check the headspace, will the bolt close on the NO-GO gauge if the chamber is cut more toward the max spec?

When resizing fired brass it is normal to set back the shoulder approximately .003" to .005." If every dimension is more toward the big size, could I just resize the brass with no, or just a hair, set back? How will this effect brass life?

All comments welcome!

[grayfox]

Since Saami is 1.1901" as min spec, as long as there is no tolerance delta/stacking, looks like that no-go (at 1.185) might close on a saami chamber, which is not so good. However, the hornady gauge is not as exact as the mansons are in measuring; also your calipers are not as exact either. So there could be some delta in the measurements, that won't show in reality when you put the mansons into that chamber.
Just be aware and see what the chamber actually does with both go and no-go, then you can evaluate what you really have, from there.

Ps, a different thought... do we know if the manson dies were cut for a 0.136 bolt face recess? They should be, but do we know.

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Biggest potential source for error is the bushing opening on the comparator gauge, sometimes it sits on more like a 0.351-0.352-whatever line, just because it's not a $200 precision cut tool. It's more of a consistency measure for your barrel, not an exact perfect precision 0.350".

[Old Bob]

grayfox...  Your thoughts concerning the precision of the Hornady measuring tools are pretty much the same as mine now. I did the same checks with my Grendel brass & headspace gauges & got similar results. From those measurements it also looks like the bolt might close on a NO-GO gauge. It won't close so I am feeling pretty sure my concerns about my 6mm ARC will amount to diddly-squat.

The Manson shipping invoice just says 6mm ARC. It doesn't specify bolt-face depth. You're right, they should be for .136" bolts. I've never seen anything that would lead me to believe the 6mm ARC was developed from anything other than the type II 6.5 Grendel chambering.

Thanks for commenting...

[BluntForceTrauma]

My understanding is that the headspace gauge represents an ideal round as opposed to being tied to any particular boltface counterbore depth.

It shouldn't matter whether one's bolt is a 6.5 Grendel/6mmARC Type 2 (I call mine Mk136) with a 0.136 depth or a 7.62x39 Type 1 0.125.

What matters is whether the chamber is reamed 0.011 deeper in a barrel headspaced to use a Type 1 bolt.

In either case, the headspace gauge used is the same.

But, yes, the cartridge was developed using the Mk136 or Type 2 bolt and it is the standard for the cartridge.

[CZ527 Guy]

I have the head space go gauge though no factory ammo. Looks like Brownells has the HS gauges in stock.

[r.tenorio671]

....a "Headspace" gauge measures the distance from the bolt face to the the datum line of the shoulder & base of casing neck.

...a majority of ammo manufactures will bump the shoulder (datum line) of cartridges  to the SAAMI spec, typically closer to the minimum spec of 1.183" to ensure it will properly chamber in ANY manufacturers  barrel as long as the barrel maker kept to the SAAMI specs when reaming the chamber.

...if you have already measured the referenced ammo and the measurements fall within that 1.183" - 1.190" range, you should have no issues chambering & firing those rounds.  What you want to do is after firing those rounds is to measure the cases afterwards  to determine what your chamber was reamed to by the manufacturer so you can adjust your sizing die accordingly.


....brand of HS gauge will be a factor, some gauge manufacturers will machine to various specs within that 1.1901 - 1.2001" SAAMI Chamber specification allowance.

...won't let me add graphics again, dang it!  I post graphics separately of the CARTRIDGE and CHAMBER specs.

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Cartridge spec, note colored boxes...
   

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    Chamber spec, note colored boxes:

[r.tenorio671]

Very helpful information.
Thanks for posting.
Looks like I'll need an exact .350 dia. gauge; that should get me very close for comparisons.

...Hornady makes the .350 inserts for their L-N-L gauge set.  The Sinclair inserts are compatible as well, both available from Brownell's.

...Brownell's also carries the Clymer GO & NOGO headspace set if someone is looking for the GO/NOGO set, excellent pricing for the set, less than buying individual gauge.  I personally use both, not just a a single or a FIELD gauge...the FIELD just measures if its still "safe to use" but doesn't give any reassurance to how close to out of spec it may be, no bueno if it's a brand new barrel you're checking IMHO.