how to use headspace gauges?

AR/M4 AR Technical Discussion
1

I built an upper with a kotonics/cardinal 6.8mm SPC barrel that had the “improved” chamber. I used a Stag lefty upper and a stag lefty 6.8mm BCG. All 3 components were bought from separate places.

So… How are you supposed to use HS gauges? Do I need to take the barrel out of the upper? Do I need to use the HS gauges with just the barrel & bolt, but no bolt carrier? Do I need to take the firing pin, extractor and ejector out of the bolt? Why do my HS gauges have this big slot cut into the bolt-facing end of the gauge? Do I need to try to align that slot with anything?

And why couldnt i find any 6.8mm field HS gauges?

My 2 manson 6.8mm headspace gauges didnt come with any instructions. Now on the manson site they had a blurb about some extra things you had to be careful of when measuring headspace on improved chambers. Im not sure if that situation applies to the 6.8mm improved system.
http://www.mansonreamers.com/Instructions/Ackley%20Gaging.pdf

2

Barrel can stay in the receiver. Generally on Go/No Go gauges, you just need to remove the Extractor spring and insert and then put the bolt back into the bolt carrier (do not re-install the firing pin or cam pin).
Slide the BCG back into the receiver. The end of the bolt carrier should poke out the back of the receiver slightly. If it sits flush with the receiver, then you have a problem.

C4

3

i’ve never removed the extractor when checking headspace although i’m willing to be educated…

4

I’ll run it down the way it is in the TM and how it is done in the military.

Insert charging handle, bolt assy. (that means complete), and bolt carrier into upper receiver.

Insert headspace gage into chamber area.

Press the rear of the bolt and carrier assy. and charging handle forward using light finger pressure into the chamber area.

Bolt should not rotate into the locked position and the bolt carrier must protrude from the rear of the upper receiver assy. for proper headspace.

5

The bolt should lock up on the “go” gauge. it should not lock up on the “no-go” or “field” gauge.

Most guys just use a “field” gauge with 5.56 chamber.

I’m ignorant about your improved 6.8 chamber. I would call Manson and ask them directly about that chamber and their gauges.

6

If you’re using the .223 Forster gauges (the whole set), many 5.56mm NATO chambered guns will close on a NO-GO but won’t (and shouldn’t) on a FIELD. I usually just use a USGI FIELD to make sure there isn’t excessive headspace (the dangerous kind).

7

FWIW the Brownell’s website has lots of good “how to” pages not just for this but for lots of other stuff.

8

The reason is that can wear/damage the gauge.

Just to be clear, you leave the extractor in, but remove the spring and insert.

C4

9

Agree. I think people might be talking about a couple different things here (Go/No Go) and how to properly fit a bolt to a barrel following a set criteria by the manufacturer.

For instance, Novekse has a set criteria for EACH of his barrel lengths. He has his own HS gauges that I am required to use on his barrels.

C4

10

I guess I should mentioned that I only use a “Field” gage as well. Ooops…

11

Grant,

I am assuming that this applies to the “civilian” gauges as they have a rim. If I remember correctly the military “Field” headspace gauge has no rim, it is tapered.

12

Correct.

C4

13

gotcha, thanks!

14

Could someone educate those of us who are lacking knowledge, on this whole HS issue? I mean, is it necessary, or is it standard practice to match a bbl up with a matching bolt when building a custom AR?
I mean, WHEN LMT, COLT, etc, are building rifles/carbines, they just grab a bolt off the assembly line, and chunk it in the next gun (bbl) that comes down the line, right?
I am just trying to figure out when HS is an issue, and when checking HS or matching up bolts with bbls is necessary??

15

Paul,

When LMT, Colt etc…are slapping guns together yes you are probably right. However, all the guns are inspected to include headspace checks before they are shipped out. Since the relaible manufacturers keep their weapons within tolerances and all the parts are new there shouldn’t be an issue with it not headspacing correctly.

16

I gotcha. You see where I’m going with this?
Basically, what I’m trying to determine, is IF I am buying uppers, lowers, and bcg’s from various sources, then do I need to be checking HS on all my builds??
OR, do I just need to go shooting, and call it good if the weapon performs reliably.???
Thanks.

17

The latter. The only time I might check is if I replace a bolt that was shot in a barrel that had a lot of wear. Bore/chamber wear has nothing to do with it though, it would the wear on the barrel extension. And even then I’d probably just slap a new bolt in and declare it GTG. If you ran into a situation where you had a new bolt that would close on a Field Reject gauge, there’s no way really to adjust things other than try different bolts, with the hope you’d get one that reduced headspace back within the Go/No Go area.

18

I’m pretty sure the Kotonics Improved chamber doesn’t alter HS, it only affectes the leade and freebore areas. HS guages aren’t designed to extend into this area.

What your Manson instructions refer to is using these in Ackley (or similar) chambers where the shoulder angle is changed. Again, I’m pretty sure the 6.8 improved retains the same shoulder angle and shoulder position so SAAMI HS gauges will work perfectly.

19

I use the Forster gauges and have declared a few rifles as “unsafe to fire” because it closed on the field gauge.

Now my questions is, Is it still possible that those rifles will not close when using a USGI FIELD? There by making it still safe to fire?

20

NATO is a bit more generous in its Field Reject than SAAMI. You can fail a SAAMI guage and still pass a NATO gauge.