6.5 Grendel Bolt face depth. Not your normal question.

[Trapeze]

I have 4 different AR-15 6.5 Grendel bolts from four different manufacturers. Two of them came matched to barrels. I am not going to name the manufacturers until I have a chance to speak with each of them, but they are well respected in the 6.5 G world.

I do have a question for you.

Using my Starrett 797B-8/200 W/SLC Electronic Calipers I measured the face depth of each.
• A: .1355
• B: .1360
• C: .1365
• D: .1370

I understood the specification for face depth of a 6.5 G bolt to be .136, with a tolerance of -.003 and +.000
This would tell me a face depth as short as .133 would be within tolerance but anything at .1361 or deeper would be out of specification.
I am waiting on my go/no go gauge for a true headspace check, then I will speak with the respective manufacturers of the “C and D” bolts that measured above.

What say you?

[LRRPF52]

I've seen the same variations. Have even seen the AA hard use at .137".

AR15s have usually ~.003" in slop in the headspace in order to be able to run, otherwise you would have too close of a fit between the bolt and the extension that would resist rotating into lock.

I wouldn't worry about it, other than keep track of your headspace on fired cases and make sure you are setting back far enough, without over-working. In the end, we're talking about a thousandth in variation between those, .0015" extreme, which is inconsequential in the big picture.

[Texas]

When we were assembling uppers with milspec components, occasionally there would be a bolt that would not headspace, so we would change it for another milspec bolt which would headspace and set that bolt aside. Later it might headspace on another milspec barrel. it is a combination of the tolerances of the barrel and tolerances of the bolt being at the extremes but still within spec that would cause the issues. I am speaking of 5.56 components since at that time there was no milspec for 6.5 Grendel.

[ricsmall]

No worries on those measurements. That little bit of variation could be any number of things, including a minute variation in caliper positioning on bolt head. A true depth micrometer would be more accurate. .0015" is just about equivalent to the thickness of a human hair, so it's inconsequential in the end. If it were +.003-.004" I'd want to measure with a headspace gauge for verification. It's especially moot if the bolts are matched to the barrels.

Richard

[Trapeze]

I'm not too concerned at this point, I will know more after the headspace gauges arrive, as long as the bolt's headspace to my barrels, all will be good.

My thoughts about the bolts are related to the wide allowable tolerance of the chamber. The distance from the bolt face to the datum point can vary ,0100, that's a huge allowable tolerance. If my chamber is at the long side of that tolerance, then add another .001 of an overly deep bolt and suddenly my bolt will close on my field gauge. This is a brand new bolt and barrel, they are only going to get deeper as time goes on.

Yes a mic would measure more accurate, my calipers are high end, my bolt face measurements are within .00025

Thanks for all the input, good stuff.

[Trapeze]

LRRPF52, Texas, ricsmall,

Thank you, I am happy to report that the bolts and respective chambers close smoothly on the go gauge and do not close on the no-go gauge. all is good.

[mirion]

Trapeze, Which headspace gages did you go with?