ARC Bolt stuff

[pmc847]

I have a 6 mm ARC AR15 but using 6.5 Grendel brass necked down.  This rifle has  26” Bartlein barrel by Craddock and mainly used for steel out to 1,000yrds (once I get load development done).  The dies, FL neck bushing from L E Wilson, were over sizing down by .0075 vs my 223 by .003.  So I’m having Whitten make me custom dies to be more inline with SAAMI specs and about .0025 down from fired brass.  This should cut stress on the brass and better fit chamber.

Next inquirer is bolt questions.  I have the JP Grendel bolt so I think I have one of the best but still feel the bolt is the weak link for both Grendel and ARC.  The Grendel bolt face ID measures .4525 and the 223 measures .378 so the the wall thickness has been reduced by  about half.  Why isn’t the Grendel bolt face based on SAAMI spec like the 223?  What does your bolt face measure out to?

For you engineers and machines out there.  Can you add back the lost thickness of the bolt face wall to the OD of the bolt face and to the OD of the lugs?  Is there enough room to add those dimensions to the barrel extension? Would this bring the Grendel bolt strength back to a point that it could be considered safe at 60-62k psi capable platform?

Current OD for lugs is .740  Plus lost wall thickness of .0315 (x2)  (if you make bolt id .441 not .4525) would make lug od .803.  The current extension id is .830 should make enough clearance.

 Current bolt face OD is .5250 plus .0315 (x2) is .588.  plus needed clearance.

I think the extension can handle that change.

What are your thoughts?  This is just an exercise of what if to start with.

Cheers

Philip

[grayfox]

The 6Arc bolt, like the grendel bolt, is a wider inside diameter because the case is thicker 0.441, so yes the "face wall" is thinner. The designers wanted to keep the 223/556 dimensions all the same as much as possible. That JP bolt is one of the best, I have not used it but all the users on 65Grendel will say that. The monster bolt is also right up there.
No you cannot add back the wall material in that bolt face area, to do so would either prevent the case from inserting into that area or cause you to need a non-AR15 spec bcg/upper/barrel extension/etc re-design.
The saami max pressure for the 6.5 Grendel and the 6mm Arc has been lowered due to the larger surface area and thus the larger bolt thrust during the firing sequence. 223 is 56,000 roughly, 556 maybe 58-60 (the exact # escapes me), but the 6.5 grendel and 6Arc are 52,000 psi. Thrust/force = PxA, so a larger case diameter (0.441 vs 0.378) means the P must be reduced to prevent excessive thrust and broken bolts/lugs/kabooms in your face.

I just today found out about a newer bolt design, patented, by SRC arms...
https://www.srcarms.com/bolts/6.5-grendel-type-2-bolt

you can look at it via the link. Don't know anything more about it.

The bolt face depth for a true grendel/6Arc is 0.136, this in order to again, maintain the firing pin as a standard AR15 pin, the bolt body has been lengthened in the rear by that 0.136, so the bolt is "longer" but the pin works fine, this in order to allow a deeper seating of the case into the bolt, allows for a better extractor geometry. Again over on 65Grendel there are several threads that tell all about this, better than I can.

Really a lot of design and engineering work went into the grendel design and the 6Arc just brought those design parameters and benefits over to itself.

[pmc847]

Good info and I don't disagree. I'm a gear head so I see something well designed and think how can it be better based on long term use data, not to say it can thus the conversation .

I might not have been clear about where the lost thickness should be added, not putting it back to where it came from (ID) but to the outside (OD) of the bolt face. The same with the lugs, to the outside of the lug thus keeping the lugs and wall thickness to 223 configuration. Wall thickness of .076 vs .0350ish and lug hight of .179 vs .1455. I think the barrel extension can handle the change. I guess the question should be is a redesign worth the effort.

Cheers and thank you for your response.

Philip

[grayfox]

Take a look at that SRC arms bolt. If I come up with a few bucks I might spring for one, but for now in the current unpleasantness, well....