12-10-2022, 10:55 PM (This post was last modified: 12-11-2022, 07:26 PM by 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 Im having Whitten make me custom dies to be more inline with SAAMI specs and about .0025 down from fired brass (minus spring back of .001+-). 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 .444 and the 223 measures .382 so the the wall thickness has been reduced by about half. 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 60k psi capable platform?
Current OD for lugs is .740 Plus lost wall thickness of .0315 (x2) That would make lug od .803. The current extension id is .830 should make enough clearance.
Current bolt face OD is .530 plus .0315 (x2) is .593. plus needed clearance.
Keeping the lugs and wall thickness to 223 configuration. Wall thickness of .074 vs .0427 and lug hight of .179 vs .1475. I think the barrel extension can handle the change. I guess the question should be is a redesign worth the effort.
I think the extension can handle that change.
Measurement taken with Mitutoyo calipers and barrel extension drawing spec sheet.
12-11-2022, 08:21 PM (This post was last modified: 12-11-2022, 08:28 PM by Old Bob.)
Tried to download an additional PDF with more bolt dimensions but it was too big a file. I'll see if I can extract the pages I need to download. Can't do it now 'cause I need to got out awhile.
I refuse to be victimized by notions of virtuous behavior.
You also have to account for the thread relief cut in the barrel tennon, where the barrel extension mates to the barrel.
Bill A. went through this exercise in the beginning. If you run the MAP at 60,000psi, what is the next part that will fail?
The chamber walls will egg-out quickly, especially with stainless barrels.
So then you design a different extension and bolt set to handle the pressure, and what performance benefits do you get, aside from single point muzzle velocity figures?
Is it worth the effort down-range, and what fps gain do you have to jump to in order to achieve that?
Is it 200fps more mv, 300fps, 400fps?
The case wont really support that with the volume, so then youre back to looking at an AR-10, with all of its penalties.
What several companies have done now with small bolts that support .308 is to use AerMet alloys for the bolt and extension.
FN did that with the SCAR-H/SCAR-17. POF did that with the Revolution and Rogue. And now Ruger has done it with the SFAR.
The Revolution, Rogue, and SFAR are all AR-15s chambered in .308 Winchester, with lowers that have an AR-10 magazine well, FCG moved back .625, longer firing pin tails, enlarged mag well transition port on the bottom of the upper receiver, larger bolt faces, still using AR-15 diameter barrel extensions.
You could look at an AerMet bolt and extension for 6.5 Grendel and 6mm AR if you wanted to push the pressures 10,000psi higher, for maybe 150-200fps gains.
150-200fps mv increase isnt doing you many favors downrange, and your brass life will diminish rapidly.
Everything is a trade-off.
NRA Basic, Pistol, Rifle, Shotgun, RSO
CCW, CQM, DM, Long Range Rifle Instructor
6.5 Grendel Reloading Handbooks & chamber brushes can be found here:
Here is a pic of what I was trying to explain. I added .0315 to radius (.063 diameter) indicated by the blue. All clearances stay the same as .223/556. Lug dimensions stay the same.
The same for the barrel extension.
I was tracking from your description, basically adding diameter to the lug root body of the bolt like they did on the SFAR and POF to create a thicker wall, then increasing the ID of the extension teeth clearance.
Another approach is the Fosberry Shotgun Bolt and Barrel Extension of 1891 (notice the ejector as well):
NRA Basic, Pistol, Rifle, Shotgun, RSO
CCW, CQM, DM, Long Range Rifle Instructor
6.5 Grendel Reloading Handbooks & chamber brushes can be found here:
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