stancrist said:

I think it is not a crack at the "6.8x51" mark, but perhaps an impression of the extractor groove on the bolt face.

Could be. My experience is that the extractor mark is generally found inside the groove, where the extractor try to shear the rim.

Looks like that this case is operating just below the threshold of catastrophic failure.

EmericD said:

My experience is that the extractor mark is generally found inside the groove, where the extractor try to shear the rim.

Not the groove in front of the cartridge case rim.  The groove / channel machined into the bolt, into which the extractor is fitted.

Although to make that shape of impression on the case head, the extractor channel bottom would have to be rounded instead of flat.

Sweden reportedly adopting a new 7.62x51 infantry rifle, with an eye to possible future conversion to 6.8x51.

https://www.ar15.com/forums/General/Sweden-goes-ahead-with-new-service-rifle/5-2633465/#i103427018

https://forums.delphiforums.com/autogun/messages?msg=7952.27

So the list of potential NGSW users is now up to three (US, Australia, Sweden).  Who will be next?  

Didn't Sweden just go with 7.62 NATO?

graylion said:

Didn't Sweden just go with 7.62 NATO?

Uh, yes.  

stancrist said:

Sweden reportedly adopting a new 7.62x51 infantry rifle...

At this year's SHOT show, Geissele stated that they are using a proprietary upper, forged bolt, barrel extension, barrel nut, gas system and magazine on their new 6mm ARC.  They said the bolt is forged from a new "Carpenter 158+ alloy".    That alloy does not yet show up on Carpenter Steel's website, so I don't know any more about it.  

 They have also modified the upper, moved the foreward assist, riveted a steel insert in the cam-pin race to prevent steel to aluminum contact from bolt movement, and goodness knows what else.

They have modified the rifle so much that many parts are not compatible with other AR15's.  They did not mention widening the magazine well, but it is not out of the question.  Their estimated sales price is $3,500 to $3,800, so about 50% more than their own boutique "Super Duty" 5.56 rifles.

nincomp said:

At this year's SHOT show, Geissele stated that they are using a proprietary upper, forged bolt, barrel extension, barrel nut, gas system and magazine on their new 6mm ARC.

They have modified the rifle so much that many parts are not compatible with other AR15's.  They did not mention widening the magazine well, but it is not out of the question.

According to this report, Geissele's "new" 6mm ARC magazine is identical to ASC mags, differing only in a very minor alteration of the follower.

YouTube

6mm ARC vs 6.5 Grendel magazines. Geissele, ASC, Duramag, AR Stoner, Elander {UPDATE}

I don't recall if this has previously been covered, but news to me is that 6.8x51 case construction has been simplified; the original 3-piece case was succeeded by a 2-piece design.

SIG SAUER Supports SOF Weapon Needs | Defense Media Network

Also, 6.8x51 cartridges produced for the Army have darkened case heads.  Empty case weight = 10 grams.  

https://forum.cartridgecollectors.org/t/actual-sig-sauer-6-8x51mm-next-generation-squad-weapon-composite-cartridge-case/52615

And a nice pic of a 6.8x51 loaded round with GP bullet, in case anyone would like to crop it.  

Are those two rounds to scale?

When i was making my rifle magwell i bought all sorts of mags and  6.5Grendel ,6.8SPC , 6ARC was just and engraving on same magazine no alteration whatsoever, the body of the mag is actually designed for 6.8SPC .