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 .

autogun said:

Are those two rounds to scale?

stancrist said:

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

Superb find Stan.

Estimated 6.8 weight:

Hybrid Case: 155gr 

Large Rifle Primer: 5gr

EPR Estimate*: 135gr

Powder Estimate*: 51gr

= 346gr / 22.42 grams per 6.8x51 cartridge 

Biggest variable is what the actual GP projectile weighs, could be lighter.

But it's clear we're dealing with a cartridge very marginally lighter then 7.62x51. The 'lightweight case' is really not that light at all. 

Cropped image high rez:

https://i.ibb.co/59FPQsW/6-8x51-NGSW-EPR.png

And reasonable forum size

When two rounds of the same purpose (i.e. infantry rifles and LMGs) are shown side by side, apparently in a comparison photo, there is a normal assumption that they will be to scale. 

gat i had 2 questions.

what's the approx. weight of a polymer cased 7.62x51 or 6.8?

did textron's CT round use a plastic case and was it lighter than the others?

assuming a full power muzzle energy, what ME would the 6.8 have at 350meters or so?

random question. what was the weight of GD's rifle? i think i read it was 3.3kg with suppressor, which seems awfully light.

also, if you had a NGSW rifle where the barrel recoiled a long way, like 4 inches, would it reduce recoil more than GDs offering?

smg762 said:

random question. what was the weight of GD's rifle?

Specific answer to your random question:  I don't know.