There would be no need to "develop completely new guns."  Several companies have built versions of the AR15 with stronger bolts and barrel extensions: Colt, Olympic Arms, ARPerformance, and CMMG to name a few.  It is not very difficult, given the current state of CNC machining.   Heck, the CMMG "mutant" guns use an oversized bolt and barrel extension for the 6.5 Grendel and 7.62x39 and are in current production.   The 6mm ARC is designed around a weakness inherent in a 60+ year-old platform designed for a less powerful cartridge.  As an engineer, it is almost insulting that so little has been done to optimize something called the "Advanced Rifle Cartridge".  THAT is what makes it particularly interesting.   Note that I wrote 'interesting", not "unbelievable" or "shocking!"    Furthermore, the 6mm ARC is not even anything new, since it is in between the 6mm PPC and 6mm AR both in size and performance.   Those cartridges have been around for a number of years.

A variation of the SIX8 form LWRC with a stronger bolt and barrel extension shooting a shortened 6mm Optimum utilizing SIG high-pressure hybrid cases would be VERY interesting.  The 6mm ARC is a bit ho-hum in comparison.

nincomp said:

As an engineer, it is almost insulting that so little has been done to optimize something called the "Advanced Rifle Cartridge". THAT is what makes it particularly interesting

Developing a new cartridge for the existing platform is more or less like putting the cart in front of the horse

First you get a desired ballistic solution for your proposed range of scenarios. Then you design a cartridge for this solution, with desired weapons' features in mind. And only then you start designing weapons for the cartridge

At least, it should be done in that way, in theory

In practice, you get failures like 6.5 Gr or 6.8 rem Spc.

nincomp said:

I find it interesting that the US's NGSW program concentrates on improving performance with new platforms and (relatively) high chamber pressures whereas the 6mm ARC goes the opposite direction and utilizes chamber pressures below that of current NATO military cartridges in order to use an older, existing platform.

The goals were simply different.

The US Army wanted to achieve results that no other portable system could provide, so they needed something totally new.

I think that the 6 mm ARC started it's life as an answer to the question: "what would be the most effective round that could be fired out of a C7/C8 rifle/carbine?" 

Again, I find it interesting that a 6mm cartridge based on the 6.5 Grendel case became popular only after Hornady tweaked it for their own bullets and gave it a flashy name.  Marketing, or Merchandising (as per the clip from the movie Spaceballs) seems to be more important than anything else.  Granted, the shoulder was moved back 0.030" (0.762mm), so it not exactly the same as the various "6mm Grendel" variations like Robert Whitley's 6mm AR,  Les Baer's .243 LBC-AR and ARP's 6mm Predator.  On the other hand, the 6mm AR has been publicly available and used in competition since 2006.

https://youtu.be/HnXKE0nfAjI?t=36

nincomp said:

Again, I find it interesting that a 6mm cartridge based on the 6.5 Grendel case became popular only after Hornady tweaked it for their own bullets and gave it a flashy name.

You probably mean "after one manufacturer standardize a drawing and produce ammo for it?" :-).

The .300 Whisper existed long before the .300 AAC, but the round found a wide acceptance only after standardization.

I think a lot of people fail to understand just how dramatically you could improve things with a new optimized bolt and bbl extension and frankly I don't blame them because unless you understand some of the specifics of how it works the increases in performance seem pretty unbelievable.

Whether it's Emeric's B2 (baby balle D) or one of sidewinder's VKO BAT's the increased peak pressures, increased ogive space, and etc are suddenly giving you numbers that seem terrifyingly too good to be true. (This includes optimizing for much higher penetration through things like structural 13500 psi CMU's at close to m80a1 levels of turning cover into concealment)

For a pretty wide variety of reasons my feeling for a long time has been that the best path forward Is limp along the 5.56 guns with the new optimized bolt and extension lightweight cases optimized new projectiles and along with the fancy new electro optical FCU's while putting the real development funds into a proper hi cartridge replacement and new suite of belt feds 

Once we have a proper beast of an absolute wrecking ball lightweight hi cartridge and new MG's to fire it out of (though you could theoretically do the cartridge I'm thinking of in 240's)  that get 338 norma mag performance out of something a bit larger than x51 but same weight or a bit less then we can use the fire control to our best advantage.

Hunters avoid pointy tips when fighting bears....a flat nose penetrates better

Who is sidewinder.. Is it  Nathan from tfb

roguetechie said:

...my feeling for a long time has been that the best path forward Is limp along the 5.56 guns with the new optimized bolt and extension lightweight cases optimized new projectiles and along with the fancy new electro optical FCU's while putting the real development funds into a proper hi cartridge replacement and new suite of belt feds 

Once we have a proper beast of an absolute wrecking ball lightweight hi cartridge and new MG's to fire it out of (though you could theoretically do the cartridge I'm thinking of in 240's)  that get 338 norma mag performance out of something a bit larger than x51 but same weight or a bit less...

If the plan is to "limp along" with current weapons while doing concurrent development of a new cartridge and machine guns, it seems very wasteful to expend resources upgrading 5.56 guns and ammo via the changes you describe.  For that reason, I also think it's extremely unlikely that the Army would follow your "best path" forward

If your "beast of an absolute wrecking ball lightweight hi cartridge" is ever to become reality, you'll have to develop it yourself.  So, when do you start?  

I wish I was a cartridge designer but I ain't that smart Stan.

Nor do I have the means to get sample projectiles manufactured or even get the powders I'd need much less the lightweight cases.

Really good ammo has entered the space age my man, it's not something a basement Enterprise can do. Or not something my basement Enterprise can do.