.280 Enfield...Ahead of its time

[Tony Williams]

Why should that be a limiting factor?

Paul's MRMG concept requires a cartridge solely for the infantry machine gun, not carbines or rifles.

And in the event the military should want a universal cartridge, they could develop new rifles with longer mag wells.

I was merely pointing out that the perceived need to fit in any AR-15 action was the limiting factor which determined the characteristics of both 6.5mm Grendel and 6.8mm SPC. I entirely agree with you that the common assumption that any new cartridge should be able to fit in a rebarrelled AR-15 needs to be challenged.

I wasn't talking about Paul's concept, but about a general-purpose round which could replace both 7.62mm and 5.56mm in portable weapons carried by dismounted infantry. After all, if you're going to develop a new long-range cartridge which is significantly lighter than 7.62x51 (and if it isn't, why bother?) it will also have significantly less recoil. So if you have such a cartridge, why bother with 5.56mm? You can reduce the number of different weapons and calibres in a squad (which helps with training, support and supplies) and enable ammo-sharing between members of the squad, as well as ensuring that all of the riflemen have weapons which are capable of engaging the enemy at any feasible small-arms range, especially when they get DInGO computer sights. As the US Army's PEO Soldier organisation put it last year:

A Soldier must be able to engage the threat he’s faced with – whether it’s at eight meters or 800.
and:

Ultimately, Army service rifles must be general purpose in nature and embody a series of tradeoffs that balance optimum performance for a wide range of possible missions in a range of operating environments. With global missions taking Soldiers from islands to mountains and jungles to deserts, the Army can’t buy 1.1 million new service rifles every time it’s called upon to operate in a different environment.