.280 Enfield...Ahead of its time

[adamjp]

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.

Agreed. Why is the AR15 a limiting factor? There are plenty of other rifles on the market, many of them have features that are improvements over the AR design.