I've long advocated a 6-7mm cartridge (the US Army 6mm SAW round had development potential) for man portable Platoon weapons, 10mm for a pistol or SMG and an enhanced 7-8mm cartridge for MMGs (a Co/Coy weapon IMO), some Sniper applications and vehicle weapons. A pepped up 8mm Remington Magnum (with an unbelted case), would work...

The .338 is a good cartridge but I think a bad compromise between 7.62 and 12.7; it's a little too big and like FNs abortive '15.5mm' too close to the 12.7 to be a really meaningful alternative.