The big problem with military propellants for small arms is that you are limited to the pressure gradients and envelopes from when that cartridge was developed and adopted for a service weapon.

While Joe Q Hunter can take his .30-06 cartridge and handload it with hotter propellant, and fire it from a custom made beefed up Mauser action; the US Army doesn't have that option, because it would be a nightmare trying to keep track of which weapon can fire what:

DO NOT FIRE 5.56MM M1400 ENHANCED AMMUNITION FROM ANY M-16, M-16A1, M-16A2, M-16A3, OR M-16A4 RIFLE WITH A SERIAL NUMBER OF LESS THAN 34210. WEAPON EXPLOSION DUE TO OVERSTRESS IS PROBABLE.

Propellant advances in military service small arms only happen when a brand new caliber is introduced generally; like for example; the M-14; firing the brand new caliber of 7.62x51 NATO; basically using advances in propellant to shrink the old .30-06 round down significantly, and still retain it's power.

So most likely, if the Army ever choses to formally adopt a 6.8mm or 6.5mm round; it will use advances in propellant technology allowing for a smaller case relative to it's kinetic energy compared to the older 5.56mm rounds (which were designed and specced in the late 50s.)

However; there is one area where propellant technology is constantly updated -- Tank Guns. It's cheaper to modernize a couple thousand tank guns to fire hotter, higher pressure sabot ammunition than it is to modernize 1,000,000 M-16s to the same level.