According to this article in Popular Mechanics:
And then there is the bane of every technology user: batteries. “Almost everything a soldier carries today requires batteries,” notes James King in a piece for the Modern War Institute. Batteries for the platoon’s AN/PRC-117 radio weigh four lbs. each, and the radio burns through them rapidly. King estimates that the average soldier goes into action with a hefty 20 lbs of batteries.
Add in body armor at 20 pounds, M4 + ammo load at 15, personal gear, grenades, etc. and the personal kit for an infantry soldier tips the scales at 75 pounds.
Then add squad weapons and ammo. Belts of ammo, base plates for mortars, manpads.....and the grunt is staggering out into the field carrying 150 pounds or more.
Not too many petite cheerleaders humping that load, no matter how fit.
You can see why there is much incentive to make the personal weapon more effective. One reason for the Squad Automatic Weapon (machine gun) and mortar are the range and penetration limitations of the 5.56mm round.
A magazine of 5.56mm with thirty rounds weighs about a pound. Thirty aimed rounds can do a lot of damage and ditching ten pounds of something means another three hundred rounds for the soldier's personal weapon. That has to be a comfort.
Maybe it is time to upgrade the 855 round to include a carbide tip in a hardened steel carrier. Another proposal would be to upgrade to something similar to the .243 Winchester although you would be hard pressed to keep a magazine with 20 rounds under a pound.