I recall vaguely that they were carrying these huge packs on their backs. Maybe the idea is that the batteries were in the packs.
I'm kind of torn on a substitute. Call me old fashioned but maybe an M-14 and a few LAWs or modern equivalent?
When I'm running around in the woods with no resupply, I'd want to carry as much ammo as I could. For the equivalent weight of an M-16 and 1250 rounds of ammo, I'd only get 500 rounds with the M-14. And against an adversary that I can't even see - meaning there's a fair amount of spray and pray - I'd want to increase my chances of a hit simply by having more lead available to throw at him, in the direction of suspicious noises.
OK, name the novel, which has a theme highly apropos of this discussion:
Yo's proud Thunderbolt gun was nowhere in sight, and the corpse was not wearing the powerpack that energized it.