The minute you give other units new roles is the minute other units lose relevance. HMG with bazookas defeats the purpose of AT Gun, and AT gun with LMG defeats the purpose of the former. Even the Captain cannot pickup weapons since he comes with two bazookas, which fills the heavy slot(s).
Giving units new roles is the entire purpose of the usf-versatility over raw power.
Sherman's can change shells and act as a smoke mortar.
REs with lmgs can act as HMG teams.
Riflemen with Zooks are the only at snare+at weapon squad ingame.
Taking medics out of ambulance and inserting res inside
Resigning captain to focus production duty lategame
And so on. If anything, support weapon racks (which mechanically I accept can't be implemented) would build on the U.S. theme of utility and versatility, while also adding more strategies to the faction and buffing debatably the weakest support weapons.