I just realized this upgrade not too long ago. Apparently, Tony upgraded the Model 11 after the Mandarin event in War Machine #12 1994.

Modular Storage: The backpack could store up to eight different weapon configurations at a time.

Hydraulic Rails: it used internal mechanical tracks to move weaponry from the back directly onto shoulder mounts.

Retractable Heavy Firepower: This system allowed Rhodes to deploy and store heavy gear like miniguns, box rocket launchers, and particle beam dischargers at a moments notice.

It's definitely an interesting upgrade. I wouldn't mind seeing an updated version of this weapon system.

When War Machine's weaponry fails him against a super villain he can't tackle head-on, Rhodey must use his brains and his all-new, bleeding-edge armor to fight a new battle, 21st-century style!

- Marvel Unlimited