I went through the rules again and yes the Hummer can go down to small arms. There is a basic 12% penetration chance for small arms against class 2 armour. This basically induces a vehicle stopped and bail out. In the narrative nature of Sangin I would venture to suggest this might change the game to one of recover the Hummer, or destroy all its secrets type of game.
The big mortar danger zone is 30 where mere mortals take 1 D10 of damage. Sangin really is up close and personal. Nice for detailed solo play as far as I am concerned. But I like Ambush Alley and CWC as well.