I will go re read, but I think that you played it right. Rolling fire on a light vehicle is going to do damage eventually. ( An arrow off 55pound pull compound bowstriking in the engine will take out a a Nissan Sedan – I know because I have seen it happen when an idiot drove one across a West African archery range at speed during shooting practice.) And don’t forget that with Skirmish Sangin you are very very close and 1 for 1. Big bangs really will affect the run of play.
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.