Fall or, Dodge in Hell. Neal Stephenson

Tom and Kevin had opened up the locker in its back, taken out the machine gun, and mounted it on the tripod. They’d kept it under a blue tarp until the caravan had got out of town, then pulled over onto the shoulder long enough for Kevin to hop out, remove the tarp, and take a seat atop the locker. For the time being, the barrel was still canted sharply upward and Kevin was keeping his hands off the controls. Apparently the sole point of all this was to make everyone in their vicinity aware that they were a hard target.