No, they're a standard special weapon choice too. Several imperial infantry units can take one. Terminators have them as standard, and some of the most elite ones use them like a pistol.
As for vehicle-mounted ones, that's usually on a rail around the hatch, the same place you'd find a small machine gun on modern tanks. If the tank is using storm bolters as it's main weapon, you're looking at a bank of several.
Anyway, the new rules do make a lot more sense. Hitting with every bullet on full-auto didn't feel right.