Pintle-mounted storm bolters are fixed to the outside of a vehicle and can either be used by a crewman from an open hatch or by remote control from inside the vehicle. They are treated as an extra storm bolter that can be used in addition to any other weapons the vehicle has. Note that this means that a vehicle that moves can fire one weapon and the pintle-mounted storm bolter.
Pintle-Mounted Storm Bolter
Source: Codex: Space Marines, p. 35