Rigger 0.5 adds (needless) complexity to this situation. But the Core rules pg 461-462 says that you have (Body / 3, round down) worth of mounting space. Basically points spent on purchasing weapon mounts.
Standard weapon mounts require 1 of those spaces, and can support an Assault Rifle or smaller, as well as 250 rounds of ammo.
Heavy weapon mounts take up 2 of those spaces, and can support any size weapon, as well as 500 rounds of ammo or (Body) worth of rockets / missiles.
Each of those weapon mount types can be operated remotely, and covers a 90 degree firing arc.
These rules apply both to arming your drones, and arming your vehicles. So your GMC Bulldog (Body 16) has 5 spaces in which to fit weapon mounts.