There are no Rules as Written for starships mounting external personal weaponry. I as a GM would let you have it, but would make you pay for it. I would extrapolate some info from the old Supplement 6 Military Vehicles to define your weapon.
1.Mount - Takes up a hardpoint so as the weapon might survive the rigors of atmospheric flight. The weapon could then be turret mounted and thus enjoy the benefit of protection (damage dice divided by 50). If no turret great, choose an facing like a vehicle. Still make you use the hardpoint for hull integrity purposes. Or at least I would make you install it as 1 of your weapons on a turret.
2.Mass - Page 14 has a conversion of weapon mass to cubic meters. One starship displacement ton is 14 cubic meters (or 13.5 forgot what Mongoose's rule is, versions varied...). Pay space in half ton increments.
Or if you do it as one of the weapons on an existing turret:
Since every turret is one displacement ton(the extra ton on a popup is is for the popup machinery). I would limit the personal weapons to either 14, 7, or 3.5 cubic meters depending on your usage.
3.Fire Control - If it is an automatic weapon, I have you install the appropriate software with Rating into the ship's computer. The Rating might make a difference in some situations depending on what Model you have. I would not have you task that mount for both personal and starship at the same time, if you went the existing turret route.
Facing and Range: If on a turret, the turret can only traverse to one adjoining facing per round. So if you are surrounded on all sides, you fire forward on round one, but not switch to rear in round two for example. You could go to left or right. On range, a turret could not hit anything closer than Medium range as the ship's profile would get in the way for anything closer. Unless it is a fixed mounting.
Or something like that.
