Allocating an entire weapons system for weaponry in spaceships is actually arming it pretty heavily. A Major battery is enough to give a tank its main gun, and if building battleships you shouldn't allow them to use major batteries because then they would literally tip over.
Just allocating cargo space for the chain gun should work. If you want a fancy turret, figure out the weight for that: 200lbs or the weight of the gun again is probably a good starting point (whichever is higher).
As a note, spaceships guns don't scale down as fast as they should. I have an
alternative SM to Caliber System for that. If you use a 1/3rd size system (spaceships 7) and a very rapid fire gun with the above table you have a 15 mm cannon with ROF 10 doing either 15d pi++ or 20d pi++ depending if you use the stingy damage calculation or the generous one.