different ship, same model
-Werewolf- Tartarus Cannon refuses to unequip with a full hold (Armor upgrade fills my hold) yet I can unequipt everything else perfectly fine.
I get the "Insufficient Cargo Space" Message
There is currently a tractoring bug, which causes you to only tractor 1 Ammo (etc) at maximum.
About the cargo:
Tartarus Cannon takes up some cargo on your ship, so if the ship is full, it can't be unmounted because to unmount a gun that takes up cargo you need as much space on it as it takes up.