[DavidG]

In the ship design window you will see a 'Weapon Mount' button in the top right corner. click on and and select the 'mount type' you want to use. If for example you are building a Light cruiser you can click Large mount (I think that's the one) The weapon icons will now have an 'L' on them if they can use the mount. The weapon will now cost more but do more damage and depending on the mount may have increased range.