[Questor]

You need to select the Heavy Mount for your weapons when you create your ship design. Before you add your weapons to your design. Click on the weapon mount button on the right and choose Heavy Mount from the list. When you add the weapons to the ship design you will notice a 'H' IIRC on the weapon. You will also notice that the weapon does more damage, takes up more space, etc. Hope this helps.

-- Questor