Sometimes you can surface for air, pop smoke, and use the gun to take pot shots at ships
BUT:
- The accuracy isn't real good at range (works good if your close)
- The gunner you need to use is high level (and adds a lot of weight)
- You could put a repairman on the slot and increase your repair
Or
- You could leave the slot open to save wieght and add some buldge or increase speed (increase speed is my favorite).
- So it really is a nice thing, but not real effective in terms of practicallity.