Thanks guys, I will check the sites you've suggested. Yes I agree, with all the room taken inside the sub, fitting a deck gun mechanism will be a challenge (but hey! I like challenges!
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You can see your sub underwater, but you have to surface in order to fire your weapons (deck gun in this case) unless you can come up with a torpedo exploding mechanism (not sure if this is allowed in the rules), kinda unfair when you think of it... oh well, welcome to the world of submarine warfare!