yeh what dave and rick said. You could, after you put the extra edge loop in...extrude the 4 faces inwards...adjust the verts to make them more square, extrude 1 more time to create a clamping edge then delete the 4 faces. Then extrude inwards to make the canon hole or outwards.

EDIT...that didnt work for me?? it did last time LOL

cheers bullet