Paul, obviously I wasn't suggesting that the barrel played a major part in offsetting the mass of the gun installation, just that when submerged the buoyancy it offered was considered to help the handling of the boat when submerged. David Brown is an expert on RN submarines and provided the MEX judges with very comprehensive information about his models and the original prototypes.
You had me worried there for a moment, I really thought that you were suffering from one of my senior moments as it would be like trying to float a challenger tank with a balloon on a stick.