In actuality, the guns look pretty much the same in both cases. Take for example the US 5"/38 caliber, whether you are looking at enclosed gun mounts (the Fletcher class) or open mounts (the Enterprise class) it is exactly the same gun. The choice to enclose it or not was most often made on the basis of weight, or on exposure to the elements (spray, waves, etc.) not protection. The enclosed mounts were madeof relative thin steel which couldn't stop much more that a rifle bullet. Take a look at the US destroyers built in the 1930s (Farragutt, Mahan, Craven, Sims & Benson classes) many of them came with gun shields/enclosed mounts forward and open mounts aft. Again, the exact same 5"/38, if you removed the shield/enclosed mountfrom the forward guns they would look identical to the open mounts back aft.