As with most things in life, there is [usually] always a reason ...however to train a 4.5" Gun Mount to 180 degrees requires one weapons computer systems 'pointing/non pointing zone' attributes to be racked out, with then a second computer controlled electro-hydraulic system override manually activated to enable the mount with the barrel to be trained and pointed toward the Bridge
In our RAN, such an exercise requires the explicit physical permission of the vessels Captain/Commander who then must also be in command of the vessel [on the bridge] during such an activity
After any maintenance work is carried out on a mounts training system, tests are conducted to prove that the mount cannot exceed its point positions
Could I suggest this image would not emblazen the walls of the Wardroom onboard HMS D23
Derek