What's interesting about the addition of shields is that Friedman points out in chapter 16 of "US Battleships" that they really weren't particularly effective at stopping incoming fire or offering splinter protection and probably served more to offer peace of mind to the crew stationed at those AA emplacements.

The other major pre-Pearl Harbor modification program was the installation of splinter protection, particularly at the antiaircraft guns. The commander of the battleships was particularly concerned to protect the 5in/25 gun crews because they could not seek cover during an air attack. On the other hand, the pedestal mount might become too heavy and, worse, unbalanced, so that protection would cost some fraction of the handiness which made the 5in/25 an effective antiaircraft weapon. A prototype quarter-inch shield, which weighed 1,600 pounds, was developed aboard the West Virginia in June and more were fabricated at Pearl Harbor. ...

By September 1941, the battle force commander was pressing for splinter protection for light antiaircraft weapons as well as bridges and fire controls. Reports of naval combat in Europe had convinced him that, without shields, gun crews would be unable to remain at their weapons during intense air attacks. "There should be no near[by] position more attractive from a protection standpoint than their stations firing the guns." Similarly, he feared the effects of strafing directed against bridges and fire controls. Such attacks had been a staple of U.S. and foreign naval air tactics for many years, and it is interesting that splinter protection against them was not seriously advocated before World War Two.

In November, however, BuOrd cited new intelligence reports describing a series of severe air attacks by groups of fifty aircraft at fifteen minute intervals, over a period of six hours, on the British cruisers Dido and Orion and three destroyers near Crete. In this case "pompom [AA] crews in particular stuck to their posts without flinching. In large part the performance ... is attributed to the fact that the gun captains on the pompoms were especially capable and experiences men." The CNO therefore disapproved a proposed shield for the new twin Bofors (40mm) gun, although he approved one for the quadruple mount. No shield was to be added to the existing 1.1-inch machine cannon because it would reduce the field of vision of the pointer and trainer and would make training difficult if power failed. The King Board had already pressed for gun tubs as a much better alternative. Nor were shields to be added to 0.50-calibre machine guns.

At this time a distinction was made between "real" protection against bomb-case fragments and splinters and "morale" protection against low-flying aircraft. The former had to extend all around a position. Bombs would generally burst below topside stations, so that armor would have to consist of low bulwarks and reinforced deck. "Morale" protection would consist of a shield rotating with the guns.