score:10
After some more research I stand by my earlier comment in general the gun ports had minimal protection, relying on small size and being in the shadow of the gun. There is a large amount of negative evidence for this, for example in his books Warrior to Dreadnought, The Grand Fleet and Nelson to Vanguard D. K. Brown does not mention protection to turret gun ports.
The positive evidence I cite the cross section of a 14" three gun turret on Eugene Solver's US Navy pages which shows no protection to the gun port. From the same site we have access to OP 769 Configuration of the Three Gun Turrets of the USS New Jersey which explains that there are upper and lower shield plates on the gun slide. These do not seal the gun port and at best provide protection from 6" projectiles and splinters. The arragements are shown in the accompanying illustration, from which their thickness can be measured which when compared to the face plate thickness is equivalent to about 3" thick. The relevant section of this document is shown below:
Further research turns up this entry on the NavWeaps site
Gun Port Shield - Curved armor plate attached to a gun barrel such that it seals the gun port in the glacis plate, regardless of the elevation of the gun. Gun ports are by their very nature weak points in the armor protection of a gun mounting or turret. Gun shields seal these openings and are intended to provide at least some measure of protection from shell splinters. In addition, many gun shields are designed so as to keep water and weather out of the interior of the mounting or turret. Some images of gun shields may be seen in these photographs of a USN 6"/47DP and a German 38 cm SK C/34.
A rough estimate of the gun shield thickness in this photo (reproduced below) can be made which is very roughly ~4" (maybe a bit more?).
The Yamato's gun shield appears to be similar in form to that of the German 38cm (15") gun shown.
Upvote:0
One factor not mentioned in the other answers is this:
Those openings are very small relative to both the size of the vehicle and the accuracy at engagement ranges of the weapons trained against it.
In other words, chances of the openings being hit are very small indeed, even if deliberately aimed for.
Hence they're typically either unarmoured or armoured against small arms (rifles, machine guns) fire only, their size being their main source of protection. A canvas sheet as seen on the guns of ships provides protection against water getting into the ship in rough seas or rain/snow conditions. Some land vehicles and even aircraft have similar sheets in place.
Upvote:2
Telling from your pictures of the Yamato and an Iowa-class, the holes were covered from the inside using steel, probably with less thickness than the turret armament. The combat environment of tanks and battleships is different, with battleships receiving more fire from higher elevation angles than tanks. This leaves mostly shrapnel from the deck as a threat to the interior of the turret.