The armament atop B turret is not a gun but an anti sub device called an UP launcher. From HMS Hood site:
http://www.hmshood.com/ship/specifications/aaa.html#UP7" Naval Wire Barrage, Unrotated Projectile
Type: Mark I
Number of Tubes: 20 (smooth bore)
Rocket: 7 in./178 mm fin stabilised rocket (3 in. / 76 mm motor)
Warhead: 8 oz / .14 kg aerial mine (CE)
Fuze: No. 700 and No. 720
Rate of Fire: Salvoes of 10
Maximum Altitude: @ 1,000 ft / 305 m
Elevation Angle (Max. / Min.): ? ° / @ 45°
Training: 360°
Comments: Hood was outfitted with 5 UP mounts which were added in 1940. One was atop "B" turret, and the remaining four on the Shelter Deck. In general, the UP, or "Unrotated Projectile," (AKA "rocket-on-a-string") was a largely unsuccessful attempt at a low altitude anti-aircraft defence system. Concept- When hostile aircraft were detected, the UP "layer" (operator) would manually launch a salvo/spread of 10 rockets.
Upon reaching @1000ft, the rockets would detonate and expel mines attached to three parachutes by @400ft of cable. Aircraft flying through the aerial minefield would snag the cables and pull the mines into themselves. In reality, the system was slow & cumbersome. The aerial minefields were clearly visible & easily avoidable. This, coupled with other factors such as the volatile nature of the cordite/rockets & the risk of mines floating back onto the launching ship, doomed this weapon to mothballs. There are no records of UPs bringing down any aircraft.