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.