I think it is the 5"/25 open mmount naval gun. There are some links to the 5"/25 and 3"/50 guns .
picture of 5/38 open gun mount on WW-II destroyer
http://en.wikipedia.org/wiki/Image:5...Open_Mount.jpg
picture of 3 Inch 50 Caliber MK21 Deck Gun
http://shock.military.com/Shock/vide...88&ESRC=navy.n
http://www.valoratsea.com/350.htm
And here are some moore pics.
http://en.wikipedia.org/wiki/3%22/50_caliber_gun
United States of America 5"/25 Ammunition Type Fixed
5"/25 (12.7 cm) Marks 10, 11, 13 and 17 Ammunition Type All except the Mark 17: Fixed Weight of Complete Round about 80 lbs. (36.3 kg)
Mark 17: Separate
Ship Class Used On Nevada (BB-36), Pennsylvania (BB-38), New Mexico (BB-40), Tennessee (BB-43), Colorado (BB-45), Northampton (CA-26), New Orleans (CA-32), Portland (CA-33) and Brooklyn (CL-40) classes
Submarines in the 1940s
http://www.geocities.com/usstuscaloo...40_15jun44.jpg