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12-inch/50-caliber gun
Reference material by Wikipedia contributors. This retains its source identity and is not 411 news reporting.
There have been a number of 12" guns with 50 caliber length:
12-inch/50-caliber Mark 7 gun, 1910 US naval gun
12-inch/50-caliber Mark 8 gun, 1939 US naval gun
12-inch/50-caliber gun (Argentina), probably based on the 1910 US naval gun, used only in the Argentinian Rivadavia-class battleships
BL 12-inch Mk XI – XII naval gun, British naval gun
30.5 cm SK L/50 gun, German naval gun
Obukhovskii 12-inch/52-caliber Pattern 1907 gun, Russian naval and rail gun