the first number is the shell diameter in inches. For the metric people like me: 3 inches is 76 mm, 4 inches is 102 mm and 5 inches is 127mm the second number is the "caliber length" that means the lenght of the barrel. so a 4 inch caliber 50 weapon has a diameter of 4 inches and a lenght of 50 x 4 inches.
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