Naval Guns are usually classified by Caliber (diameter of the bore), Calibers (length of the barrel described in multiples of the diameter of the bore) and a Model or Reference designation.
For example, the description: USN 16"/50 Mark 7 Mod 0 means that the gun was developed by the Navy of the United States of America, has a bore 16 inches (40.64 cm) in diameter, a barrel length of 16 x 50 inches = 800 inches (20.320 m) long and is the seventh version of the 16 inch gun with no modifications to the original Mark 7 design.
Real guns still exist
Too bad nobody ever heard the words in the song
video killed the senses