The SS 393 insignia.

Gun's Caliber, Bore And Length

The SSN 651 insignia.

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 The relationship of caliber in bore and length of gun. The length of the barrel (especially for larger guns) is often quoted in calibers. The effective length of the barrel (from breech to muzzle) is divided by the barrel diameter to give a value. As an example, the main guns of the Iowa class battleships can be referred to as 16"/50 caliber. They are 16 inches in diameter and the barrel is 800 inches long (800" / 16" = 50 caliber). 
The 6 common calibers of U.S. submarine guns were: 20mm/70cal, 40mm/70cal, 3"/50cal, 4"/50cal, and the 5"/25cal, which is seen in the example below.

An exception to all this is the .50cal or "50cal" Machine gun*, which is a 0.50inch or 12.7mm bore gun and caliber has no reference to barrel length. It's barrel is 1.143m or 45" long, so it would actually be a .5inch/90cal or 12.47mm/90cal gun.
*Refers to the Browning M2 .50cal hb machine gun.

This information was found in and is from an entry in Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors.

caliber_bore_length.jpg is an adaptation of Caliber_bore_length_gdl. png. Which is an Illustration of the relationship of calibre to bore and length of gun.
Created by Graeme Leggett 8th February 2005.
Copyright Free.
Original created in Adobe Illustrator.
From Wikipedia, the free encyclopedia 

Drawing adaptation by Jerry Uffelman 11 February 2006

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