Naval Gun Terminology & Definitions

Page last updated: 24 September, 2016

Much of the information below was extracted from Tony DiGiulian's excellent treatise on Naval Guns which can be found at:


Naval Guns are usually classified by "caliber" (the diameter of the bore) and "calibers" (the 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 United States Navy, has a bore diameter of 16 inches, a barrel length of 800 inches (16 inches x 50) and is the seventh version of the 16 inch gun with no modifications to the original Mark 7 design.


AA - Anti-Aircraft.

Bag Guns - Weapons that used powder bags rather than a cartridge case to hold the propellant.

Bayonet Joint - A method of attaching the gun barrel to the housing in order to make for easier barrel replacements.

Bolt - The mechanism that positions the cartridge in breechloading guns, closes the breech, and ejects the spent cartridge case.

Bore - Inner Diameter (ID) of the barrel as measured from land surface to diametrically opposed land surface.

Breech - The rear of the gun

Chamber - Part of the gun in which the propellant charge is placed.  In a bag gun, that space between the obturator or breechblock and the forcing cone.  In fixed or semi-fixed ammunition, the space occupied by the cartridge case.

DP - Dual Purpose.  Guns  used against both surface and aircraft targets.  Maximum elevation of these guns is usually greater than 50 degrees.

Grooves - The spiral grooves cut into the surface of the bore and into which the soft metal projectile driving bands are forced during its travel down the bore.

IV - Initial Velocity of the projectile upon leaving the barrel of the gun.  Equivalent to "Muzzle Velocity."

Lands - The bore surface between rifling grooves inside the gun barrel.

Muzzle - The fore end of the gun where the shell comes out.

RF - Rapid Fire.  The USN term to describe large caliber guns with automatic shell-handling equipment.

ROF - Rate of Fire.  Usually shown in terms of RPM - Rounds per Minute.

RPM - Rounds (per gun) per minute.  The rate of fire of each gun in a multiple gun mounting.

SP - Single Purpose.  A gun that is intended for use only against surface targets.  Maximum elevation of these guns is usually less than 45 degrees.

Tampion or Tompion - (Pronounced "tom-kin.") A plug that goes into the muzzle of the gun to keep sea spray out of the barrel.

Trunnion - The cylinders upon which the gun barrel pivots up and down.

Working Pressure - The pressure generated inside the barrel by the burning propellant measured at the breech of the gun


Barbette - The fixed armored ring around the trunk of the mounting.  This usually extends from the gunhouse down to the armored or protective deck.

Base Ring - The bearing race upon which the entire gun mount turns. All of the fully enclosed and most of the open mountings for the 5"/38 were base ring types. This design distributed the weight of the gun and mounting better than did a pedestal type.

Blast Bags, Bloomers or Gun Bucklers - Canvas, rubber or neoprene covers around the barrel of a gun where it enters the gun port.

Casemate - An armored enclosure containing a gun mounting.  Unlike a turret, this enclosure does not rotate.

Elevation - The angle to which a gun can be moved on the vertical axis past the horizontal.

Gunhouse - The armored portion of the rotating structure extending above the barbette.

Handling Room - The compartment just below the gun mounting where ammunition brought up from lower storage locations is loaded into hoists or scuttles for transferring up to the guns.

Magazine - Compartment on a ship where ammunition is stored.

Passing Box - A small container mounted between compartments with a flap or door on each end and used to provide a flash-tight method of moving powder bags from one compartment to another.  Usually there is a mechanical connection such that only one flap can be open at a time.

Pedestal - The mounting on which the entire gun carriage turns on a roller-race.

Powder Room - A compartment where propellant charges are stored.

Sighting port - An opening for a gun-sight in the front of the gun-shield or turret glacis plate.

Superfiring - A gun mounted such that it can fire over another without elevating its gun barrels from the horizontal is said to be superfiring. On most ships with two forward turrets mounted on the centerline, the second turret is mounted higher than the first so that it may fire forward at almost any elevation.  The second turret is thus superfiring.

Train - The angle to which a gun or turret can be rotated on the horizontal axis.  For instance, a bow gun or turret pointing directly forward is said to be trained to 0 degrees.  If it could rotate to point directly astern, then it would be trained to 180 degrees.

Turret - There is always a controversy about whether a particular rotating enclosed gun emplacement should be called a "Turret" or a "Mount."  In the USN, the difference between a turret and a mount is that a "Turret" is built into the ship and extends well below the weather deck and includes a barbette. A "Mount" is not part of the ship's structure and does not include a barbette.  As a general rule, 5 inch and smaller guns are in "Mounts, " while 6 inch and larger guns are in "Turrets.

Turret Definitions - In the USN, when multi-gun turrets are described as "two-gun" or "three-gun" it means that their guns are individually sleeved and that each gun can elevate independently of the others.  When the mounting is described as "twin" or "triple" it means that all guns share a single slide or cradle and that individual guns can not elevate independently from the others.

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