5"/38 caliber gun

The Mark 12 5"/38 caliber gun was a US naval gun. The gun was installed into Single Purpose and Dual Purpose mounts used primarily by the US Navy. On these 5" mounts, Single Purpose (SP) means that the mount is limited to 35° elevation with no provision for AA shell fuze setters, and is designed to fire at surface targets only, while Dual Purpose (DP) means that it is designed to be effective against both surface and aircraft targets because it can elevate to 85° and has on mount AA shell fuze setters. The 38 caliber barrel was a mid-length compromise between the previous United States standard 5"/51 low-angle gun and 5"/25 anti-aircraft gun. United States naval gun terminology indicates the gun fired a projectile 5 in in diameter, and the barrel was 38 calibers long, making the 5"/38 dual purpose midway in barrel length between the 5"/51 surface-to-surface and the 5"/25 anti-aircraft guns. The increased barrel length provided greatly improved performance in both anti-aircraft and anti-surface roles compared to the 5"/25 gun. However, except for the barrel length and the use of semi-fixed ammunition, the 5"/38 gun was derived from the 5"/25 gun. Both weapons had power ramming, which enabled rapid fire at high angles against aircraft. The 5"/38 entered service on USS Farragut (DD-348), commissioned in 1934. The base ring mount, which improved the effective rate of fire, entered service on USS Gridley (DD-380), commissioned in 1937.

Among naval historians, the 5"/38 gun is considered the best intermediate-caliber, dual purpose naval gun of World War II, especially when it was usually under the control of the advanced Mark 37 Gun Fire Control System which provided accurate and timely firing solutions against surface and air targets. Even this advanced system required nearly 1000 rounds of ammunition expenditure per aircraft kill. However, the planes were normally killed by shell fragments and not direct hits; barrage fire was used, with many guns firing in the air at the same time. This would result in large walls of shell fragments being put up to take out one or several planes or in anticipation of an unseen plane, this being justifiable as one plane was capable of significant destruction.  The comparatively high rate of fire for a gun of its calibre earned it an enviable reputation, particularly as an anti-aircraft weapon, in which role it was commonly employed by United States Navy vessels. Base ring mounts with integral hoists had a nominal rate of fire of 15 rounds per minute per barrel; however, with a well-trained crew, 22 rounds per minute per barrel was possible for short periods. On pedestal and other mounts lacking integral hoists, 12 to 15 rounds per minute was the rate of fire. Useful life expectancy was 4600 effective full charges (EFC) per barrel.

The 5"/38 cal gun was mounted on a very large number of US Navy ships in the World War II era. It was backfitted to many of the World War One-era battleships during their wartime refits, usually replacing 5"/25 guns that were fitted in the 1930s. It has left active US Navy service, but it is still on mothballed ships of the United States Navy reserve fleets. It is also used by a number of nations who bought or were given US Navy surplus ships. Millions of rounds of ammunition were produced for these guns, with over 720,000 rounds still remaining in Navy storage depots in the mid-1980s because of the large number of Reserve Fleet ships with 5"/38 cal guns on board.

Mark 12 5"/38 cal gun assembly
Each mount carries one or two Mk 12 5"/38cal Gun Assemblies. The gun assembly shown is used in single mounts, and it is the right gun in twin mounts. It is loaded from the left side. The left gun in twin mounts is the mirror image of the right gun, and it is loaded from the right side.  The Mk12 gun assembly weighs 3990 lb.  The Mark 12 Gun Assembly was introduced in 1934, where it was first used in single pedestal mounts on the Farragut class destroyers, but by the time of World War II they had been installed in single and twin mounts on nearly every major warship and auxiliary in the US fleet.

Characteristics
The major Mk12 gun assembly characteristics are:
 * Semi-automatic : Some of the recoil energy is used to prepare to gun for the next round. The firing pin is cocked, the breech is opened, the spent powder case is ejected from the chamber, and a compressed air valve is temporarily opened to expel residue from the previous round through the muzzle.
 * Hand loaded : A projectile-man and a powder-man are stationed at each gun assembly. Their job is to  move the round consisting of a projectile and a powder case from the hoists or racks to the rammer tray, then start the ram cycle.
 * Power rammed : An electric-hydraulic ram is bolted to the top of the slide. This 5 to 7.5 hp system is designed to ram an 93 lb, 47.5 in long round into the chamber at any gun elevation in less than a second.
 * Vertical sliding-wedge breech block (See drawing): The breech block closes the chamber behind the powder case. It also holds the firing pin assembly.
 * Hydraulic recoil : Two hydraulic pistons in the housing absorb the major shock of recoil. They also buffer the end of counter-recoil for a soft return to battery.
 * Pneumatic counter-recoil : A chamber in the housing is filled with high pressure air. Projecting from the rear of this chamber is a piston. This piston pushes against to rear of the slide to return the housing back into battery (all the way forward; ready to fire).  When the gun is in battery, the pressure in this chamber is about 1500 psi.  During recoil, the pressure rises to about 2200 psi.
 * Barrel : The barrel of a 5"/38 caliber gun has a five inch (127 mm) diameter bore and a length 38 times the bore (190 in). It has chrome plated rifling of 45 grooves with a uniform right hand twist of one full revolution in 150 in.  The initial velocity of a new gun is 2600 ft/s.  It is connected to the housing with a bayonet joint that allowed for replacement by destroyer tenders in the theater of operation.
 * Ballistics : Maximum horizontal range, with a 55 pound projectile, is 18000 yd. In the antiaircraft role, it had a ceiling of 37,200 feet (11,300 m) at 85 degrees elevation.
 * Armor Penetration with 54 lbs. (24.5 kg) Special Common Shell:
 * 5.0" (127 mm) belt armour at 4,000 yards (3,660 m)
 * 4.0" (102 mm) at 5,400 yards (4,940 m)
 * 3.0" (76 mm) at 7,400 yards (6,770 m)
 * 2.0" (51 mm) at 11,000 yards (10,060 m)
 * 1.0" (25 mm) deck armour at 13,800 yards (12,620 m)
 * with 55.18 lbs. (25.0 kg) AA Common Shell: 1.5" (38 mm) at 10,000 yards (9,140 m)


 * Range with 55.18 lbs. (25.03 kg) AAC Mark 49 (792 mps):
 * 10° 9,506 yards (8,692 m)
 * 15° 11,663 yards (10,665 m)
 * 20° 13,395 yards (12,248 m)
 * 25° 14,804 yards (13,537 m)
 * 30° 15,919 yards (14,556 m)
 * 35° 16,739 yards (15,298 m)
 * 40°17,240 yards (15,764 m)
 * 45° 17,392 yards (15,903 m)
 * AA Ceiling 37,200 feet (11,887 m)

US ships during World War II carried only small quantities of Special Common ammunition as the AA Common were considered more useful as all around multirole ammunitions, even if this meant much less armour penetration. Bursting charges were 7.25 lbs.(3.3 kg) Explosive D Composition A, the Special Common had only 0,9-1,2 kg due to its thicker walls (AP).

Loading and firing
At the "load" command, the powder-man slips a primer protector off the end of the powder case, extracts the case from the scuttle or rack, and lifts it into the gun's rammer tray. Meanwhile, the projectile-man pulls a projectile out of the hoist or fuze setter, places it in the rammer tray in front of the powder case, and then pulls down on the rammer lever. This causes the power rammer to ram the projectile and powder case into the chamber. As the powder case clears the top of the breech block, the block automatically rises to seal the chamber. The gun is ready to fire. The case combination primer in the base of the powder case can be fired either electrically or by percussion. When the gun fires, the recoil’s rearward motion returns the rammer lever to the up position, and the rammer drives back to the rear of the rammer tray. During counter-recoil, the breech block is automatically lowered and the spent powder case is ejected from the chamber. When the gun has returned to battery, a blast of compressed air is sent down the bore to clean it out. The gun is ready to be reloaded.

Types
There are four basic mount types:


 * Twin
 * All twin mounts were enclosed, sat on a base ring stand, and had an ammunition-handling room, called the Upper Handling Room, below the mount. This type of mount was the standard installation on battleships, cruisers, early Destroyer Leader classes (these mounts on the DL's were single-purpose/anti-surface ship only), and later destroyers. It was also used on the island (starboard) side of the Essex-class aircraft carriers.


 * Enclosed single
 * All enclosed single mounts sat on a base ring stand, and had an upper handling room. The enclosed single mount was used on some early destroyer classes, but by the end of World War II, it was mainly found on the many minecraft and auxiliaries which were developed from the older classes of destroyers, as well as on most of the destroyer escorts, and many large auxiliaries (repair ships, destroyer tenders, etc.).


 * Open single base ring mount
 * Used on the port and starboard gangways, just below the flight deck, on escort carriers. Aft mounts of destroyer escorts.


 * Open single pedestal mount
 * This was the first 5"/38cal type installed. It was put on the Farragut class destroyer in 1934.  Some of these mounts placed on ship's forecastles were partially enclosed to protect the crew against bow spray, but they were still considered an open mount.  Since these  mounts did not have Upper Handling Rooms, they could be installed on ships without extensive reconstruction. For that reason, they were frequently used on armed merchant ships.

There are several models of the 5"/38, differentiated by the word Mark (or its abbreviation MK) and a number. Variations to the basic design are called Modifications (or its abbreviation Mod). For instance, 5"/38 MK 21 is a single-barrel open pedestal mount widely used on amphibious ships, auxiliaries, and merchant ships. The 5"/38 MK 30 is a single enclosed base ring mount widely used on Destroyer escorts. The 5"/38 MK 38 is a twin mount specifically designed for newer destroyers.

Ammunition delivery
Since this gun fires semi-fixed ammunition, each round is delivered to the gun in two pieces — a projectile and a powder case.


 * Base ring mounts : The ready service ammunition is kept in the upper handling room just below the mount. The projectile travels up to the gun room (also called the gun house) through an electric-hydraulic hoist. It arrives next to the projectile-man nose down and waist high.  If the projectile has a time fuze, the fuze is automatically set as it goes up the hoist, and the hoist maintains the ordered fuze setting from the fire control system as long as the projectile stays in the hoist.  The powder case is sent up through a powder scuttle in the gun room's deck just next to the powder man's feet.  It arrives with its base up, and the primer covered with a protector called the "Butterfly". The men in the upper handling room hand carry the projectiles and powder cases from the ready service racks to the lower ends of the hoists while avoiding the equipment hanging down from the rotating mount.  In a twin mount executing "Rapid continuous fire" (The firing keys are held closed, and the gun fires as soon as the breech closes), the crew move 30 to 44 projectiles and powder cases per minute.


 * Pedestal mounts : The ready service ammunition is kept in lockers or compartments arrayed around the mount. The projectiles and powder cases are hand carried from ready service to the left side of the moving mount.  The projectile is placed nose down in one of three Fuze Setter Mechanisms (commonly called fuze pots) on the mount.  If the man is delivering a projectile with a mechanical time fuze, he then spins a hand crank just in front of that fuze setter mechanism. This would dial in the ordered fuze time into that projectile.  The powder case is placed in a rack bolted to the mount's deck just behind the powder-man's feet.

Mount crew
Depending on the mount, a 5"/38 caliber gun could have a crew of from 15 to 27 personnel in the gun room and upper handling room. This does not include the personnel needed in the magazines during extended actions. There were two modes of mount operation that the crew was trained and expected to know.  The primary mode was "automatic control", where the mount was slaved to the fire control system.  But if the fire control system was damaged, or if the ship's power was out; the mount could continue the action in "local control".  In US service, most gun crews were US Navy personnel.  Even the civilian Merchant Marine ships had a small detachment of the Navy Armed Guard on board to operate the 5"/38 and other guns. One exception to this was on ships with a Marine Detachment, where the Marines manned one of the mounts, usually decorated with the Marine emblem. (See the USMC emblem on the USS New Jersey's Mk 28, Mod 2 mount picture above.)


 * Mount captain : A senior Petty Officer or Gunnery Sergeant who was in command of the mount. In enclosed mounts, he stood on an interior platform that was located half way up the back bulkhead of the enclosure. There was a hatch on the top of the enclosure where he could stick his head and shoulders out the mount's top.  On some mounts, this hatch had a steel hood welded around the back and sides (See an example of this hood in the USS New Jersey picture.) This hood protected the Mount Captain from the muzzle blasts of adjacent weapons.  He was wearing a sound powered telephone so that he could receive action orders from the battery commander, and send mount status reports back. Covering the telephone headset, there is a helmet specially designed to fit over the phone.  Around his neck, he has a pair of gunnery binoculars which had a reticule scaled in angular mils.  By ducking his head down into the mount, he could see the entire interior of the mount from his platform.  Next to him was a voice tube down to the upper handling room.  At arms length, he had switches for controlling communication, emergency lighting, and battle lanterns.  His duties during Automatic Control was to receive action orders from the battery commander (e.g.: "Mount 51, plot. Surface action starboard.  Target destroyer. Bearing 060. Range nine thousand, five hundred yards. Slow salvo."), give the appropriate orders to his Gun Room and Upper Handling Room crews (e.g.: "Match pointers.  Switch to automatic. Handling room fill the hoists with able able common and full service charge. Standby for slow salvo."), verify that his orders were being followed, and report his gun's status back to the battery commander (e.g: "Plot, mount 51 in auto. Bore clear.").  In Local Control, he aims and fires his mount's gun(s) — sometimes without external help.  With his gunnery binoculars, he estimates the range to the target, and its bearing rate.  He then mentally converts these into range and deflection orders to his Sight Setter.  After firing, he observes the fall of his shot, and makes sight corrections to his Sight Setter if necessary.


 * Gun Captain : The Gunner's Mate(s) responsible for maintaining the mount. Daily, he goes through a process of checking fluid levels, lubricating bearings, cleaning gun sights, cycling powered equipment, testing firing systems, checking gas pressures, and verifying that all the equipment that will be needed in an action is in his mount. In twin mounts, there will be a gun captain assigned to each gun assembly. The gun captain usually stands on his foot high tool box welded to the mount's deck, and offset from the gun's centerline. (See the man in black standing on the box in the MK21 open mount picture.) This places him high and aft of the rammer motor. From there, he can watch the actions of the powder-man, projectile-man, breech block, and rammer.  He can verify that the gun returns to battery before the next round is loaded. If something goes wrong, he is free to move around his gun to fix the problem.  He knows everybody's job, and can step in if necessary.  At the mount captain's command, he manually opens the breech block before the first round is loaded, and reports if the bore is clear.  His duties are the same in automatic or local control.


 * Pointer : 5in38calElvIndReg.jpg Controls the mount's elevation and firing. He sits in the left front corner of the mount. In front of him, are his optical sight, hand wheels, and elevation drive controls.  To his right, about elbow high, is a large box called the Elevation Indicator Regulator. (pictured) This box controls the elevation power drive. Through a window on top of the box, he sees a set of dials that indicate the elevation of the gun, and the automatic elevation orders coming from the Fire Control System. Above the Indicator Regulator is the Electrical Fire Select Switch. It is a rotary switch with three positions: Off, Local, and Auto. Off disables the electrical firing system on the mount.  Local enables the electric firing key on his right hand wheel.  Auto enables the off mount electrical firing circuit from the Fire Control System.  Just outside of his right knee, is the Percussion Select Lever. This mechanical lever has two positions: Safe, and Armed.  When it is in the armed position, the mechanical linkage for percussion firing is enabled.  His right footrest is at one end of this linkage, and the firing pin sear in the breech block is at the other.  By rocking his right foot forward, he fires the gun.  Electrical firing is the primary firing method.  When the Mount Captain commands, "Match pointers. Switch to Automatic.", he looks down at the dials on his Indicator-Regulator. The dials tell him the difference between the gun's present elevation and the ordered elevation electrically coming from the Fire Control System by synchro.  He changes the elevation of the gun, by moving his hand wheels, until the dial difference is zero.  His dials are now "matched", and he switches the elevation drive into Auto.  This disengages his hand wheels, and gives elevation control to the Fire Control System.  Then, he moves the Electrical Fire Select Switch to "AUTO", and reports back to the Mount Captain, "Elevation in auto."  He may now look through his sight, and if the Sight Setter has matched the sight-setter dial pointers, he will see the target in the cross-hairs.  When the Mount Captain commands, "Switch to Local", he switches the elevation drive and the Electrical Fire Select Switch to local.  In local control, he controls the gun's elevation with his hand wheels to keep his sight's horizontal cross hair on the target.  At the command of the Mount Captain, he fires the gun by squeezing the firing key on his right hand wheel.


 * Trainer : 5in38calTrnIndReg.jpgols the mount's train angle (bearing). He sits in the right front corner of the mount. (See the far right man in the Mk21 open mount picture.) In front of him, are his optical sight, hand wheels, and train drive controls. Between his knees is a large box called the Train Indicator Regulator. (pictured) This box controls the train power drive. On top of the box, is a window with a set of dials that indicate the train angle of the gun, and the automatic train orders electrically coming from the Fire Control System by synchro. When the Mount Captain commands, "Match pointers. Switch to Automatic.", he looks down at the dials on his Indicator-Regulator. The dials also tell him the difference between the gun's present train angle and the ordered train angle.  He changes the bearing of the gun, by moving his hand wheels, until the dial difference is zero.  His dials are now "matched", and he switches the train drive into Auto.  This disengages his hand wheels, and gives train angle control to the Fire Control System.  Then, he reports back to the Mount Captain, "Train in auto."  He may now look through his sight, and if the Sight Setter has matched the sight-setter dial pointers, he will see the target in the cross-hairs.  When the Mount Captain commands, "Switch to Local", he switches the train drive to local.  In local control, he controls the gun's train angle with his hand wheels to keep his sight's vertical cross hair on the target.


 * Sight setter : 5in38calSghtSttrDials.jpg Operates the sight setting equipment. In single mounts, he stands just behind the trainer (See the man with headphones in the Mk21 open mount picture), while in twin mounts he sits between the guns, just forward of the projectile hoists.  The sight setter moves the sights' reticules relative to the barrel's axis.  In early open mounts, this was done by moving the platforms to which the sights were bolted.  Sights in enclosed mounts have movable prisms in their optical paths.  The sight setter has three dials, and two hand cranks. (pictured) The two right dials and right hand crank control the elevation reticule offset (called Sight Angle ). The upper right dial is scaled in minutes of arc, and the lower right dial is in yards.  The left dial and hand crank control the reticule's left and right offset (called Sight Deflection ), and the dial is scaled in angular mils.  When the mount is in Automatic Control, he turns his hand cranks to keep index marks on his dials matched to lines on the central disks of the dials.  These disks are electrically controlled by the Fire Control System by synchro. This is called matching the pointers, and it allows the sights to remain on the target while the mount is controlled by the Fire Control System.  In local control, he takes sighting orders from the Mount Captain in yards of range and mils of deflection.


 * Fuze setter : 5in38calFzIndReg.jpg operates the equipment which sets the fuze time on projectiles with mechanical time fuzes. On a single enclosed mount, he sits below and just outboard of the Pointer's seat.  Under the Pointer's seat, and in front the Fuse Setter, is the Fuse Indicator Regulator. (pictured) It is a box with a window, a hand crank, and a selector lever.  On a twin mount, he and his Fuze Indicator Regulator sit next to the Sight Setter between the guns.  When the mount is put in Automatic control, he flips the selector lever to Auto, and this electrically powered Fuze Indicator Regulator automatically follows the fuze setting orders sent from the Fire Control System by synchro.  In Local Control, he follows the Mount Captain's fuze orders by spinning the hand crank until the dials in the window read the correct fuze time.


 * Powder-man : Slides the primer protector off the powder case, and then lifts the case from the powder scuttle at his feet to the gun's rammer tray.


 * Projectile-man : Moves the projectile from the hoist to the rammer tray, and then he pulls the rammer lever to load the projectile and powder case into the chamber.


 * Hot case man : When the gun fires, he catches the ejected powder case and throws it out of the mount.


 * Check sight : He verifies that the mount is aiming at the target.

Ammunition
This gun uses semi-fixed ammunition. (Pictured) (Also called Separated Ammunition.) Each round consists of a projectile and a powder case. The two parts of the round are kept separate until they get to the gun. At the gun, they are first combined on the rammer tray, and then power rammed into the chamber together with one ram cycle. The powder case completely fills the volume of the chamber, and its length seats the projectile's rotating band into the bore's rifling. This is different from a naval bag gun. In a bag gun: (1) The projectile, by itself, is power rammed into the chamber until its rotating band is seated in the bore's rifling. (2) The rammer is retracted. (3) The powder bags are then rammed in. This is also different form some artillery field guns: (1) The projectile is hand rammed into the chamber with a ramrod until its rotating band is seated in the bore's rifling. (2) The ramrod is retracted. (3) Next powder is placed in the chamber either as a bag or primed canister with the required charge. Note that the single ram operation used in semi-fixed guns decreases the loading time, and therefore increases the firing rate on medium and large caliber guns. For example, each of the nine 8"(203mm)/55caliber Rapid-Fire guns installed on the Des Moines-class cruiser used auto-loaded semi-fixed ammunition and had a firing rate of 10 to 12 rounds a minute. See also the 8"/55 caliber Mark 71 gun.

Projectile
The Projectile (pictured) has three major parts: the body, the fuze, and the explosive charge.

The body is basically a machined steel tube with an ogive shape at one end. At the ogive and rear ends are threaded openings used to the install the shell's filler and hold the fuzes. Around the tube near the base is a copper alloy ring called the Rotating Band. This band has a diameter larger than the bore, and when the projectile and powder case are rammed into the chamber, the band is jammed into the grooves of the bore's rifling. It forms a gas seal between the projectile and the bore. Also, as the projectile travels down the barrel, the band grips the rifling to impart spin to the projectile.
 * Projectile body

The fuze detonates the projectile to cause maximum damage to the target. Different targets required different fuzes. The most important requirement of a fuze is that it remains unarmed until the projectile is well clear of the gun's muzzle on the firing ship. To do this, the fuze remains unarmed until a series of events take place. The first event is the shock of firing. The 5"/38 cal projectile experiences at least a 14,000 g acceleration as it is pushed down the barrel by the burning propellant. This acceleration causes parts of the fuze to setback due to their inertia, and they shear pins that hold them in the unarmed position. The second event is the spinning of the projectile due to the bore's rifling.   The 5"/38 cal projectile spins at more than 200 revolutions per second when traveling at 2600 ft per second (New gun initial velocity). The centrifugal force of this spin causes other parts to move outward. The third event is the projectile's deceleration after it leaves the muzzle, and other parts creep forward. All of these events must take place in the correct order to arm the fuze.
 * Fuze

List of fuze types:
 * Mechanical time fuze
 * A nose time fuze that detonates the shell after an adjustable time interval has elapsed since firing.


 * Base detonating fuze
 * A base impact fuze screwed into the rear of a projectile to protect the fuze during impact. It delays the shell's detonation about 25 ms after impact, allowing the projectile to penetrate the target prior to detonation.


 * Point detonating fuze
 * A nose impact fuze. Very fast detonation on the surface of the target.


 * VT fuze
 * The VT (Variable Time) fuze is a proximity fuze. It is a nose electronic fuze that does not require impact to trigger.   Designed to detonate close to the target.  It was originally intended to be used against air targets.  Now it is also used in shore bombardment and surface actions against fast boats.  This is because the VT fuze has proved well suited for bursting the shell at the correct distance above the ground or water for maximum damage to lightly armored targets over a large area.


 * Auxiliary detonating fuze
 * In the projectile drawing, an Auxiliary Detonating Fuze is screwed onto the bottom of the Mechanical Time Fuze. This is because the time fuze primer does not have enough explosive shock to detonate the relatively insensitive Explosive "D". The Auxiliary fuze uses an intermediate explosive that is set off by the time fuze primer, and in turn, sets off the Explosive "D".

Powder case
The powder case is a brass or steel alloy cylinder closed at one end. It holds the propelling charge and a case combination primer. The charge is held packed around the primer by a wad, distance piece (not in clearing charge), and plug. When rammed in the chamber with the projectile, the Full and Reduced charge cases are designed to completely fill the volume of the chamber from the breechblock face to the base of the projectile when its rotating band is jammed into the bore's rifling. In other words, the powder case acts a rammer extension for the projectile. There are three types of powder cases:


 * Full service charge (pictured)
 * A 26.7 inch (679 mm) long, 12.3 pound (5.6 kg) brass case with 15.5 pounds (7.0 kg) of smokeless or flashless (used at night) powder. The Full Service Charge new gun initial velocity is 2,600 fps (792 mps). It is used in surface and anti-aircraft actions.
 * Reduced charge
 * The Reduced Charge uses the same case as the Full Service Charge, but with only 3.6 lbs (1.6 kg) of power and a longer distance piece.  The Reduced Charge new gun initial velocity is 1,200 fps (366 mps).  It is used in shore bombardment to lob shells, like a mortar, over obstacles to hit targets on the opposite side, and for propelling star shells at a lower velocity to protect the parachute from being shredded while it is deployed.
 * Clearing charge (Pictured)


 * The clearing charge (also called, "the short round") is a short case; plugged just above the wad. The distance piece between the wad and plug is left out. Its powder charge is less than a full service charge. This case is essential for the safety of the mount because it is needed to clear a gun after a misfire. Due to its importance, it is kept in a special container in the mount whenever the ship is in a combat zone. A misfire is especially dangerous in semi-fixed guns. When the breech is opened after the misfire, the faulty powder case can be extracted, but the projectile will remain jammed in the rifling. Also, all or part of the extracted case's cork plug may still be wedged in the chamber behind the projectile. This "fouls" the chamber because it decreases is volume. And, since Full and Reduced charge cases take up the full volume of an unfouled chamber (See loaded Semi-Fixed round drawing above), any residue left from the previously extracted case will prevent them from fully loading into the chamber. Therefore, the clearing charge is made short so that it will fit into the fouled chamber.


 * After hand extracting the bad case from the chamber, the clearing charge is removed from its special container and is hand rammed into the chamber. With the clearing charge sealed in the chamber, the projectile is fired out the muzzle. It is important to clear the projectile through the muzzle because it is not easy nor safe pushing a bore rod down the barrel to force a fuzed projectile back through the chamber and into the gun house. Also, if the gun has fired a number of rounds just prior to the misfire, time is critical because the barrel may be hot enough to cook off the high explosive in the projectile. This would destroy the mount.

Weapons of comparable role, performance and era

 * QF 4.5 inch Mk I – V naval gun : British equivalent dual-purpose gun.
 * 12.7 cm SK C/34 naval gun : German equivalent limited to low-angle fire.