Updated: October 16, 2001
6-inch Rapid Firing Gun, Model M1900, at Fort Hancock, NJ
Photo: RW - 1986 Model M1903 6-inch Rapid Firing Gun on Shielded Carriage
The 6-inch guns were medium range (15 miles) guns used for coastal defense against light cruisers, destroyers, and other light craft. These guns were mounted on disappearing carriages or shielded barbette mounts. The M1900 carriage only provided a frontal shield, but later carriages included a wrap-around style shield that protected the gun and crew from three sides and from above. These guns had a rate of fire of 4 round per minute, and were called "Rapid Fire" guns.
Various type of 6" guns were utilized for coastal defense:
M1903A2 (M1903 modified for air scavenging)
M1905A2 (M1905 modified for air scavenging)
Photo: Model M1900 6-inch Rapid Firing Gun on M1900 Carriage
Various type of carriages were utilized to mount these guns. All
carriages required prepared concrete emplacements.
M1 Barbette (Waterbury Electric Elevation Drive)
M2 Barbette (Atlantic Elevator Company Electric Elevation Drive, Gun automatically pointed in elevation)
M1900 Barbette (All manual operation, front shield)
These guns utilized either a electric or friction primer that was
inserted into the obturator spindle at the rear of the breechblock and retained by the firing lock. This primer was either electrically fired or initiated by pulling a lanyard to ignite the small powder charge located within the
The breechblock of these guns was the Interrupted-thread type, mounted on a hinge at the right side of the breech. Compressed air was used to close the breech.
The breeches of these guns were equipped with a "DeBange
Obturator". This device was used to seal the breech to prevent the damaging escape of hot propellant gases during
firing. These guns used a powder charge contained in cloth bags instead of a one-piece brass cartridge. When a gun with a brass cartridge is fired, the obturation is performed by the case
expanding and this prevents any gas from escaping through the breech. Other types of guns using brass cartridges do not require an obturator mechanism.
The nitrocellulose powder charges were contained in cloth bags that were made from a special raw silk known as "cartridge cloth". This cloth burns without leaving any smouldering
residue in the barrel which would present a safety hazard when loading the subsequent round. These powder charges were stored in separate magazines from the shells for safety reasons. A 32 pound power charge was used with the HE shell, and a 37 pound charge with the heavier AP shell.
Two basic types of projectiles were manufactured for these guns. Armor Piercing (AP) shells containing 4.5 pounds of Explosive D and weighing 108 pounds, and High Explosive (HE) shells
filled with 14 pounds of TNT and weighing 90.5 pounds (unfuzed). The AP shell used a M47 Point Detonating Fuze, had a thicker shell, and used less explosive filler.
Photo: HE and AP Shells Fuze, Primer, and Propellant Bag for 6-inch Guns
The caliber of these guns were usually about 50. The term
"caliber" in this case, refers to the ratio of the length of the barrel to the diameter of the bore. A 50 caliber gun with a 6 inch bore will have a barrel length of 300 inches or 25 feet. (6" x 50cal = 300 inches/12 inches per ft = 25')
The Gun Carriage:
A manual crank handle was used to aim these guns in azimuth. Later carriages were equipped with the M7 automatic data transmission system that allowed the crew to simply match the gun position to the pointer. The pointer was remotely set by the plotting room after the correct values were plotted and corrected.
The gun was elevated using a manual hand wheel, or on later model guns, an electric-hydraulic elevation motor system was installed. Maximum elevation was 47.5 degrees, maximum depression was -5 degrees. The M2 carriage was equipped with the Atlantic Elevator Company Electric Elevation Drive, which automatically pointed the gun in elevation. A safety interlock prevented the gun from moving while the breech was open.
Hand ramming was used to load the gun. No power rammers were required or installed. The M1903 and M1905 guns used a loading elevation of 10 degrees.
The recoil mechanism utilized spring type recuperator cylinders and dash-pot counter-recoil buffers to absord the recoil of firing and to return the gun to battery. The recoil cylinder was filled with 4 and 1/2 gallons of heavy oil.
A 75mm gun could be mounted on the main gun tube for training purposes to allow gun crews to practice firing drills without using expensive ammunition. The 75 mm T16 guns (for the
M1903A3), or the 75mm T17 guns (for the M1905A2) were used for this purpose.
Firing the gun:
When the firing lock hammer was tripped by a lanyard or by the
application of an electric current, the small primer cap within the primer ignited the black powder in the primer, which fired a jet of flame through the vent in the breech block, which in turn ignited the igniter charge of black powder located on the back of the powder bag.
This ignited the main charge in the powder bag, which continued to burn while the projectile was forced out of the barrel. The soft copper rotating band on the projectile engaged the lands of the rifling in the barrel and forced the projectile to rotate. This rotation stabilized the projectile in flight and provided for accuracy. Once the projectile cleared the muzzle, any unburned powder continued to burn, but did not increase the velocity of the projectile since the expanding gases in the barrel could no longer exert any force on the rear of the projectile. An air scavenging system was installed on later model guns (A2 versions) to clear the breech of any burning residue with compressed air after the gun was fired. This protected the crew from the potential hazard of residual burning embers igniting the new powder charge while the breech was open for loading.
Cleaning the Gun:
These guns required cleaning after firing to protect the barrel from corrosion. Long rammers with burlap tips were dipped in a solution of soap and water to clean the barrel. All parts were cleaned and oiled to prevent the salt air from corroding the steel parts. Sources of Data:
ROTC Manual, Coast Artillery, Basic, 10th Edition, 1938
US Army TM 9-2300, pages 130-131