Big Bullets for Beginners
Guns are generally classified according to use, size, and tradition. This varies among the military services. The basic distinction is between small arms and artillery. Any gun below a 20-millimeter bore size is generally classified as a small arm. The US Army distinguishes among mortars, howitzers, and guns. Mortars give high trajectories with short range and are usually loaded from the muzzle. Howitzers give medium-to-high trajectories, and guns provide flat-to-medium trajectories of longer range. Bore size is usually given in millimeters.
A gun can be considered as a particular kind of heat engine. In operation, the propellant charge located in the gun chamber is ignited by the primer. Gases produced by combustion of the propellant grains cause a rapid buildup of pressure. When a certain pressure is reached (shot-start pressure) which overcomes the forces of projectile weight and engraving of the projectile in the rifling, the projectile begins to move toward the muzzle which causes an increase in chamber volume. A maximum pressure is reached a few inches from the origin of rifling followed by a decrease in pressure all the way to the muzzle. At the muzzle, the pressure is 10 percent to 30 percent of the maximum pressure, depending on the geometry of the propellant grains.
Artillery ammunition can be classified in many ways. Another classification is based on the manner in which the components are assembled for loading and firing. Complete rounds of artillery ammunition are known as either semi-fixed or separate loading. In contrast, small arms rounds are FIXED ammunition, with which it is not possible to adjust the amount of propellant in the cartridge case).
Semi-fixed ammunition is characterized by an adjustable propelling charge. The propellant is divided into increments, or charges, and each increment of propellant is contained in a cloth bag. All of the cloth bags are held together by an acrylic cord, and are stored in the cartridge case. The primer is an integral part of the cartridge case, and is located on the base. Semi-fixed ammunition may be issued fuzed or unfuzed. Semi-fixed ammunition is used in 105mm howitzers. The ammunition is shipped in a wooden crate, with two fiber tubes in each crate. The fiber tubes are sealed at each end with tape. Upon removing the tape, the cannoneer will place the heavy end down first, and remove the projectile from the fiber tube. Next, the cartridge case is removed. Both the projectile and canister MUST REMAIN in their fiber cups until firing.
Separate loading ammunition has four separate components: primer, propellant, projectile, and fuze. The four components are issued separately. Upon preparation for firing, the projectile and propellant are loaded into the howitzer in two separate operations. Separate loading ammunition is used in 155mm howitzers.
There are two explosive trains in each conventional round of artillery ammunition; the PROPELLING CHARGE EXPLOSIVE TRAIN, and the PROJECTILE EXPLOSIVE TRAIN. The projectile reaches the target area by the power obtained from the propelling charge explosive train. The function of the projectile in the target area depends on the type of projectile explosive train.
The propelling charge explosive train consists of the primer, igniter, and propellant. The propelling charge explosive train is initiated by the primer, which is a small amount of very sensitive explosive. The primer is very sensitive to shock, friction, spark, and heat, and must be kept protected and away from other ammunition components. In separate loading ammunition, the primer is a separate item of issue. The igniter provides hot flaming gases and particles to ignite the propelling charge. The igniter consists of black powder or Clean Burning Igniter (CBI). The igniter is very hygroscopic and subject to rapid deterioration on absorption of moisture. If kept dry, however, it retains its explosive properties indefinitely. The igniter for semi-fixed ammunition is an integral part of the primer. It consists of a perforated tube filled with black powder and is permanently mounted in the cartridge case. In separate loading ammunition, the igniter is in a circular red pancake shaped bag sewn to the base increment of the propellant. When ignited by the primer, the igniter sends hot flaming gases around the charge to ignite the propellant.
A propellant is a large amount of insensitive but powerful explosive that propels the projectile to the target. Semi-fixed ammunition propellant is generally issued with seven increments numbered 1 through 7, and connected by a thin acrylic cord. Each increment is a different size because each increment has a different premeasured amount of propellant. Increment 1 and 2 are single perforated and increments 3-7 are multi-perforated. Separate loading ammunition propellants are issued as a separate unit of issue in sealed canisters to protect the propellant. The amount of propellant to be fired with artillery ammunition is varied by the number of propellant increments. The charge selected is based on the range to the target and the tactical situation.
"Smokeless powder" propellant is actually neither smokeless nor a powder. It is called smokeless to distinguish it from black powder, and it consists of grains of various shapes and sizes, up to an inch long, depending on the weapon for which they are intended. These grains are either of nitrocellulose ("single-based" powder) or a mixture of nitrocellulose and nitroglycerin ("double-based" powder). Nitrocellulose is created by soaking an organic cellulose product such as cotton or wood pulp in nitric acid. The resulting mixture is combined with chemical solvents, forming a doughy, pliable mass. This nitrocellulose is then extruded through a press into long cords, which are cut into grains of the appropriate size. The grains are then dried and the solvents removed for reuse.
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