Propelling charges are mixtures of explosives designed to propel projectiles from the gun to the target. In fixed ammunition, the propelling charge and projectile are assembled together in a case and handled as one unit. The principal component parts are the brass or steel cartridge case, the primer, and the propellant powder charge. In the separated ammunition, the propelling charge and projectile are assembled separately-they are stowed and handled as separate units until they are loaded into the gun. The propelling charge of the separated ammunition round consists of the propellant primer, details, and closure plug assembled into the metal case. The propelling charges of separate loading ammunition are made up in sections separate from the projectile and primer. Propelling charges for all calibers of ammunition have some common features. The basic type of charge is case ammunition. Saluting, reduced, and clearing charges have components that are the same as case ammunition, so they are included with case ammunition.
Propelling charges for small and medium caliber guns are assembled with primer and powder enclosed in a brass or steel container called a cartridge case. Assembly of the entire charge in a single, rigid, protective case increases the ease and rapidity of loading and reduces the danger of flarebacks. Also, the case prevents the escape of gases toward the breech of the gun; it expands from the heat and pressure of the burning powder and forms a tight seal against the chamber.
In case-type propelling charges, the propelling charge and primer are contained in a cylindrical metal cartridge case. This ammunition is of two types-fixed and separated. In fixed ammunition the primer, propelling charge, and projectile are assembled into a single unit that may be loaded into the gun in a single operation, In separated ammunition, the primer and propelling charge are contained in a cartridge case as a separate plugged unit; the projectile is also a complete, separate unit.
A complete round of separated ammunition consists of two pieces-a projectile and a cylindrical metal cartridge case sealed by a cork or plastic plug. Figure 2-4.-Tank-type 5AV54 cartridge case container.
Separated ammunition is used in 5-inch guns and their cases are kept in airtight tanks (fig. 2-4) until they are to be fired
A complete round of fixed ammunition is one piece, with the cartridge case crimped to the base of the projectile. Fixed 76-mm rounds are also kept in tanks, but smaller calibers and small arms are stowed in airtight boxes, several rounds to a box.
The insides of both the fixed and separated ammunition cartridge cases are quite similar. Figures 2-5
Figure 2-4 - Tank type 5/E/54 cartridge case container
Figure 2-5.-Typical cartridge case for separated ammunition, sectional view.
and 2-6 show the main components of both types of cartridge cases. The base of the primer fits into the base of the case so that the firing pin of the gun lines up with and contacts the primer when the breech is closed. A black-powder ignition charge runs the full length of the perforated stock or tube of the primer.
The 5-inch ammunition being issued to the fleet is assembled with case electric primers. The most notable exception to this practice is the 76-mm round that uses a percussion-only primer.
Look at the cartridge case in figure 2-5 again. When the gun fires, the case expands under the powerful pressure of the burning propellant gas, then must contract so that it can be removed from the chamber. It must not stick to the chamber walls nor may it crack. For a long time, only seasoned brass cases could be relied on to perform correctly. During World War II, when the supply of brass became critical, metallurgists developed a steel case that has since almost completely replaced brass. Regardless of what cases are made of, used cases are often called "fired brass." Steel cartridge cases are no longer reloaded and reused; however, since the cartridge tanks are required for reuse, the cases maybe returned in the empty tank for the scrap value.
Immediately after firing and before returning the cases to their tanks, the ejected cases (76mm and larger) should be stood on their bases to permit residual gases (small amounts left over after firing) to escape completely. Other cases should be replaced in the original containers, tagged, and stowed.
In the center of the base of the case is the threaded hole for the primer. The case tapers slightly toward the forward end so that it can be withdrawn from the chamber without binding. A rim at the base is engaged by the extractors of the gun. In fixed ammunition, the case often has a bottleneck in which the projectile is crimped.
Figure 2-6.-Typical round of fixed ammunition, cartridge case assembled with projectile, sectional view.
The propellant powder in the case is the seven-perforation kind we have already discussed. (Small caliber grains have one perforation.) The powder is weighed out with great precision and loaded into the case at the ammunition manufacturing facility. Since it does not take up all the space inside the case and since it would be dangerous for the powder to have a lot of room to rattle around in, it is tightly packed and sealed under a cardboard or pyralin wad The wadis kept tight by a triangular cardboard distance piece. The distance piece bears up against the plug that closes the mouth of the case. Infixed ammunition, the case is sealed by the projectile base.
A small amount of lead foil included in each propelling charge functions to clear the bore of the metal fouling that scrapes off the projectile rotating band onto the rifling as the projectile passes through the barrel.
Reduced Charge.- A reduced charge is one that contains less than the service load of powder. Reduced charges are often used to fire on reverse-slope targets and may be used in target practice to decrease wear on the gun.
Clearing Charge.- When a round fails to seatfully upon being rammed into the gun chamber (preventing closure of the breech) or when the propelling charge fails to function, the projectile maybe fired by extracting the full-sized case and loading a clearing charge that is shorter.
Saluting Charge.- These are charges used when firing a gun to render honors. Since no projectile is involved in such firing, the charge consists of a cartridge case containing a black-powder load and a primer. Ships normally employ 40-mm for saluting. Saluting charges for these guns are issued completely assembled, with no replacement components.