21 GRAIN PERCUSSION PRIMER MARK II-A
Primers. The devices for initiating explosions of propelling charges in military guns are called primers. With fixed and semi-fixed ammunition the primers are seated in the base of the cartridge case. In the case of separate loading ammunition the primers are inserted separately in the breechblock, the expanding gases of the detonated primer forcing the walls of the primer case tightly against the bore through the breechblock and thus sealing this channel of escape for the gases from the powder chamber. This necessitates a much larger and stronger case for separate loading primers than for those inserted in the base of a cartridge case.
Classes of primers. Primers are divided into three classes according to the method by which they are fired: (a) friction primers, (2) electric primers, (3) percussion primers. Combination primers are made which may be fired by any two of these methods, usually electric and one of the others. The characteristics of a good primer are, certainty of action, safety in handling, no deterioration in storage, simplicity in construction and be cheap to manufacture. They are also divided into obturating and non-obturating depending upon whether they close the vent during discharge or not.
Primer pressing. Primers for fixed ammunition are inserted in the base of the cartridge cases by means of a special press for this purpose. The primer body is a trifle larger than the seat in the cartridge case provided. This seat is rough bored to a diameter less than the finished size and then mandreled to finished dimensions with a steel tapered plug. This process toughens the material of the case around the primer seat and prevents the expansion of the primer seat under pressure of the expanding gases.
Percussion primers. Except for very heavy siege guns and railroad artillery the guns handled by the Field Artillery use percussion primers. The 110-grain percussion primer is the one in use in our service and as typical will be described. The charge consisting of 110 grains of compressed black powder makes the charge burn like a torch rather than explode, which facilitates the ignition of the charge of smokeless powder, with which the flame comes in contact. The diametral holes spray the flame in several directions thus insuring ignition at many points simultaneously. The percussion element consists of a percussion primer cup, the percussion composition and an anvil, all of which are assembled together in a cup in the rear face of the primer case. The percussion composition is made up of chlorate of potash, sulphide of antimony, ground glass and sulphur. A blow upon the cap by the firing pin detonates the percussion composition and the flame from this detonation ignites the black powder which in turn explodes the charge of smokeless powder.
The General Shape and Nomenclature of Projectiles.
The reason for the particular shape of shells may not be clear to all. In the first place all matter has the property known as inertia, which we may define as that tendency of matter to remain in a state of rest or to continue at a uniform velocity if in motion. It offers a resistance to any change in the state of either rest or motion whether of amount or direction. Consequently when we apply a sudden and tremendous force to the base of a projectile by means of the expansive force of exploding powder gases, there will be set up in the metal a resistance to this force in which every particle of the projectile will resist by an amount proportional to the mass of particles beyond the point of application of the force to itself. The actual force will be proportional to the weight and acceleration produced by the applied force in the projectile. This explains the reason why the walls of the projectile are thicker near the base. It also explains the method of calculating the thickness of walls, for if we know the weight at any cross section and the co-efficient of strength of the metal we may calculate the thickness of walls necessary to withstand the pressure for any given muzzle velocity which is fixed by other considerations. It explains also the preference for steel in projectiles as for the same weight the steel is much stronger making it possible to throw a greater amount of shrapnel or high explosives in shell.
155-MM COMMON STEEL SHELL MK IV