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
The necessity for compact loading, especially in the case of high explosive shell is also noted as otherwise the shock due to inertia would break up the charge and perhaps cause a premature explosion. Hence it is very necessary to guard against airholes in filling shell cavities.
In order to secure regular and uninterrupted movement of the projectile through the bore it is necessary that the projectile and bore have the same geometric axis. Also the projectile must be seated exactly and uniformly for succeeding rounds in its seating in the bore. This latter is necessary in order that the powder chamber may not vary as this would give irregular pressures. The liability of strapping the rotating bands or setting off the fuze in certain kinds of fuzes are also explained by inertia. It might even cause sufficient shock to detonate the charge in the shell. The remedy is accurate seating of each projectile by reason of trained gun crews using the same amount of force at each ramming. The first condition, coincidence of axes, is obtained by means of the ogival head which has a diameter some tenths of a millimeter smaller than the diameter of the bore, and serves as a front support for the projectile while the rotating bands center it in rear. Were it not for the bell the projectile, held only by the soft material of the rotating band, would wabble in its travel through the bore and tumble soon after leaving it. It is also necessary that the center of gravity of the projectile be on its geometric axis. Otherwise it will travel on a spiral of the same pitch as the grooves and knock the tube walls as it travels through the bore and without the support of the bell might cause a premature explosion by actuating the fuze.
155M/M SHRAPNEL—95 LBS.—MARK-I.
COMMON STEEL SHELL MARK I.
COMMON STEEL SHELL MODEL OF 1905.
SHRAPNEL.
CARTRIDGE CASE.
4.7” Gun Ammunition.