Gun Construction

In the diagram (fig. o) the pressure developed along the bore is given for two different chambers with charges adjusted to give the same maximum pressure and muzzle velocity. In the case of the smaller chamber, the weight of charge is less, but it occu pies a relatively larger volume of its chamber and this increase in the "density of loading" results in a more rapid rate of com bustion through the greater confinement of the gases. This is modified to some extent by the lighter charge being composed of individual cords or elements of larger size, thereby exposing less surface to the igniting gases and tending to reduce the rate of burning. Features of importance in these pressure curves are the position of maximum pressure in the bore and the extent of the muzzle pressure. Both these features affect the position of the gun's centre of gravity as the pressure curve is reflected in the thickness of the gun walls. At first sight it would appear that all the advantages are with the lighter charge giving the lower forward pressure, which reduces the muzzle vibration and, in combination with the earlier maximum pressure, brings the centre of gravity towards the breech end, but consideration must be given to the effects on the muzzle velocity of usage and of small variations incidental to the gun, the charge and the projectile. The smaller chambered gun is naturally more sensitive to such changes and its regularity in muzzle velocity may be inferior. The life of such a gun may be shorter through loss in accuracy, though the measured wear or enlargement of bore be less than with the larger chambered gun.

The regularity in the muzzle velocity from round to round is an important characteristic of a gun and generally defines the minimum size of chamber and weight of charge. The "form" of the chamber as opposed to its capacity is to some extent in fluenced (particularly at the forward end) by the type of driving band fitted to the projectiles, and this also affects the character of the rifling. The form of chamber and shot seating is respon sible for the centring of the projectile and its initial steadiness which are of importance as affecting both accuracy and wear. Generally modern gun chambers are of medium capacity obtained by length rather than width and with small changes in section.

During the evolutionary epoch the value of shrinking tubes or hoops over each other to obtain increased circumferential strength in a gun had gradually emerged and been confirmed by experience. It was appreciated that in a monobloc gun such as the cast-iron gun of the early i9th century there was a limit to the increase of circumferential strength obtained by increasing the thickness of the material (this limit is practically reached when the thickness of the gun walls equals the radius of the bore). Towards the end

of the first epoch, Thomas Barlow had shown that the stresses produced by pressure in a cylinder are greatest on the interior surface and diminish in a ratio affected by the square of the dis tance from the centre to the exterior. Thus, as the thickness of a cylinder increases, the value of the material towards the exterior rapidly diminishes.

Basic Law.

It was further realised that the bore of the gun must not be permanently expanded on firing, in other words, that the gun must work elastically and in no circumstances must any of the material be permanently strained (stressed beyond its elastic limit). This is the basic law of gun construction. Steel has a certain elastic range, depending on its quality, both under tension and under compression stress. Thus if it is put in a state of com pression it will have an increased elastic range for subsequent ten sion stresses. The principle of initial tension or compression is the principle of making use of the compressive as well as the extensive elasticity of material such as steel. Initial compression is generally obtained by shrinking one tube on another. This produces compressive stresses in the inner tube at the expense of tension stresses in the outer. The theoretically ideal gun would be one in which the circumferential stresses produced on firing were uniform throughout the thickness of the walls. Such a gun would be in initial tension on the exterior gradually passing to compression in the interior, and on firing, all parts of the gun would be equally stressed circumferentially. This ideal effect can be partially achieved by shrinking a series of thin tubes or hoops over each other.

Wire Winding.

The effect can be more nearly reached through a definite section of the gun by winding wire at varying tensions for successive layers. It can be still more completely achieved in a monobloc steel gun by the cold forging process, sometimes known as "auto-frettage." Auto-frettage.—This process takes advantage of Barlow's law and by the application of fluid pressure to the bore of the gun or tube permanent expansion is produced through overstraining the inner layers of the material. The pressure is so controlled, however, as not to overstrain the outer layers which remain stressed within the elastic limit. After removal of the pressure the inner layers do not return to their original form, and the outer layers therefore remain in a state of tension and act compres sively on the inner. This condition is stabilised by "ageing" the metal by suitable mild heat treatment.