Diagram showing design of shell for the German "Kaiser Wilhelmgeschutz" or "Paris Gun", as far as was known by the French and British in early 1918.

Accompanying text :

The shell presents features that are distinctly novel. In the first place it is built up, instead of being made as a whole; in this way manufacture and filling are much facilitated. Secondly, it is divided into two principal parts, entirely different in characteristics and functions.

These are: (a) The body or shell proper; (b) the head.

The Body. - The walls of the shell are abnormally thick, tapering towards the front, where the danger of deformation due to "set back" diminishes. This is an indication that the pressure to which the gun is worked is in all probability greater than usual, but it also serves another purpose - one that is evident in the entire construction of the shell - and this I will refer to later. The function of the two copper bands AA is that of centering and steadying the shell in the bore and of effecting the gas seal. Since they take the rifling, they indirectly assist in rotating the shell, but this is not their "métier"; and to effect this the shell itself is rifled, which is a reversion to old practice. Rifled projectiles were used by ourselves many years ago, but the design was naturally more crude. The thrust of the rifling of the gun on the shell, in imparting the necessary rotation for stability of flight, is thereby much more evenly distributed along the shell, and failure to rotate, which might result if the two copper bands were alone called upon to do this work, is thus eliminated.

Assuming a maximum working pressure of 21 tons/in², the acceleration of the projectile at the point of maximum pressure is about 250,000 feet per second, which is enormous, and consequently the "set back" pressure is unduly high.

The diaphragm B increases the strength of the shell, and by dividing the bursting charge into two parts, lessens any risks there might be due to the "set back" of the explosive towards the base. It is understood that it is also called upon to support an additional "impact" fuze inside the shell, which tends to obviate all possibility of "blinds," and serves to ensure the bursting of the forward compartment. It appears to me that this last-mentioned function is its real "raison d'être." The screwed socket C also holds a base impact fuze.

The capacity for high explosive is small, and the burster would weigh about 33 pounds, or 10 per cent. of the weight of the shell. With such thick walls and a small burster, the projectile would break up into a few large pieces, the resulting damage in all probability being small.

The Head. - A curious feature, although not entirely novel, is a false head, which is screwed on to the body or shell proper. It is a comparatively thin-pointed steel dome, and I should say is struck with a radius of about 10 calibres. I have assumed such a radius in estimating the ballistic efficiency of the projectile. I am not aware if the head has been "reconstructed" from parts which may have been found, so that this value is hypothetical, although a reasonable one. The function of this head is to diminish the air pressure, as I have previously explained, but it also serves another purpose - that is, to throw the centre of pressure well forward of the centre of gravity - a principle which, given the requisite twist of rotation for stability, increases the steadiness of the projectile during flight. The analogy of the peg top and the teetotum serves us here. Given the requisite spin, the peg top in which the C.G. is situated high up, is much steadier and maintains its steadiness better than the teetotum in which the C.G. is low down near the point. The pressure on the head of the projectile when leaving the gun is about 1 ton, and the head must be strong enough to withstand this pressure. There is one more point which I might conveniently mention here - and that is this: In a vacuum, a rotated projectile would travel on its path in the same relative position to that at starting, and its axis would become more and more oblique to the trajectory. The presence of air resistance causes a rotated projectile to keep its point nearly in the trajectory. Hence, although the air must be very rarefied in the upper part of the trajectory which I have described, yet there must be sufficient resistance to keep the "yaw" within limits until on descending again into the denser strata the projectile becomes steadied, otherwise it would become quite unstable, the result being "blinds" and short ranging. This I believe not to be the actual case.