It is interesting to mention that the propellent "powder" has combined in it some vaseline or other greasy matter which acts as a lubricant between the gun and the shell when firing takes place.
Shrapnel is so different from the other types of shell that it merits a short paragraph or two to itself. Instead of being filled, as the others are, solely with explosive, the front part of it accommodates a considerable number of small round bullets, behind which comes a charge of gunpowder. The front half of the shell is separate from the back part, the two being connected by rivets of soft iron wire, so that a sudden shock can rend them apart. The shell is fired from the gun and comes flying along: suddenly, owing to the action of the fuse, the gunpowder
explodes: the case then flies in two, the bullets are liberated and fall in a shower. In the South African War, where fortifications were few, these shells were very effective, but against fortifications, and particularly against trenches and barbed wire, big explosive shells are of much greater value.
CHAPTER XI
WHAT SHELLS ARE MADE OF
The body of a shell is made of steel of a fairly strong variety. That is to say, it is stronger than that used for shipbuilding and for bridges and such work: but it is less so than some of the higher grades of steel, such as that used for making wire ropes. Owing to so much of this steel being rolled during the war, "shell quality" has come to be as well known to the general engineer as any of the many varieties which he has been accustomed to since his apprentice days. Many people wondered, at one time, why the cheaper and more easily worked cast iron could not be used for shells. There was a period when the steel works were quite unable to cope with the demands for steel, yet the iron foundries were crying out for work. This question then arose in many minds, Why not make cast iron shells? The answer is that cast iron is too weak: it would blow into fragments too soon.
Just think what a shell is and what it has to do. It is a metal case filled with explosive. It is thrown from a gun and is intended to blow itself to pieces on arrival at its destination. It is that self-destruction
which carries destruction to all around as well. It is necessary, in order to obtain the best result, that an appreciable time should elapse between the ignition of the explosive and the bursting of the case. The force of the most sudden explosion is not really developed at once, but takes an appreciable time. After ignition, therefore, as the explosion gradually becomes complete, the pressure inside the shell is growing, and too weak a shell would go to pieces before the maximum pressure had been attained. Thus much of the energy of the explosion would simply be liberated into the air instead of being employed in hurling the fragments of shell with enormous force.
That is, of course, not a complete explanation of the whole action of a high-explosive shell, but it indicates generally the reason why a special quality of steel is required in order to get the best results.