The material of which these guns are made is nickel chrome gun steel. Steel is ordinarily an alloy of iron and carbon, but this metal also contains traces of nickel and chromium, which make it specially suitable for its special purpose.
Each of the tubes of which the gun is formed start as an ingot, a mere lump of metal, but roughly shaped. The requisite mixture is obtained in a furnace and the molten metal is run out into a mould. The ingot is heated again and pressed under enormous hydraulic presses until it is approximately the shape required. This pressing not only produces the desired shape, it also improves the quality of the metal.
The rough forging is then bored out, to make it into a tube. One is inclined to wonder why the ingot is not cast hollow to commence with, and so save the labour of boring it all out later. The explanation of this is that certain impurities are always present in the metal and these always gather together in the part which sets last. Now in a solid block or ingot it is clear that the centre is the part which will set last, and hence that is the part where the impurities will congregate. Then, when the centre part is all bored out the impurities are entirely removed.
The tube is shaped externally by being turned in a lathe.
The innermost tube is not simply smooth. There is a spiral groove, called the "rifling," running round and round, screw fashion, inside it. The purpose of this is to give the shell a spinning action which causes it to keep point foremost throughout its flight. But for this the shell would tend to turn over and over, resulting in uncertain and inaccurate flight.
The shell is a little smaller than the bore of the gun, but near its base it has an encircling band of soft copper, which band is a tight fit in the gun. The soft copper crushes into the "rifling," whereby the shell obtains its spinning action.
The large guns are mounted in pairs, each pair on a turntable, by the movement of which to right or left they are trained upon the distant target. The turntable is surrounded by a wall of thick armour and is covered by an iron hood or roof.
In addition to being turnable to right or left, there is, of course, provision for raising or depressing the direction in which each gun is pointing. They need always to point more or less upwards, and the particular angle depends upon the range or distance of the object aimed at. This is ascertained by range-finding instruments and communicated to the officers in the turrets, as the covered turntables are called. The guns are then elevated or depressed to suit the range.
Each gun rests upon a cradle which is itself fitted upon a slide. When it is fired it "kicks" backwards, against the force of a buffer of springs, or a hydraulic or pneumatic cylinder. Thus after each shot the gun moves backwards upon the slide, but the hydraulic apparatus brings it back again into position for firing almost instantaneously.
In naval guns all the movements, including that of the turntable, are by power, either hydraulic or electric, or a combination of the two. The loading is also by power.