The _jacket_ is usually in two pieces and is shrunk on the tube; it extends the entire length, and its rear end is threaded in the inside for the attachment of the “breech bushing.” _Hoops_ are shrunk on over the jacket and in a big gun are sometimes as many as six or seven in number. The liner, tube, jacket and hoops are made of the finest quality of open hearth steel, and the steel must conform to specifications set by the government. [Illustration: Photo by Bethlehem Steel Co. This photograph shows a mould for a gun ingot under hydraulic press for fluid compression.] The chemical composition having been determined, the necessary elements are weighed out and the whole charged into an open hearth furnace. When the furnace is ready to be tapped the molten metal is run into a large ladle, which in turn is taken by a crane to the casting pit, where the mould is filled. The ingots for the large calibre guns run from 42-inch to 48-inch in diameter, and after being poured they are immediately run under a hydraulic press, where they are subjected to a pressure of about six tons per square inch to drive out the gases, and then lowered to about 1500 pounds pressure per square inch for a certain length of time during the cooling. This pressure tends to make the ingot solid, by expelling the gases, which would cause blow-holes, and by preventing “piping” and “segregation.” When a metal cools, the top and sides cool first, and this outer layer shrinks and pulls away from the centre, with the result that a cavity or “pipe” would be formed, but the hydraulic pressure forces fluid metal into this cavity and so prevents the “pipe.” The cooling also causes the various elements to solidify separately, and they tend to break away from the mass and collect at the centre; this is called “segregation,” and is also partially prevented by fluid compression. A solid ingot, however, is obtained, and this is absolutely necessary. After the ingot has cooled sufficiently it is “_stripped_,” that is, it is removed from the mould, and then it is sent to the shop to have the “discard,” or extra length, cut off. When the ingot is cast, an extra amount of metal is poured into the mould to permit this discard, the theory being that the poorer metal, together with gases and other impurities, rise to the top. The government specifications require that there shall be a 20% discard from the upper end and a 3% discard from the lower end. The discard having been cut off, the ingot is “cored,” that is, its centre is bored out, the diameter of the hole depending on the size of the ingot. [Illustration: TAKING THE BORE OF A BIG GUN Photo by Bethlehem Steel Co. This photograph shows gun ingot in boring mill being cored.]