Guns of this type consist of a cast-iron body strengthened by a steel tube and steel hoops. The steel tube, which is about one quarter of a calibre in thickness, is inserted into the bore from the rear, and is set fast by a heavy male screw-thread worked at its rear end. It extends a short distance forward of the trunnions. The steel hoops are shrunk on over the cast-iron body, and cover all that part occupied by the tube. For the 14-cm. calibre there is but one row of hoops; for the higher calibres two rows breaking joints. The trunnions form a part of the hoop next to the forward one. In all calibres above 16-cm. this trunnion-hoop is thicker than the others, and forms a salient on the surface of the gun. [Illustration: 27-cm. French Gun, Model 1870.] The bore is rifled on the multigroove system, with an increasing twist of from 0° at the breech to 4° at the muzzle, the direction of the twist being from right to left (contrary to the invariable rule in other countries). The chamber proper is divided into four parts: the shot-chamber, the powder-chamber, the gas-check seat, and the breech-block seat. The shot-chamber is conical in shape, small end forward, the rifling vanishing at this point in an easy slope. The diameter of this end is the same as that of the bore measured across the grooves. The rear end is joined to the powder-chamber by a second small cone, against which the rear ring of the projectile takes when home. The powder-chamber is cylindrical, and of a diameter slightly greater than that of the bore. The gas-check seat is of considerably greater diameter than the powder-chamber. It is conical in shape, small end forward. [Illustration: Breech-Block (Gun open).] [Illustration: Breech-Block (Gun closed).] The breech-block seat is cylindrical, having a heavy screw-thread around its wall, which is cut into three equal sectors, leaving three blanks of the same width. One of these blanks comes at the bottom of the seat, in order to allow the breech-block to slide back and forth. The breech mechanism consists of two main parts, the block and the console or bracket, which is hinged to the face of the breech and holds the block when it is withdrawn from its seat. [Illustration: Gas-Check (1871).]