All naval guns except the converted 15-cm. calibre have a steel body strengthened by hoops. The converted gun has no hoops, it having been strengthened by boring the body up to form a casing, which was shrank over a steel tube. This tube extends from the breech-block to about one calibre beyond the muzzle. The 30½-cm. gun has three tiers of hoops, the 26-cm. and 24-cm. guns have two tiers, and the remainder one tier. The trunnions in all cases are in one with one of the hoops. The hoops are prolonged forward of the trunnions, diminishing rapidly in thickness. The bore is rifled on the multigroove plan, the rifling differing slightly according to the projectile which the gun was intended to fire. Formerly all projectiles were provided with a zinc rifling-jacket, and for such, a smooth shot-chamber was necessary, the rifling stopping at its forward end. The French style of copper bands being now used, the rifling is continued through the shot-chamber to the opening of the powder-chamber. With the zinc jackets, the grooves decreased in width from the breech towards the muzzle in order to keep a firm grip on the easily yielding metal throughout the bore. With the copper rotating belt the grooves are of the same width throughout. [Illustration] The powder-chamber is cylindrical, and about the depth of a groove wider than the diameter of the bore across the grooves. It is connected with the shot-chamber by a short cone which brings up the rear rifling-belt of the projectile in loading. In general the powder-chamber is concentric, but there are two calibres (26 cm. and 15 cm.) in which it is eccentric, its axis being slightly above that of the bore. The powder-chamber ends in a gas-check seat, which is coned and slightly countersunk forward to receive the Broadwell gas-check. Just behind the gas-check seat is the transverse breech-block seat, three of its sides being flat, and the rear or bearing side being hollowed out to a semicircular wall. The bore of the gun is continued straight through the breech, forming a loading-hole in rear of the breech-block. [Illustration: Broadwell Gas-Check, Seat, and Face-Plate.] The breech-blocks of all guns traverse the breech transversely, and they are all single blocks. There are two patterns, differing only in the shape of the rear portion, one being cylindrical and the other flat. The latter type is found only in guns where formerly a double wedge (the Kreiner system) was used, so that in changing to the single wedge there was not metal enough left in rear to permit hollowing out that face. The breech mechanism consists of the breech-block, the covering-plate, the transporting-screw, the locking-screw, the curb-chain, the gas-check, the face-plate, and the vent.