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.
Breech-Block (Gun open).
Breech-Block (Gun closed).