The breech-block is a heavy solid steel block, generally cylindro-prismatic, sometimes square in section. The rear of this block is not parallel to the front, but inclined in wedge shape at an angle of 1° 55´. The prismatic part of the block is slightly wider than the diameter of the cylindrical part, forming at the junction an edge at the top and bottom, which serves as a guide for the block in sliding in and out. The block is much shorter than the width of its seat, so that it is not necessary to entirely withdraw it in order to unmask the bore. Except for special guns, the breech-block always draws out to the left, and its left end has bolted to it a steel plate of the same size, called the covering-plate. This plate merely serves as a holder for the locking and moving mechanism. With light breech-blocks a shackle-handle bolts to the centre of this plate for withdrawing the block. With the heavier calibre, where the block cannot be moved by hand it is worked in and out by a screw. This transporting-screw lies along the top of the block from end to end, revolving in journals; one half of the circumference only lies in the block, the other half, projecting, takes in a half-female thread in the upper wall of the gun. The end of the screw projecting beyond the covering-plate is squared so as to permit a crank to be shipped. In this manner, by revolving the screw, the block is worked out or in. [Illustration: Breech-Block (front).] [Illustration: Breech-Block (rear).] The locking-screw, as its name implies, serves to lock the block. It is on the rear part of the block, placed like the transporting-screw only half in the block, so that its thread will take in the gun-wall. Besides locking the breech, it forces the block close home and releases it, taking the strain from the transporting-screw, which might otherwise be bent by the shock of firing. The threads of the locking-screw, except the first or outer turn, are cut away for one third of the circumference, so that the action of locking and unlocking is similar to that of the French mechanism. The outer thread is left full to cover the joint when the breech is closed. A stop on this thread limits the revolution of this screw by catching on the covering-plate, so that when brought up at one point it shows the breech to be locked, and at the other that it is unlocked. The same crank is used to work the transporting-screw and the locking-screw. The curb-chain is a short chain which limits the withdrawal of the block and prevents it from being pulled all the way out. One end is fast to the lower end of the covering-plate, and the other to the gun just underneath.