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parts, the block and the console or bracket, which is hinged to the face of the breech and holds the block when it is withdrawn from its seat.
Gas-Check (1871).
The breech-block is a steel cylinder having heavy screw-sectors about its surface to correspond with those in the seat. The last turn of screw-thread is left entire in order to relieve the shock of closing the breech. The forward face is made movable in order that when it becomes worn and scored by the action of the powder-gas it may be changed. This face is a steel disc which sets flat against the face of the block, and is of the same diameter. It seats in the face of the block by means of a smaller cylinder or trunnion, and projecting from the rear of this trunnion is a steel rod, which passes completely through the axis of the block and carries the vent. In order to prevent the disc from having any independent motion, a small tenon on its rear face enters a socket in the face of the block. The disc is keyed fast by a small pin which is screwed radially through the block near the middle, the end of the pin taking in a score cut in the vent-rod. A small catch, projecting from the rear lower side of the block, holds the console in place when the breech is closed. The rear face of the block is provided with two parallel handles screwed to it, to assist in moving it, and a long crank-handle between them for revolving the block. At the end of this crank-handle there is a slight projection made to receive the blows of any instrument that may be used to drive the block around when it works stiffly. Along the two lower blanks of the block slots are cut, in which traverse small projections of the console to hold the block steady. About the middle of the lower screw-sector a small hole is cut into which a small stop springs when the block is withdrawn to its limit on the console. The vent is provided with a copper bush at its forward end and a steel one at its rear.
The console is semi-cylindrical, and is hinged to the face of the breech so that when the breech-block is withdrawn it may be swung to the right and unmask the bore. It is provided with a spring stop and two small projections taking in the block, which limit the movement of the latter and hold it steady when resting on the console.
Two small pins, screwed into the face of the breech, limit the arc of movement of the crank-handle, one at the vertical point and the other 60° to the right.
A small stop attached to the face of the breech holds the crank-handle fast when the breech is closed. This stop allows the handle to pass in closing, but must be raised by hand in opening.
The gas-check is of copper, and is a fixture in its seat. It is a ring in reality, the centre being cut out to permit the passage of the charge through it in loading; the edge is turned up, forming a cup, the outer part being conical to fit the seat. The bend of the rim is quite thick, and a gutter is cut around it in order to make the powder-gas seal the joint properly. The bottom of this ring is provided with three concentric grooves to break the force of the gas that may escape through the joints. When the breech is closed the face of the block presses directly on the gas-check, and a copper ring is countersunk in the face of the block which forms the contact with the check. This ring has grooves to correspond with the grooves on the back of the gas-check.
The vent is horizontal, and is situated in the axis of the breech-block. Its outer end is formed in double-cone shape (throughout the steel bush), the small ends of the cone being joined. Percussion primers are used in connection with a spring gun-lock. ([See Primers].)