The vent being axial in the model of '90, enters the powder-chamber through and along the axis of the spindle of the obturator. In front of the gas-check seat in the model of '85 there is a cylindrical surface leading to the ellipsoidal powder-chamber, which is connected with the shot-chamber by a slope, and the radial vent enters 12½ inches from the rear face of the jacket and at the maximum diameter of the powder-chamber. The vent-piece, made of copper, is screwed into position, the part projecting above the surface of the piece being rectangular. THE BREECH MECHANISM. The principal parts are the breech-block, the carrier-ring, the obturator, the lever-handle, the bronze handle, and the vent-cover. [Illustration: THE BREECH-BLOCK. FIG. 30.] =The Block=, Fig. 30, is threaded for a distance of four inches from its forward end, and the circumference of the thread is divided into six equal parts. The threads of the block are removed along three alternate sectors, as is usual in the interrupted-screw fermature, to allow the block to slide in the corresponding slotted sectors in the jacket (or base-ring in earlier model). The threaded sectors engage in the corresponding slotted sectors in the breech-block, but one sixth of a turn of the block will engage the screw-threads of both. The interior of the block is bored out to form the obturator-spindle recess. At the end of the block are two lugs for the lever-handle. On the right one is a shoulder which limits the motion of the lever-handle. =The Nose= is the front end of the block, slightly reduced in diameter in order to partly enter the gas-check seat. =The Stop-groove=, into which the stop on the carrier-ring enters, is in the planed sector on the left-hand side. [Illustration: FIG. 31.] =The Latch-groove= (Fig. 31) is in two parts, one longitudinal (_d_), at the front end of the block, and the other transverse (_a_), at the rear end. The stem of the latch drops into the front groove (_e_) when the block is withdrawn, and into the rear groove (_b_) when the block is revolved into its firing position, in each case unlocking the carrier-ring from the jacket--in the first case, so that it maybe swung back; in the second, to prevent the breaking of the latch when the gun is discharged. =The Locking-recess= (_e_) is at the front end of the longitudinal latch-groove. The stem of the latch drops into it when the block is withdrawn, and thus locks the block positively to the carrier-ring. =The Guide-groove= is the cylindrical recess at the rear of the block in which the guide-sectors move when the block is rotated. [Illustration: THE CARRIER-RING. FIG. 32.] =The Guide-sectors= (_b_, Fig. 32) are three projections from the interior of the carrier-ring (_h_), which fit in the slotted sectors of the breech-block and guide it during its motion through the carrier-ring.