The Broadwell gas-check is a steel ring, fixed in the gun semi-permanently; that is, it does not move with the breech-block, although it may easily be knocked out. This ring is coned around its outer edge, which is turned up cup-shaped. The inner side of this cup-rim is shaped in gutter form, in order to make the best distribution of the gas pressure. The whole middle is cut away, of the same size as the powder-chamber, so that the charge can be passed through it. Three concentric scores are cut around the back of the check, the idea being to break the force of any gas that may escape by making it suddenly expand and contract as it forces its way by them. The rear of the gas-check projects slightly beyond the rear of its seat. The face-plate is a steel disc attached to the forward face of the block, and forming the bearing surface for the gas-check. This plate has a slightly greater diameter than the height of the block. It fits into a seat cut for it, and is prevented from turning or falling out by a little dowel and a spring-catch on the upper part of the block. As this plate wears, thin washers of brass or copper are fitted behind it. The rear face of this plate is fitted with grooves, which, from the shock of firing, attach and hold fast these washers. Each gun is provided with a spare face-plate and gas-check, and aboard ship a reserve gas-check is supplied in addition for each pair of guns. [Illustration: Vent-Bush and Vent Gas-Check.] [Illustration: Vent-Piece, showing Hook for Head of Primer.] The vent is pierced in a steel vent-bush, which traverses the breech-block in the axis of the bore. The forward part of the vent is provided with a simple arrangement for preventing the escape of gas. A small vertical chamber contains a steel ball, which in its normal position covers the vent completely. When the primer is fired, the flame drives the ball up and passes on to the cartridge, but the back-flame also striking the ball drives it down over the mouth of the vent again and seals it. The rear end of the vent-bush is provided with a hook for holding the primer in place for firing. The hook itself completely covers the mouth of the vent, having a slit for the friction-bar of the primer. It is so shaped on the rear side that the back-flame through the vent throws it back. A small knob is fixed to it for convenience in hooking and unhooking. This hook is so screwed on the end of the vent-bush that it may be easily removed, and after removing the vent-bush itself may be easily backed out of the block. Aboard ship each gun is provided with two reserve vent-bushes. The vent of the 8-cm. boat-gun is a right-angled one, pierced from the face of the block to its centre, and then, turning at right angles, passes up through the block and the wall of the gun. The joint at the top of the block is made tight by copper bearing surfaces. This vent has neither the ball for checking back-fire nor the primer-hook.