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.) _Breech-Loader, Model 1864-67._ These guns are hooped like the model of 1870, but are not tubed. [Illustration: 14-cm. French Gun.] The vent is pierced vertically near the bottom of the powder-chamber, instead of being in the breech-block. The grooves are of the pattern known as the “basket-handle,” for mechanical fit projectiles (see French Groove, English Ordnance), having an increasing twist of from 0° to 6°. The 14-cm. and 16-cm. guns have three grooves, the others five. The width of the grooves is the same throughout the length of the bore, being cut back on the loading side near the powder-chamber to facilitate loading. The depth of the groove, however, diminishes towards the muzzle in order to pinch the studs and steady the projectile. At the powder-chamber and running forward for a short distance in the centre of each band is a supplementary groove, in which travel the rear studs of the projectiles, which are made so small as to easily sheer off. These secondary grooves prevent the projectile from being pushed too far into the bore. With the 19-cm. gun, the supplementary groove is made by cutting back the loading side of the regular groove. There is no shot-chamber proper. [Illustration: Groove (1864).] [Illustration: Groove (1871).] [Illustration: Rear end of Groove (1864), showing Stop for Projectile.] The powder-chamber is cylindrical, of the same diameter as that of the bore across the grooves. The bottom groove is produced through the powder-chamber, to serve as a directing groove for the projectile in loading. In rear of the powder-chamber there are two gas-check seats, conical in shape, with the small diameter forward. The rear and larger one is intended for service in case of accident to the other. [Illustration: Gas-Check (1864).] [Illustration: Breech-Block (Section).] The breech-plug seat is similar to the model 1870.