The breech-closing mechanism consists of two main parts like that of the model 1870, the breech-plug and the console or bracket. The breech-plug differs from the model 1870 in being lighter, by having the interior cut away as much as possible consistent with strength. It has two movable faces or discs to correspond with the two gas-check seats. These discs are secured to the face of the plug in a similar manner to the model 1870, except that the disc in this case is allowed to revolve freely. The gas-check is carried on the face of the disc instead of being a fixture of the gun. It is quite similar to the model 1870, except that the hole in the centre is smaller and serves to secure it to the face of the disc. The centre of the disc being slightly raised carries two ears which receive the inner edge of the gas-check and centre it. Over these a solid-headed nut screws, which fixes the gas-check and makes a tight joint. The gas-check is made of steel. The console is similar to the model 1870. _Muzzle-Loader, Model 1858-60._ These guns are hooped, but have no tubes. The trunnions are cast with the body of the gun, the hoops not coming so far forward. The vent is vertical near the bottom of the powder-chamber, like the model 1864. The grooves are of the “basket-handle” type, three in number, with an increasing twist of from 0° to 6°. These grooves, near the chamber, are cut back and prolonged to form seats for the rear studs. All guns of this model are converted from old smooth-bores. _Bronze Muzzle-Loader._ These guns are of the ordinary bronze type. The 23-pdr. (canon de 12) has six common grooves having a _regular_ twist from _left_ to _right_. The bottom groove is narrowed on the loading side near the powder-chamber in order to force the studs into close contact with the driving side. The 8-pdr. (canon de 4) has six grooves with a regular twist similar to the 23-pdr. The vent is pierced vertically near the bottom of the powder-chamber. _The Mitrailleuse._