Bore

Bore is the internal cavity of any kind of firearm. It is in most cases cylindrical; but in the Lancaster gun it was oval, and in the Whitworth hexagonal, both being also spiral; while in all rifled firearms it is furrowed with spiral grooves, and for the same reason—viz. to give that rotation which enables an elongated projectile to be used (see RIFLE). In most modern guns and small arms there is a chamber at the bottom of the bore. In breech-loading weapons this chamber is larger in diameter than the bore, in order that the cartridge and projectile may enter it easily. In muzzle-loaders it is generally of smaller diameter, but an enlarged chamber has been tried in some heavy guns in order to give more air space for the expansion of the gases when the cartridge is fired. The diameter of the bore is called the 'calibre.' In rifled guns it is measured, not from the bottom of the grooves, but from the smooth surfaces between them, called the 'lands.' Heavy iron guns were formerly cast solid and then bored out, but, as it is essential that the surface of the bore should be extremely hard to prevent its being 'scored' by the shot, endeavours were made in America to attain this object by casting them hollow, and cooling the inner surface more rapidly than the rest of the metal. Large guns of modern construction are, however, either made entirely of steel, as in the 'Krupp' process, or as in the Woolwich and Armstrong systems, have a steel tube, toughened in oil, for the bore, and strengthened outside by coils of wrought iron shrunk on over it (see CANON), so that the hardness of the bore is assured.