The most suitable inclination of grooves for a rifle cannon has not yet been determined experimentally; and consequently a wide diversity of twists is employed by different experimenters. The following table[4] presents a synopsis of the results in case of some rifle cannon tested at Fort Monroe, Va., in 1859, by a Board composed of ordnance and artillery officers. [4] See Table, pages 126, 127. [Sidenote: 165]] The following is extracted from the report of the Board: “The method of obtaining rifle motion in these different guns is of two kinds: “1. Flanged projectiles entering into the grooves of the gun. “2. Expanding projectiles, which are forced into the grooves by the action of the charge. Although the flanged projectile, when made with great precision, has given good results, as shown by the tables of firing, the extreme nicety in its fabrication, and the care and trouble to load the gun, particularly when it becomes foul by firing, seems to render it not as suitable for service as the expanding projectile. * * * * * “From the results obtained, the conclusion is inevitable that the era of smooth-bore field artillery has passed away, and that the period of the adoption of rifle cannon for siege and garrison service cannot be remote. The superiority of elongated projectiles, whether solid or hollow, with the rifle rotation, as regards economy of ammunition, extent of range, and uniformity and accuracy of effect, over the present system, is decided and unquestionable.” The ARMSTRONG GUN, of which so much has been said, belongs to the class of breech-loading rifle-cannon. Its projectile is made of cast-iron, surrounded by two leaden rings, placed at the extremities of the cylindrical part, for the purpose of engaging the grooves, when it is forced through the bore. The great range and accuracy claimed for this projectile, are probably derived from its great length compared with its diameter; but a gun of great strength would be required to project it. [Sidenote: 166]] TARGET 40' BY 20'.