From these earliest examples the development of artillery has been gradual until the present day of the built-up gun with an energy and range undreamed of in the earlier days. The built-up gun of today has attained to a calibre of 16 inches, a length of nearly 50 feet, a weight of 124 tons, and an extreme range at 42 deg. elevation of 20.9 miles with a maximum height of trajectory of over 5-3/4 miles. The projectile, too, has increased in size from a few pounds to the one ton or 2,240-pound mass used in the above gun. The energy imparted to it at the muzzle amounts to 6,408-foot tons assuring a penetration at the muzzle of 33.8 inches of steel, or at 3,500 yards of 27.5 inches, the muzzle velocity being 1,975-foot-seconds and powder charge 640 pounds of smokeless. The maximum pressure in the powder chamber allowed is 37,000 pounds per square inch. Briefly the modern gun is a built-up piece, constructed by fitting or shrinking super-imposed hoops or cylinders one over the other in size and number as diagramatically explained in Fig. 25, sufficient to re-inforce the bore to withstand the varied pressures. [Illustration: Fig. 25. 13-inch B.L.R. (Total Length, 40 feet.)] [Illustration: Fig. 26] The twist or rotary motion is imparted to the projectile by means of the "rifling" in the bore. Fig. 26 shows the cross-section of an 8-inch gun with the dimensions of the rifling, which is composed of two elements, the "groove" or spiral cut made in the bore and the "land" or space between two adjacent grooves. To take these grooves "rotating bands" of soft metal, generally copper, are fitted to the projectile as will be explained under "Manufacture of Projectiles." Classification of Projectiles Projectiles are classified according to their calibre, type of gun for which they are intended, material of which they are made, etc., as per the following scheme used in the U.S. Army for marking cases of projectiles: { Cast steel Inch { Rifle { Cast iron { Mortar { Common steel { Shot { Armor piercing { Shell { Rendable A.P. Weight empty--Lbs. Capped or uncapped or Capped and grooved for base cover or Uncapped and grooved for base cover Base } Fuze Point } Manufacture of Projectiles While a high state of development has been attained in the manufacture of armor-piercing shells attention will be confined to their manufacture in so much as the methods for improvement hereinafter suggested are intended to affect the physical and not the chemical properties of the material, and are, therefore, applicable to all projectiles in which the stresses to be resisted exceed the resisting powers of the projectiles as at present manufactured.