The British 13.5, which was known as the 12-inch-A until the “Lion” was launched, has a length of 45 calibers, and a muzzle-energy ten per cent greater than that of the 50-caliber 12-inch of 1909 and 1910. It may be noted that the caliber is the diameter of the bore of a gun. The statement that a gun has a length of 45 calibers, for example, implies that the gun is forty-five times the bore’s diameter. Thus a 12-inch gun of 45 calibers is 45 feet long.] These pieces are prepared with the utmost care to prevent any defective material entering into the make-up of the gun. After the parts are put together a thorough forging follows, either by use of hammer or press, the latter being now used in preference. The usual practice in forging is to continue it until the ingot is decreased to one-half its original thickness and is within two inches of the desired diameter of the finished work. It is then annealed with great care to relieve the strains set up in the metal by the forging and next goes to the machine shop to be rough bored and turned. The final boring takes place after a second annealing. The above is only a rapid sketch of the total process, in which elaborate care is taken to prevent imperfection of any kind. [Illustration: ORDNANCE PROVING GROUND View showing smoke cone occurring during the proof firing of a twelve-inch gun with brown powder. _Courtesy of the Bethlehem Steel Co._] In a wire-wound gun an inner tube of steel is thoroughly wrapped by successive layers of ribbon wire, each layer being wound with wire at a different tension. This type of gun is preferred by foreign manufacturers, but within the United States the built-up system is in higher favor and is almost exclusively employed. The makers of the wire-wound cannon claim for it a positive soundness of material impossible to secure in a built-up gun, and that it has greater firmness of material and superior tangential strength. But with this come certain disadvantages, a notable one being a lack of rigidity in the longitudinal direction, this tending to increase the “droop” of the muzzle and give a certain “whip” to the piece when fired that reduces accuracy. This and other disadvantages have given the built-up guns general preference in this country, they being found strong enough to bear any pressure desirable in service. In addition they are much cheaper to build than the wire-wound guns.