359. Temperature of the gun after repeated firing.—Firing, and especially rapid firing, heats the gun considerably. In a 5-inch 50-caliber gun, 27 rounds fired in less than 4 minutes caused at the breech a temperature of metal of 275° F. and at the muzzle 304° F. This test was made without the use of a gas ejection system, which would serve to cool the chamber and bore. In a 3-inch 50-caliber gun, 160 rapid rounds produced a muzzle temperature of 304° F. The significance of these temperatures as affecting the interior ballistics of the gun, is that the heat of the chamber and mushroom will heat the powder charge and cause an increased muzzle velocity, provided that the charge is left in the chamber for a considerable time before firing. If the gun is fired shortly after loading, no material increase in muzzle velocity will be produced.
360. Heat cracks.—At each discharge of a gun, the metal at the surface of the bore is heated to a temperature that would cause it to expand, if free, much more than it actually can expand under the condition of compression in which it is held by the outer layers of metal. (See Chap. IV.) Its effort to expand being thus prevented, the metal at the inner surface is crushed, and upon cooling minute longitudinal cracks, known as heat cracks, appear in the surface of the bore. These cracks appear most prominently at the bottom of the grooves of the rifling, at the foot of the driving edges of the lands when the grooves are hook shaped, and at both corners of square-sectional grooves. They work farther and farther into the metal as the use of the gun continues, but do not, in the “accuracy life” of the gun, become deep enough to reduce the strength of the gun.