II.

Interior Ballistics

Interior ballistics deals with the temperature, volume, and pressure of the gases resulting from combustion of the propellant charge in the gun; it also deals with the work performed by the expansion of these gases on the gun, its carriage, and the projectile. Some of the critical elements involved in the study of interior ballistics are the relationship of the weight of charge to the weight of projectile; the length of bore; the optimum size, shape, and density of the propellant grains for different guns; and the related problems of maximum and minimum muzzle pressures. The British engineer Benjamin Robins conducted many experiments in interior ballistics. His findings justly entitle him to be called the father of modern gunnery. Modern experiments verify most of Robins's conclusions, but disagree principally with respect to maximum temperature and pressure. Late in the 18th century the Anglo-American physicist Benjamin Thompson made the first attempt to measure the pressure generated by gunpowder. The account of his experiments was the most important contribution to interior ballistics that had been made up to that time.

About 1760 French ballisticians determined the relationship of muzzle velocity to length of barrel by measuring the velocity of a musket ball and cutting off a portion of the barrel before taking the velocity of the next shot. By using the results of these experiments and advances in chemistry and thermodynamics, ballisticians developed formulas showing the relationship between muzzle velocity and weight and shape of projectile; weight, type, and grain size of powder charge; pressure and temperature in the barrel; and the size of the powder chamber and the length of the barrel.