The twist of rifling is usually expressed in the number of calibers length of bore in which it makes one complete turn. The twist actually required at the muzzle to maintain the stability of the projectile varies with the kind of projectile and the muzzle velocity. If a uniform twist be used, the driving force on the rotating band will be at a maximum when the pressure in the guns is at a maximum—or near the origin of rifling (seat of the projectile). The increasing twist serves to reduce the maximum driving force on the band thus lessening the danger of stripping the band. This is its principal advantage over the uniform twist, though it also reduces slightly the maximum pressure in the gun. The principal disadvantage of the increasing twist is the continued change in form of the grooves pressed in the rotating band, as the projectile passes through the bore. This results in increased friction and a higher value for the passive resistance than with a uniform twist. (Note: greater ranges obtained by cutting grooves in projectile, principal used on the long range gun by the Germans.) If the twist increases from zero at the breech uniformly to the muzzle, the rate of change in the tangent to the groove is constant. A twist in this form offers less resistance than the uniform twist to the initial rotation of the projectile. To still further diminish this resistance a twist that is at first less rapid than the uniformly increasing twist and later more rapid has been generally adopted for rifled guns. Formerly in our service the twist was uniform; one turn in 25 calibres for guns and one turn in 20 calibres for howitzers. All the latest model army guns, however, have an increasing twist of one turn in 50 calibres at the breech to one turn in 25 calibres at a point from 2 to 4 calibres from the muzzle. In howitzers and mortars the twist is sometimes one turn in 40 calibres at the breech to one turn in 20 calibres at a point several calibres from the muzzle. Some mortars are rifled with a uniform twist and some guns have a rifling which begins with a zero twist. (The 1905 3” gun, 0 to 1 in 25.) OUTER CYLINDERS. Outside of the tube is the jacket. It extends to the rear of the tube a sufficient distance to allow of seating the breech block. In this manner the longitudinal stress due to the pressure of the powder gases on the face of the breech block is transmitted to the jacket thus relieving the metal of the tube from this stress. In all built-up guns there is some method devised for locking the tube to the jacket so as to prevent relative movement of these parts.