rounds (commonly known as "beanbag bullets") and the rubber bullet, types of reduced-lethality ammunition. The force exerted by a projectile upon tissue is equal to the bullet's local rate of kinetic energy loss, with distance (the first derivative of the bullet's kinetic energy with respect to position). The ballistic pressure wave is proportional to this retarding force (Courtney and Courtney), and this retarding force is also the origin of both temporary cavitation and prompt damage (CE Peters). Hydrostatic shock Hydrostatic shock is a controversial theory of terminal ballistics that states a penetrating projectile (such as a bullet) can produce a sonic pressure wave that causes "remote neural damage", "subtle damage in neural tissues" and/or "rapid incapacitating effects" in living targets. Proponents of the theory contend that damage to the brain from hydrostatic shock from a shot to the chest occurs in humans with most rifle cartridges and some higher-velocity handgun cartridges. Hydrostatic shock is not the shock from the temporary cavity itself, but rather the sonic pressure wave that radiates away from its edges through static soft tissue. Knockback The idea of "knockback" implies that a bullet can have enough force to stop the forward motion of an attacker and physically knock them backwards or downwards. It follows from the law of conservation of momentum that no "knockback" could ever exceed the recoil felt by the shooter, and therefore has no use as a weapon. The myth of "knockback" has been spread through its confusion with the phrase "stopping power" as well as by many films, which show bodies flying backward after being shot. The idea of knockback was first widely expounded in ballistics discussions during American involvement in Philippine insurrections and, simultaneously, in British conflicts in its colonial empire, when front-line reports stated that the .38 Long Colt caliber revolvers carried by U.S. and British soldiers were incapable of bringing down a charging warrior. Thus, in the early 1900s, the U.S. reverted to the .45 Colt in single action revolvers, and later adopted the .45 ACP cartridge in what was to become the M1911A1 pistol, and the British adopted the .455 Webley caliber cartridge in the Webley Revolver. The larger cartridges were chosen largely due to the Big Hole Theory (a larger hole does more damage), but the common interpretation was that these were changes from a light, deeply penetrating bullet to a larger, heavier "manstopper" bullet. Though popularized