After the Vietnam War, military planners developed a concept of “Casualty Reduction”. The large body of casualty data made clear that in a combat situation, fragments, not bullets, were the most important threat to soldiers. After WWII, vests were being developed and fragment testing was in its early stages. Artillery shells, mortar shells, aerial bombs, grenades, and antipersonnel mines are all fragmentation devices. They all contain a steel casing that is designed to burst into small steel fragments or shrapnel, when their explosive core detonates. After considerable effort measuring fragment size distribution from various NATO and Soviet bloc munitions, a fragment test was developed. Fragment simulators were designed, and the most common shape is a right circular cylinder or RCC simulator. This shape has a length equal to its diameter. These RCC Fragment Simulation Projectiles (FSPs) are tested as a group. The test series most often includes 2 grain (0.13 g), 4 grain (0.263 g), 16 grain (1.0 g), and 64 grain (4.2 g) mass RCC FSP testing. The 2-4-16-64 series is based on the measured fragment size distributions.
The second part of “Casualty Reduction” strategy is a study of velocity distributions of fragments from munitions. Warhead explosives have blast speeds of 20,000 ft/s (6,100 m/s) to 30,000 ft/s (9,100 m/s). As a result, they are capable of ejecting fragments at very high speeds of over 3,300 ft/s (1,000 m/s), implying very high energy (where the energy of a fragment is ½ mass × velocity2, neglecting rotational energy). The military engineering data showed that, like the fragment size, the fragment velocities had characteristic distributions. It is possible to segment the fragment output from a warhead into velocity groups. For example, 95% of all fragments from a bomb blast under 4 grains (0.26 g) have a velocity of 3,000 ft/s (910 m/s) or less. This established a set of goals for military ballistic vest design.
The random nature of fragmentation required the military vest specification to trade off mass vs. ballistic-benefit. Hard vehicle armor is capable of stopping all fragments, but military personnel can only carry a limited amount of gear and equipment, so the weight of the vest is a limiting factor in vest fragment protection. The 2-4-16-64 grain series at limited velocity can be stopped by an all-textile vest of approximately 5.4 kg/m2 (1.1 lb/ft2). In contrast to the design of vest for deformable lead bullets, fragments do not change shape; they are steel and can not be deformed by textile materials. The 2-grain (0.13 g) FSP (the smallest fragment projectile commonly used in testing) is about the size of a grain of rice; such small fast moving fragments can potentially slip through the vest, moving between yarns. As a result, fabrics optimized for fragment protection are tightly woven, although these fabrics are not as effective at stopping lead bullets.