The Small Arms Protective Insert (SAPI) is a ceramic plate fielded by the US military. It was first used in the Interceptor body armor, a ballistic vest. It is now also used in the Improved Outer Tactical Vest as well as the Modular Tactical Vest, in addition to commercially available "plate carriers". The kevlar Interceptor vest itself is designed to stop projectiles up to and including 9x19mm Parabellum submachine gun rounds, in addition to fragmentation. To protect against higher velocity rifle rounds, SAPI plates are needed.
In May 2005, the U.S. Armed Forces began replacing the standard Small Arms Protective Insert plates with the Enhanced Small Arms Protective Insert (ESAPI). An ESAPI provides protection from .30-06 M2 Armor-piercing rounds with a steel or tungsten penetrator, but costs about $600 per plate, 50% more than SAPI plates. They are produced by Ceradyne, BAE Systems, and ArmorWorks Enterprises.
A call for a next generation plate, to stop even greater velocity threats than the ESAPI plate has been issued by the U.S. Army. They have specifically allowed scalar or flexible systems, and are also calling for greater coverage, with less than a pound of additional weight.
The standard plate for the Interceptor body armor is made of boron carbide or silicon carbide ceramic. New ESAPI plates are also made of boron carbide. The standard plates are not given an NIJ rating, as they are tested in accordance with specific protocols for the military and not the NIJ's testing. Military testing calls for survivability of three hits from the round marked on the plate - for standard SAPI, of a caliber up to 7.62x51mm NATO M80 ball and of a muzzle velocity up to 2,750 ft/s (840 m/s). For ESAPI, a .30cal M2 AP (.30-06 black-tip armor-piercing) cartridge. This performance is only assured when backed by the soft armor of the OTV (or any soft armor which meets military requirements for protection. The ceramic plate is backed with a shield made of Spectra, a material up to 40% stronger than Kevlar., to trap any fragments of either plate or projectile and prevent them from injuring the wearer.
Front and back SAPI plates:
ESAPI plates are the same size but slightly greater in weight.
Torso side plates are as follows:
The mechanism of effect lies in absorbing and dissipating the projectile's kinetic energy in local shattering of the ceramic plate and blunting the bullet material on the hard ceramic. The Spectra backing then spreads the energy of the impact to a larger area and stops the fragments, reducing the likelihood of fatal injury to the wearer.
It is a false assumption that eliminating the penetration of a projectile into the body by using a personal armour system absolves the wearer from serious injury or death. 
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