SAPI means Small Arms Protective Insert.

The Small Arms Protective Insert (SAPI) is a ceramic ballistic plate used by the United States Armed Forces. 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 9×19mm Parabellum submachine gun rounds, in addition to fragmentation. To protect against higher-velocity rifle rounds, SAPI plates are needed.

ESAPI

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 Springfield M2 armor-piercing (AP) with a steel penetrator in accordance with the NIJ Level IV standard, but costs 50% more than SAPI plates.

XSAPI

A call for a next generation plate, to stop even greater velocity threats than the ESAPI plate was issued by the U.S. Army in 2008. The X Threat Small Arms Protective Insert plates are specifically allowed scalar or flexible systems, and asked for better coverage, with less than a pound of additional weight. XSAPI did in fact offer slightly better protection, at the cost of more weight and thicker armor profile.

The XSAPI is intended to protect against an "X-Threat", which is able to be inferred from another source to be the M993 7.62 NATO armor piercing tungsten carbide projectile.

Materials and capabilities

The standard plate for the Interceptor body armor is made of boron carbide or silicon carbide ceramic. 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 7.62 NATO M80 ball and of a muzzle velocity up to 2,750 ft/s (840 m/s). For ESAPI, a .30-06 Springfield M2 armor-piercing (AP) (.30-06 black-tip armor-piercing) cartridge. This performance is only assured when backed by the 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.

Sizes and weights

SAPI plates meant for body armor come in front and back plates which are identical, and smaller side plates. The front and back plates come in five sizes. Their dimensions are the following:

Front and back SAPI plates:

· Extra Small - 1.27 kg (2.8 lb) | 184 x 292 mm (7¼ x 11½ in)

· Small - 1.59 kg (3.5 lb) | 222 x 298 mm (8¾ x 11¾ in)

· Medium - 1.82 kg (4.0 lb) | 241 x 318 mm (9½ x 12½ in)

· Large - 2.09 kg (4.6 lb) | 260 x 337 mm (10⅛ x 13¼ in)

· Extra Large 2.40 kg (5.3 lb) | 280 x 356 mm (11 x 14 in)

ESAPI plates are the same size but greater in weight.

· Extra Small - 1.70 kg (3.75 lb)

· Small - 2.08 kg (4.60 lb)

· Medium - 2.50 kg (5.50 lb)

· Large - 2.85 kg (6.30 lb)

· Extra Large - 3.25 kg (7.20 lb)

Side SAPI (SSAPI, S-SAPI) torso side plates are only in 6" x 8" size, and weigh around 1 kg (2.3 lb). 

ESBI plates:

· Small - 0.75 kg (1.65 lb) | 152 x 152 mm (6 x 6 in)

· Medium - 1 kg (2.19 lb) | 152 x 203 mm (6 x 8 in)

· Large - 1.15 kg (2.53 lb) | 178 x 203 mm (7 x 8 in)

Physics

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.