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What does the National Institute of Justice rating system measure for ballistic vests?

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The National Institute of Justice (NIJ) rating system classifies the level of protection provided by ballistic vests against specific threats.1

This system, developed by the research and standards division of the U.S. Department of Justice, categorizes body armour based on the type of ammunition it is designed to stop. The classifications range from Type IIA, which offers protection against 9-mm or .40-calibre handgun rounds, up to Type IV, which is designed to be proof against .30-calibre (7.62 mm) armour-piercing rifle bullets.1

By establishing these standardized ratings, the NIJ ensures that law enforcement and military personnel can rely on equipment that has been tested to meet specific performance requirements for stopping projectiles and dissipating their energy.21