The National Institute of Justice has a rating system for body armor. As you add layers of a ballistic fiber, such as Aramid fiber, you add protection.
Vests are tested not just for stopping penetration, bu
t also for blunt trauma protection – the blow suffere
d by the body from the bullet’s impact on the vest.
Blunt trauma is measured by the dent suffered by a s
oft clay backstop to the vest – a maximum of 1.7″ (44 mm) is allowed.
NOTE: The standard NIJ test rounds are listed below –
tested vests stop many other comparable rounds, and lesser threats.