When a handgun bullet strikes body armor, it is caught in a “web” of very strong fibers (like Kevlar). These fibers absorb and disperse the impact energy that is transmitted to the vest from the bullet, causing the bullet to deform or “mushroom”. Additional energy is absorbed by each successive layer of material in the vest until the bullet has been stopped.
Because the fibers work together both in the individual layer and with other layers of material in the vest, a large area of the garment becomes involved in preventing the bullet from penetrating. This also helps in dissipating the forces but it can cause, “blunt trauma,” non-penetrating injuries to internal organs.
Currently, today’s modern generation of concealable body armor can provide protection in a variety of levels designed to defeat most common low- and medium-energy handgun rounds. Body armor designed to defeat rifle fire is of either semirigid or rigid construction, using hard materials like ceramics and metals. Because of its weight and bulkiness, it’s not practical for routine use by uniformed patrol officers. Mainly these types of vests are used in tactical situations where they’re worn externally for short periods when confronted with higher level threats, usually by the military.