Vests may be augmented with metal (steel or titanium), ceramic or polyethylene plates that provide extra protection to vital areas. These "trauma plates" have proven effective against all handgun bullets and some rifles, if the bullet actually hits the plate. These types of vests have become standard in military use, as Kevlar-only vests are ineffective against most military rifle rounds.
A vest does not protect the wearer by deflecting bullets. Instead, the layers of material catch the bullet and spread its force over a larger portion of the body, absorbing energy more quickly and hopefully bringing it to a stop before it can penetrate into the body. This tends to deform the bullet, further reducing its ability to penetrate. While a vest can prevent bullet wounds, the wearer still absorbs the bullet's energy, which can cause blunt force trauma. The majority of users experience only bruising, but impacts can cause severe internal injuries. This is considered to be unimportant by many, as it seems guaranteed any bullets or shrapnel with sufficient force to cause notable injuries would do more damage without the vest. Most vests offer little protection against arrows, ice picks, or stabbing knife blows. As the force is concentrated in a relatively small area with such bladed weapons, the tip of the object can push through the weave of most bullet-resistant fabrics. Specially-designed vests are available that can provide protection against bladed weapons, and sharp objects; they are often used in prison-guard vests. This is done by coating the outer surface of the vest with tiny crystals of a sandpaper-like material or placing a very thin plate of resin hardened glass-fibre sheet between the Kevlar layers. This is important for the safety of law enforcement and prison guard personnel.