Bullet proof items (i.e. vests) are designed to trap and slow bullets down as they attempt to pass through the clothes. Bullet Proof Clothing come in two different types; hard and soft. The soft armors are designed to protect against most handgun and small arms ammunition, whereas hard armors are designed to stop high caliber and armor piercing rounds.
NIJ IIIA rated armors can stop up to .44 magnum bullets and has an Areal Density of 5.9 kg/m and a thickness of 6mm.