Bulletproof vests work by catching the bullet in a "web" of very strong network of fibers. These fibers absorb and disperse the impact energy of the incoming bullet. This causes the bullet to deform or "mushroom." Additional energy is absorbed by each successive layer of material in bulletproof vest, until such time as the bullet has been stopped. Bulletproof vests vary widely in the quality of materials used and become less flexible to wear as the quality of protection increases.
The design of bulletproof vests is effective in stopping many bullets and obtrusive objects. Because the fibers are webbed and layered, higher quality bulletproof vests will prevent many bullets from penetrating. However, it is possible that any bullet or sharp object would penetrate a bulletproof vest if it hit the vest at the right angle, sliding between the fibers.
Bulletproof vests can be penetrated by a number of bullets. In particular, bulletproof vests may be vulnerable to penetration by rifle rounds, unusually high velocity pistol rounds, pistol ammunition when fired from a rifle barrel, armor piercing ammunition or other unusual forms of ammunition. It is for this reason that the term bullet-resistant is preferred rather than the misleading term: bulletproof.
Sharp-edged or pointed instruments can penetrate bulletproof vests. Bulletproof vests are particularly vulnerable to objects such as ice-picks and long knives. Knives have the ability to slip through the fibers of the vest and long knives have the ability to penetrate deeply through many layers. Additionally, a vest does not cover the rest of your body leaving you vulnerable to injuries where protective clothing is not worn.