No body armor is "bullet proof" as some people have inappropriately dubbed body armor.  Any vest on the market can be defeated, pierced, if hit with enough or a high velocity penetrating round.  When purchasing body armor it is important to make sure that you understand the types and abilities available.

The National Institute of Justice (NIJ) has produced a threat Matrix which rates armor based on the effectiveness.  Below is a brief explanation of the varies threat levels:

NIJ Level I

• .22 long rifle 40 grain  1050 Feet per second (FPS)
• .380 ACP Full Metal Jacket (FMJ)  95 grain 1025 FPS

NIJ Level IIA

• 9mm FMJ 124 grain  1090 FPS
• .40 S&W FMJ 180 grain   1025 FPS

NIJ II

• 9mm FMJ 124 grain  1175 FPS
• .357 Magnum 158 grain  1400 FPS

NIJ IIIA

• 9mm FMJ  124 grain  1400 FPS
• .44 Magnum  240 grain  1400 FPS

NIJ III  (Considered Tactical Level)

• 7.62mm FMJ 150 grain  2750 FPS

NIJ IV  (Considered Tactical Level)

• 30-06  166 grain  2850 FPS

In addition to rating body armor, NIJ has conducted many studies on longevity of body armor.  Most manufacturers stand behind a five year warranty for body armor effectiveness.  NIJ has determined that body armor does not expire, however it does age.  If you are purchasing body armor that is being sold due to the "expiration date", then it is perfectly functional as long as it was stored in a cool dry location away from the elements.  If the body armor is being purchased used, then the effectiveness has been diminished.  Exposure to the elements and body sweat has degraded the fibers which compose the body armor panels.  In most cases the body armor should still work, but remember Murphy is never on your side.  You may be getting a good deal as far as price, but your life should be worth more than a few bucks. 

After determining the threat level that you need, then you have to decide on the style.  The two most common styles are the contour, open sides, and full coverage.  The contour provides the same front and rear protection as the full coverage, however the sides are left vulnerable.  The full coverage style wraps around the sides to give some side protection, but you give up some mobility.  Tactical vests typically provide full coverage in addition to neck, throat and groin protection.  Also avaiable are several vest carriers that incorporate cooling packs to assist with the trapped heat from the body.

When looking at Trauma and Shock Plates, NIJ uses the same Threat Matrix.  The main difference between these plates and soft body armor is that the plates do not degrade due to the elements.  Most plates are made of Hardened Kevlar, Steel, Titanium or Ceramic.  Each plate has its benefits and its shortcomings.