The most important thing to understand is that there is no such thing as a "Bulletproof" vest.
There are varying levels of bullet-resistance as standardized by the National Institute of Justice (NIJ).
NIJ BODY ARMOR CLASSIFICATION
Personal body armor covered by this standard is classified into five types (IIA, II, IIIA, III, and IV) by level of ballistic performance.
Soft armor vests stop pistol and fragmentation threats, but it takes a hard armor plate to stop a rifle bullet.
- Level IIA Stops 9 mm and 40 S&W
- Level II Stops .357 Magnum
- Level IIIA Stops .357 SIG and .44 Magnum
- Level III Stops Most Rifle calibers, including .308 FMJ (Will not stop .223 or 5.56mm)
- Level IV Stops Armor Piercing Rifle, including Armor Piercing .30-06.
Most hard armor plates are meant only to be used in conjunction with soft armor underneath. (I.e. A level III plate over level IIIA soft armor.)