There are two basic classes of body armor. The first is known as “soft body armor” and is what most cops wear under their uniform shirts. This armor is reasonably light and flexible and made of synthetic materials like Kevlar and Spectra Shield. This type of armor is classified by threat level (the type of bullets they protect against). Current NIJ threat levels are (in order from least protection to most protection) Level IIA, Level II, and Level IIIA.
These vests stop pistol and shotgun rounds reliably, but they generally won’t stop centerfire rifle rounds. It’s a velocity issue. The faster the bullet, the harder it is to stop.
In order to stop rifle rounds, you’ll need “hard” or “plate” armor. This armor is made of ceramic, a solid composite, or steel. It is rated with the NIJ classifications of Level III and Level IV. See the chart below from Howard’s Uniform Supply to see the particulars for each type of armor.
FK – Time to practice those head shots…