The main problem with current body armor, for example with the Interceptor Body Armor that the US Army is currently using, cannot stop any round higher than a 5.56 NATO round. Such as a .308 round or similar rounds which are considered high caliber rounds and are commonly used in sniper rifles and hunting weapons.
But that might change in a couple of years. You see, this company in Poland had developed a body armor that uses Liquid instead of traditional SAPI plates. But how is a liquid going to stop bullets? You may ask. Well, this isn't water, they're using what is knows as a Sheer Thickening Fluid (STF). It's a Non-Newtonian Liquid which means that it will harden and strengthen instead of dissipating like water does. Plus it's also reportingly much more comfortable than traditional body armor. And hopefully will protect our future soldiers from harm as they do their duty to protect our world.
I've only tipped the surface of this topic. If you want to learn more, then look at this link to learn more.
http://science.howstuffworks.com/liquid-body-armor1.htm