.50 BMG exploding hearts??
Mate I've heard heaps of .50cal myths. I can tell you that it's not possible for a heart to explode, they can rupture but not explode.

If a .50cal round hits you in the arm or leg it will rip it clean off and the person will go into shock , loose blood rapidly and die.

Most people die when hit by a projectile by what is called "hydrostatic shock"

I know if a large caliber (like for example 105mm) projectile flys past you (but it has to be VERY close, no more than 15-20mm) it can rupture blood vessels, just from the air pressure.