Hopefully this hasn't been shared before.
Check out this crazy double bullet .308 - The "7.62x51mm M198 Duplex"
This is a screenshot/x-ray from the video.
They shoot out behind the other so the first penetrates and loses all it's energy creating a clear path for the second bullet.
Watch the video tests here - http://www.liveleak.com/view?i=8b9_1387869456
Crazy.