As seen in Captain America: The First Avenger, Cap's shield has some incredible properties. It's made out of a fictional metal known as vibranium which has the ability to repel all vibrations making it virtually indestructible. Newton's 3rd law applies to this because if a bullet were to be shot at it, the force it hits the shield with is equal to the amount of force the shield hits the bullet. Just in the opposite direction.

Interesting take on the movie -- I haven't seen it yet.  I'd best move it up on my list.