Microscopic view of imperfections in the rifling of a barrel
Rifling on a fired bullet
These pics from that link, with their captions, are misleading. I always chuckle at it on TV too. All those little parallel marks at 90 degrees to the rifling don't print on the bullet. The bullet is dragged across those marks as the bullet passes down the barrel. The "signature" left on the bullet is nothing resembling the pattern seen in that photo. It's a signature all right, but it has waaay less detail than that to match against a barrel.
Those breechface (boltface) marks on the head of the case is a lot more of what you would think of as a direct "print".
Good link, Buzzcook.