The above posts that claim that the 5.56 is only devistating at close range are correct. However, penetration (in soft tissue) is actually greater at lower velocity, when the bullet stays in one piece.
The 5.56 round will kill from as far away as it can penetrate deep enough to destroy vital organs. It will have poor "stopping power" at these distances, but most likely, that's a moot point.
Certainly, short barreled 5.56 carbines are not an ideal choice for long range. There are some circumstances where they might be best used at long range, however. For example, if being persued by a larger group that you probably can't outrun, the ability to hurt them from a distance could be advantageous.
A real world example: during our incursion into Mexico during the early 1900s, some US troops were being chased by the Mexican military. One American popped a Mexican off of his horse with one shot from about 800 yards, with his 1903 Springfield. The chase ended there.