Very short AR's don't develop enough gas port pressure because the bullet exits the barrel before the pressure has time to go down the gas tube and cycle the action.
With a standard 16" carbine barrel, the gas port is about 7.5" ahead of the breechface, with another 8.5" of barrel ahead of the gas port. When you cut the barrel off 5", there is so little dwell time with the bullet still in the barrel past the gas port that the action won't cycle.
This is addressed by drilling out the gas port to give higher pressure over a shorter time frame.
If you know what you are doing, or don't and get really lucky, you may hit on a combination that works most of the time without beating the gun to death.
Or not.
rc