I was not able to break the rotors of the Littlebird helicopter with any rifles. I tested with the M4A1 and the AS50 and neither did any damage to the rotors of the helicopter despite direct hits. During testing I also found that it was impossible to shoot the copilot because of an invisible wall covering him.
Further testing revealed as well that the AH-6 variants have similar issues. It seems that the littlebird is immune to all bullet damage, as not even a minigun was able to damage any part of it.