User blog comment:Traffy731/luffy's red hawk attack/@comment-121.96.170.241-20120923061118

Notice how he used 2nd Gear only at his arm and cocked it like 2 times before engulfing it with Armament Haki. Maybe that's how it got on fire and the Armament Haki is a way to protect his arm from burning, and also of course giving a more heavier punch.