I've had some ideas about this subject in the past, although the idea was for a 2D sidescrolling RPG. Its a gun combo system that revolves around very smooth and fluid timings of a "Maneuver" (for example, a backflip or barrel roll) and a series of queued weapon actions. The general idea is that each maneuver will have a certain amount of "Trigger Frames" where during specific frames of the animation a "ray" is cast from the player from a direction based on the current maneuvers animation frame and if the ray hits an enemy the queued shot in that "Trigger Frame" will fire. So for example the Backflip Maneuver has 3 "Trigger Frame", so the player can queue 3 weapon actions, and in each of those actions the player can preform multiple shots from the same weapon based on their "Reflex Points" (basically an action point system). So the player is surrounded by 2 enemies and queues up a shotgun blast, a knife throw, and 3 shots from a pistol. When the Backflip is used the player will start to jump up and backwards and the first "Trigger Frame" activates and cast its ray which hits the enemy in front of the player and he/she fires the shotgun blast. The player is now mid air and starting to spin and the 2nd "trigger Frame" begins and hits the enemy behind the player, so the knife throw happens. Then when the player lands facing the enemy who just got staggered by the knife throw the final "Trigger Frame" activates 3 shots from the players pistol into the enemies face... well shit, that was a mouthful... hope that made sense, its pretty difficult to explain.