MF and Lucian wouldn't really need to line it. They could just aim and keep firing.
Graves could just shoot anywhere, it's a shotgun after all.
Your reasoning of trying to use logic for this wont work.
I do believe something along this line would be good though. Something like attack-move. Right now they can do this anyway, but maybe if they had to wait less time before moving? Would probably require a lot of animation work to make it so the attack continues even if they moved. So I doubt they would really add anything.
Maybe a new mastery in the offensive tree.
"Into the Fray"
Melee: Your first attack on an enemy champion increases the range of the attack by 75 unities and you lunge at the target. This cannot happen to the same champion more than once every 30 seconds.
Ranged: Every 4th auto attack's range is increased by 50 when attacking the same champion repeatedly.