What you say about the overlap of rectangles for hitting with ranged attacks may well be true for autotargeting, but it's certainly not correct when you pick out a target with the mouse, and then click to attack. The target is not one simple rectangle; you have to actually click on some part of the monster you're fighting. I tried it with a Titan, and trying to fire at a point between his legs didn't work (although in real life it might have given him a very nasty moment 
)! I'm no expert in this, but possibly there's a whole assembly of smaller rectangles, and your missile sprite has to intersect with one of them.
