League of Legends (LoL) Question: How RFC interacts with Graves.

So Rapid Fire Cannon is a recommended on graves, but I am not exactly sure exactly how the item works with his passive. I would have assumed it tightened the cone, and increased the range, but from what I've seen if you have an rfc proc, and shoot aimed at a target at max range with no other enemies in the way, the distance the pellets travel doesn't actually increase, hitting nothing, and ONLY the bit of magic damage from rfc is done. from this it would seem like the item is Garbage tier, unless you are pulling something with frozen mallet or some such, which isn't exactly optimal. I mean I guess its a bit of guaranteed unbody-blockable damage, but, meh. On to the ACTUAL bloody question, is that actually how it works? and if not, how does it? and if so, any theory on why the item would be recommended?(the graves rework and rfc were introduced in the same patch, the marksman update, so graves new auto attacks had to have been taken into account when adding the item to his recommended)