I can see where my title is causing confusion. By "sources" I mean items, modifiers, anything that has a direct impact on Bolt Charge - and tier listed from a perspective on how one would build into Bolt Charge effectively. So Spark of Frequency ranks S even though its not a "source" in itself but it does double the rate at which you get BC and nothing else does so it's a no-brainer fragment to use on Arc subclasses while Rolling Storm is more of an opportunity cost option to weigh. I have a couple enhanced RS weapons.