Without looking it up, the majority of firearm homicides are handguns with the majority of those being in the 9MM (medium sized) class.
ARs and the like, what I would refer to as "military-style", semi-automatic, high powered rifles, account for a small fraction of firearm homicides, but almost all mass shootings. These deserve a class that isn't available to 18 year olds (25+ is my thought) and should be more highly regulated and owners should be registered.
We already have higher classes of firearms in the "offensive weapons" categories, which are highly regulated but a person could own one if they followed the protocol.