Quoting here from another thread:
"There will be a change to make the challenges in the Book of Weapons XP-based instead of the current specific challenges. We had to use the existing system (which makes sense for daily and weekly challenges as well as Book of Monsters unlock) since the new XP system was not available before we went on test server.
So, with one of the next updates on test earning XP with a weapon (or its variants) fills the bar towards the next unlock. This will make the progression system feel less like "side quests" and allows players to simply unlock new variants by playing how they like as long as they are using a weapon from the same unlock category."