Currently it is totally opaque apart from crowdsourced community efforts on when you unlock new weapon types.
It would be nice to have a thing on the mission completion screen when you level that tells you that you have unlocked a new type of weapon, and a track somewhere (in the shop/inventory) where you can see what weapons unlock when. Inquisitor - Martyr has this where you can see when you will unlock weapons, perks and equipment for every level.
This feature isn’t very useful for player who has everything already levelled, but it will make the new player experience a lot nicer if they can see when the weapon unlocks so they have a visible incentive to keep levelling.
As an aside, guaranteeing a newly unlocked weapon at the shop at the next refresh would help a lot too.