Weapon Tiers
All weapons in the game have a Tier that determines the maximum level of the weapon itself.
Tiers are directly tied to the current world level you are on. You will only unlock certain tier weapons at certain world levels. I.e Tier Uncommon at World Level 2, Tier Legendary at World Level 5, etc.
Not all weapons are included in a tier. For example, the Machine Gun may only have tiers of Common, Epic, and Mythic.