In Season 4 of Call of Duty Modern Warfare and Warzone, a new way was introduced to show off in the presence of other players elements that attest to their ability with a specific weapon: let's talk about Master challenges.
For those who do not know what it is, the Master Challenges are mini-objectives through which each player can unlock emblems and business cards starring one of their weapons. The requirement to access a weapon's Master Challenges is to have unlocked its Gold camouflage, which we remind you of by completing every single challenge related to basic camouflages. At this point you should be able to see an additional menu in the custom class creation screen, by accessing which you can check the progress of the three types of challenges: Killing, Head shot is Skills. If the first two types of challenges simply ask you to accumulate kills and headshots to get increasingly rare business cards, those skills serve to unlock emblems and can be completed by making eliminations in particular conditions: some skill challenges require you to eliminate enemies in a slip, others using weapons without accessories and so on.
Speaking of weapons, have you already read our guide on how to unlock the Grau in COD Warzone Season 4?