If you’ve reached max XP in CoD and are looking to find something to work on in game, then there are plenty of challenges to unlock fancy skins and charms for weapons.
Each weapon has four standard skins to unlock, a gold, platinum, atomic and orion skin challenge and then prestige challenges to get kills with each of those four skins equipped.
The initial four skin challenges vary somewhat by weapon. The gold skin challenges require you to unlock the previous four skins and then get three kills with the weapon without dying, ten times.
The platinum skin requires all gold skin challenges in that weapon category to be unlocked, then you need to get 25 longshots with the weapon.
The atomic skin challenges need 51 platinum skins unlocked, which is effectively all the weapons. Then you need 25 headshots with each weapon.
Completing 51 atomic skin challenges unlocks the orion skin for all weapons.
Then there are the challenges to unlock emblems and calling cards by getting kills with each weapon with each of the four special skins equipped.
Gold requires 100 kills with the skin on, platinum 200, atomic 300 and orion 400 for a total of a thousand kills per weapon, or 50,000+ kills total.
For each weapon, completing the above four challenges gets you a charm featuring the weapon and it’s number out of 51.