There are 12 mastery camos in Modern Warfare 3, each with its own challenges for players to complete based on multiplayer and zombies. With the new title came 36 new unique weapons, but also all of the weapons from Modern Warfare 2 were carried over. While players can use the carried-over weapons to unlock four mastery camos in Zombies, there are no new mastery camos for multiplayer. Players can however still unlock the four multiplayer mastery camos from MW2 including Gold, Platinum, Polyatomic, and Onion. This is how to unlock Onion in MW2!
To begin, players must unlock the Gold Camo for a certain number of weapons per weapon category. The Gold Camo requires players to complete four base challenges, level up the weapon to a certain amount, and complete the final Gold Challenge to get 3 Kills without dying 10 times. The base challenges range from kills while mounted to kills while ADSing. The challenges require the players to play differently each time, focusing on different aspects of the multiplayer experience.
Once Gold is unlocked for the MW2 weapons, the next requirement for players is to unlock Platinum for 51 weapons that were carried over. Unlocking the Platinum Camo makes players unlock the Gold Camo for a specific number of weapons in each weapon category and then complete the Platinum Challenge. For the majority of weapons that final challenge is to get 25 Longshot Kills, but for some including the melee weapons and launchers it differs.
The final stage to unlocking Onion is to unlock the Polyatominc Camo for 51 of the Modern Warfare 2 weapons. Polyatomic requires unlocking the Platinum for a set number of weapons in the class, but there is one more challenge to complete per weapon. For the majority of weapons that final challenge is to get 25 Headshot Kills, but for some including the melee weapons and launchers it differs.
After completing each of the three camos for 51 weapons that were carried from MW2 to MW3, Onion will be unlocked. This camo will take the most amount of time in multiplayer and patience out of all of the camos but will be extremely rewarding to complete. Good luck and have fun grinding for Onion!