Leveling up your weapons in Call of Duty: Vanguard can be an extremely tedious process. It can take hours just to max out one weapon, and understandably players come up with different optimized strategies to level them up in the fastest way possible.
After the Warzone and Vanguard integration, a new contract called 'Top Secret' was recently introduced in Warzone. Picking up the Top Secret Intel gives you a random contract to complete. The random contract can be any of the regular contracts such as the Most Wanted, Recon, or Scavenger.
Completing the Top Secret contract rewarded players anywhere between 1,000 to 5,000 weapon XP and players could rank up their weapons within an hour or so. It was not long before Raven took note of this and removed the contract from Warzone. They've temporarily removed the Top Secret contract until they fix the XP exploit.
Similarly, players noticed that they could earn considerable weapon XP in zombies. Although Vanguard Zombies lacked content during pre-season and season 1, several players still played it after they figured out that zombies mode was great for leveling up their weapons.