Every Mission you complete you'll get an individual Weapon Upgrade specific to each Character (i.e. Graves & Lang)! You can also use any unlocked Weapon Upgrades in any unlocked Mission at any difficulty setting under the Replay Mission Menu.

Unlockable	How to Unlock
200-Round Magazine Capacity for the LMG 5.56mm (for Lang)	Complete 3 Missions
25% increased Rate of Fire for the LMG 5.56mm (for Lang)	Complete 7 Missions
40mm Grenade Launcher Attachment for the LMG 5.56mm (for Lang)	Complete 2 Missions
Automatic Shotgun (for Graves)	Complete 8 Missions
Flashbangs (for Graves & Lang)	Complete 4 Missions
Foregrip for the LMG 5.56mm which increased stability and reduces recoil when used (for Lang)	Complete 9 Missions
Gun Camera for the 7.62mm Sniper Rifle (for Graves)	Complete the first Mission
HEAT Rocket Launcher (for Lang)	Complete the first Mission
Incendiary Grenade (for Lang)	Complete 5 Missions
Proximity Mine (for Graves)	Complete 6 Missions
Shotgun Attachment (for Graves)	Complete 2 Missions
Suppressor for the 7.62mm Sniper Rifle (for Graves)	Complete 3 Missions