Mission 3: South China Sea
Weapon 6: P90
On your way to locate hatch G-46, go up the stairs and make a left instead of going through the door on the right which leads to G-46. When you go left, it will lead to hatch J-37 and the weapon will be on a box to the right of it.
Weapon 7: U-100 MK5
When on the deck of the Titan after the ship splits, go down the stairs on the left of the deck where the F-16 fighter was and follow the corridor to the end. Take a left to find the weapon at the end of the corridor leaning against the wall.
Weapon 8: SPAS-12
After you take down the chopper on your way to find Garrison, enter the hanger on the Valkyrie and enter this Osprey that has its rear door open. Once inside, look for the weapon on the left side between the seats.
Mission 4: Singapore
Weapon 9: SCAR-H SV
When following the tank along the street at the beginning of the mission, you will eventually come to a bridge with scaffolding around one of its supports on the left side. Climb up it using the ramp on the right side to the second floor and follow the scaffolding around over a large jump and you’ll find the weapon at the end.
Weapon 10: M240B
This is one of the more tricky weapons to find in the game. When you reach the parking garage, go up the ramp to the upper floor and then swing back north to see a stack of crates at the far end. Hop on them and then hop to the left to the ledge of the building. Continue north and left and follow the ledge around until you come to the weapon lying on the edge at the far end.
Weapon 11: M16A4
When you reach the first hanger area after climbing out of the sewers, you will be able to enter a room on the left to prepare for the fight. In the left corner will be a stack of boxes that you will need to detonate with a grenade or another explosive. Once the boxes are blown up, the weapon will be behind them in the corner.