Complete the indicated number of Emergency Situations to unlock the corresponding weapon:

Shotgun Mk. 1: Complete 1 emergency situation.
Assault Rifle Mk. 1: Complete 2 emergency situations.
Shotgun Mk. 2: Complete 3 emergency situations.
Assault Rifle Mk. 2: Complete 4 emergency situations.
Shotgun Mk. 3: Complete 5 emergency situations.
Assault Rifle Mk. 3: Complete 6 emergency situations.
Thermal Breather: Complete Webster Street.
Terror Mode: Complete all story missions on any difficulty setting.