When you complete the indicated number of ‘Emergency’ Situations the corresponding bonus weapon will become unlocked.

Unlock Shotgun Mk. 1:
Complete 1 Emergency situation.

Unlock Assault Rifle Mk. 1:
Complete 2 Emergency situations.

Unlock Shotgun Mk. 2:
Complete 3 Emergency situations.

Unlock Assault Rifle Mk. 2:
Complete 4 emergency situations.

Unlock Shotgun Mk. 3:
Complete 5 emergency situations.

Unlock Assault Rifle Mk. 3:
Complete 6 Emergency situations.