Warzone 2 players who want to unlock the M13B must find the special radiation zone in the DMZ, kill the Chemist, and successfully extract with his M13B. Once you extract with the M13B, you'll have access to a highly-accurate assault rifle that's one of the best loadouts for Warzone 2.
Successfully exiting the map while havinbg the Chemist's M13B in your inventory will automatically unlock this weapon for all game modes. Keep in mind, only th person holding the M13B will actually unlock the gun. So if you're playing with friends you'll need to run DMZ multiple times until everyone has this gun.