The Liberator is the Survivors' custom-built machinegun. It was created with salvaged parts and human ingenuity. This gun has a very large clip of 600 bullets max, making it a very useful weapon in long firefights.
Its name means the exact opposite of the Oppressor's name. A liberator is someone who frees people, while an oppressor is someone who enslaves people. These names obviously refer to the roles played by both the Survivors and Watchers (as the Liberators and Oppressors, respectively).