The Artillery Turret is the only Turret or Vehicle in the game of Area 51, it is featured in "The Last Stand" and a later level.
After the last survivor of HazMat Bravo linked up with Delta (Ethan), Chew ordered Cole to walk to the armory above them and find some ammo shells for the turrets. After holding out for some more minutes, the onslaught of mutants stopped and two Delta teammates (Marko and another) loaded the turrets with the ammo shells, Cole and Chew got on the turrets and started firing. After this Marko and the other teammate got on the turrets to give cover fire to Chew and Cole to get demolitions to blow a part of the vast room.
The Artillery Turrets surprisingly have an explosive shell which seems like a tank shell. These Artillery turrets are fast firing, for safety the turret when used is raised.