Auto Turret Base is a Custom Block behavior, currently only available through the Console Command 'setname autoturretbase.' The turret's are used to shoot at zombies.
When an auto turret base is placed, an 'AutoTurret ' will automaticly be placed on top of it so only make the base. If there is no 'autoturret' custom block set, the turret used will be the default minigun model.
In future editions there will be more turret tops available to the user for a variety of tasks. It has also been stated by DjArcas that the turret system will develop and it is likely that they will cost resources to make.