The bolted variety can be accessed when standing close to it, by pressing an icon which will appear like how the melee/knife button appears.
Turrets have unlimited ammunition. However, after repeated firing they need to cool down. It is recommended that while not pointed at an enemy or a target, to let it cool down as when it reaches the overheat thresold. The temperature of the machine gun can be found in the top right hand cornor where ammunition is usually displayed.
Turrets have very important strategic significance, for example, where enemies are scripted to attack you at medium range in very large waves where your normal firearm would not be sufficient in terms of firepower, rate of fire, and clip size. They have a limited axis on which they can rotate, possibly so there are more scenarios and to slightly balance the weapon's great abilities.
A turret does not replace one of your weapons like how picking up a fallen foe's weapon replaces the weapon not backpacked. When using a turret, both weapons are backpacked. This means you do not waste potentially valuable time picking up your old weapons after disengaging the turret.
A turret uses a box clip very much like M249. However the one with the turret has an infinite capacity.