Each mech of medium or large size may have one turret. A weapon mounted in a turret has a 360 arc, but that weapon system takes up an extra 50% of space (rounded up to the next whole space). So, a turret allows weapons mounted it in a larger arc but takes up more space in the mech's frame. If more than one weapon is in the turret, all those weapons have the same 180 arc, but that arc can face any direction. A turret can change its facing at any time, even between attacks.