There are three varieties of Mech Turret. All Mech Turrets are built using the same rules as a Combat Vehicle. The weight rules still apply for any weapons mounted in said turret (the turret will weigh 1/10th the weight of the mounted weapons, rounded up to the next half ton). However, the turret is not an independent piece of the Mech and cannot be armored separately (outside of the Armored Components rule), nor does it have an internal structure value, it is only counted as a piece of the location it is mounted (Head, Shoulder, Torso). One critical hit must be allocated for the turning mechanism of the turret. If this receives a critical, the turret is jammed in the last position it was in. Weapons receive critical hits as though they are mounted normally on a standard Mech, they are mounted as normal in the piece of the mech mounting the turret, and explicitly tagged as being a part of the turret.
Quad Mech Turret
The Quad Battle Mech Turret is used explicitly on Quad BattleMech. It is capable of rotating 360 degrees, like a standard vehicle turret, with no downsides. The turret mechanism critical is placed in one of the torsos. This turret type cannot be mounted on a Bipedal Mech.
A Head Turret can be mounted on a Bipedal Mech, or a Quad Mech. It follows the same rules as a Quad Mech turret, except the critical is placed in the head. This turret tends to be smaller than most other 'Mech turrets, and usually requires a Small Cockpit or a Torso Mounted Cockpit to be effective, as otherwise there usually is no room to mount the weaponry as well as the turret mechanism.
Lastly, a Bipedal 'Mech can mount a 'Mech Shoulder Turret. These are mounted in the Left or Right torso. This turret cannot rotate the full 360 degrees that a head turret or Quad Mech turret can. It can aim to the forwards, rear, and side arc where it is mounted. It cannot be rotated towards the opposite direction of where the turret is mounted (so, a left shoulder mounted turret can't aim to the right arc of a 'Mech, and vice versa), as the 'Mech would be shooting through its own head.
TacticalOperations, p 347, 'BattleMech Turret'