Cold War MBT turret designs

This short article will try to cover the basic design philosophies of the modern main battle tank turrets. I have noticed three different philosophies from the predominant tank builders of the Cold War.
 

M1 Abrams

The Abrams' turret is massive. It is noticable wider than the turret of the Leopard 2 or Soviet tanks. Apparently the approximately one feet thick side armor covers the complete flanks of the turret.

Marked grey in this sketch is the approximate layout of the turret armor

At the right side, the thickness side armor cavities can be seen.

Photo showing the blow-off panles (center) and side armor (right) of the M1A1

Why does the M1 Abrams have such massive armor protection over it's turret bustle? It's quite simple. Unlike most other main battle tanks, the majority of the main gun ammunition is stored in the bustle (in case of the M1A1 all except six rounds). A penetration of the bustle would result in the tank being not able to continue combat.
The Leclerc with it's bustle mounted autoloader supposedly follows the same idea in terms of general turret armor layout.

Leopard 2

The turret of the Leopard 2 and some other tanks, follows a different design. While the frontal armor and the side armor covering the crew compartment are in terms of thickness similar to the M1 Abrams, the bustle armor is considerably reduced. Judging by different pictures, it seems to have only one third of the normal side armor thickness. 

Marked grey in this sketch (not very correctly) is the turret armor layout

Drawing of a Leopard 2 turret

Leopard 2 turret production. The bustle armor is thinner.

From the Leopard 2 designer's perspective, it seems that a penetration of the two separated compartmens (ammunition on the left, hydraulics on the right) was deemed to be a minor problem. Instead of armoring the turret bustle, more weight could be allocated to other parts of the tank (for example the roof armor or the frontal armor). 
Why is this possible? Redundancy. The turret hydraulics could be manually operated via hand pump (at a slower speed), while only 15 of 42 main gun rounds were stored in the turret bunker (with 27 being located in the hull).
Other tanks that follow the same general turret design include the Challenger 1 and Challenger 2, where the turret bustle is protected only by thin steel armor and external storage boxes.

Storage boxes at the Challenger 2's bustle

T-64 / T-72 / T-80 / T-90

Soviet tanks and tanks following the Soviet tank design scheme are utilizing a much different turret shape. The turret's frontal armor is wider than the rest of the turret, while the side walls are sloping "back". As a result the frontal armor can cover the whole turret along the frontal arc.

Patent drawing of a T-90A style turret. Note the thin side walls

T-72B tank. The heavy frontal armor is visible due to the welded cavity roofs

Turret of a T-90A after ballistic tests. Note: massive frontal armor, thin side armor

Why did the Soviet's opt for such a "minimalistic" design? Well, first of all they could make their turrets smaller thanks to the use of hull-mounted autloaders.
It also allows saving (probably at least a ton) of weight that normally would have been required to protect the sides.

Pro and contra

Each of the designs has it's pros and cons, otherwise the tank designers would have been stupid for not always choosing the same design.
The Soviet tank turret design is highly optimized for frontal combat - Soviet tanks are often claimed to be designed specifically for attack operations, the lack of side armor only supports those claims. The sides are covered by only 80 to 90 milimetres of steel armor, which makes them incredible vulnerable in assymetric warfare (how many tanks were lost at Grozny?).
The Leopard 2 and Challenger tanks follow a turret design that is also front optimized, but does have a hefty amount of composite armor covering the crew compartment. The bustle is rather unprotected, but this is supposedly no problem due to redundancy and the rather low probability to hit these areas.
The M1 Abrams has full protection along the turret bustle, but suffers from a less weight-efficient design, having to allocate approximately 20 to 30% more weight for turret side protection. Together with the overall huge size of the Abrams' turret, it seems unlikely that it can achieve the same level of protection with the same weight as the Leopard 2.