My question is why.
Why don't we have a gun mental? What adventages does it give? Lighter interms of weight ( I drought it)? Easier to replace if damaged in battle? IMO a gun mental will at lest help to better intergrate the frontal armor.
Both Western and Chinese/Russian tanks have gun mantlets, only difference are that the Chinese/Russian tank mantlet armour is within the turret with the canvas like covering on the outside, whereas western tanks like to have manlet armour as a block on the outside and seal with canvas from within.
The canvas like cover is made from composite material and serves to seal gaps between gun and turret from water, air and biological/chemical agents, as well as nuclear dust, as part of the NBC protection. It also serves to prevent openings when the tank gun moves backwards after recoil from firing (It's also not possible to prevent gaps from forming between the gun and turret when the gun is changing in elevation).
The tanks would still be able to operate normally without the canvas cover if one does not consider NBC protection a requirement. Below shows what the 99 looks like with cover removed, you can see the ring of armour around the gun barrel and extending into the turret.
Most western tanks adopts a externally placed mantlet outside the turret. Advantage of this is savings in work and time when changing gun barrel, as the whole barrel can simply be removed by pulling through the turret gun ring.
Soviet style tanks adopts internal mantlets with the advantage of smaller size for the same protection, and armour weight closer to the base of the gun barrel and not hung on further along the barrel, at the expense of more tedious procedure when changing gun barrels. (though I think given the cold war doctrine it's probably not expected many of their tanks to change barrels.)
Apparently Chinese newer tanks have a redesigned inner mantlet that allows the gun barrel to be changed by pulling through the turret gun ring, but I do not have a source for this.