The largest tank guns at the moment are 120 and 125mm, the difference is then in calibers, For example Leo 2A7 has a 120mm 55 calibre gun, Al-Khalid has a 125mm 48 calibre gun, while the former is 120mm it’s longer in calibres, latter is 125mm but shorter in calibers.What I am more excited is future 152 MM tank gun to be mounted on this tank.... is there any tank already in service with 152 MM gun as most of the tanks I see are always with 125 MM gun... is there any plans for your VT4 to upgrade it with this type of gun?
152mm gun on a tank is unnecessary for now. We won’t be seeing those on tanks anywhere in the near future, not even by 2030 imo. It is entirely possible, but not logical. The most we will see is 130-135mm guns like the Germans are offering for Leo 2.
A 152mm gun would massively increase the size of the turret because of the breech, and T-14 is already rather massive. Not to mention the recoil and it’s effect on the tanks systems and it’s accuracy, weight, the much bigger ammo (means low ammo capacity), changes to the auto-loader. On the other hand it would massively increase penetration and range, so it will take time to develop a gun that can balance all these, It would need a lot of changes from the ground up. It is highly unlikely T-14 with a 152mm gun will go past test stage before 2030.
On top of that modern 125 and upcoming 130mm guns are more than enough for modern armor if you have modern projectiles. So 152mm guns on tanks can become a reality when and if modern armor systems become strong enough, but then one has to consider wether it would just be smarter to invest more in Rocket projectiles and not make bigger guns. As the former might be cheaper and more reliable. That we will only see as the future gets closer.
VT-4, AK-1, T90 or any modern tank can also be equipped with bigger guns when and if it is needed, as all modern MBTs were designed with modularity in mind. That’s why the German Leo and British Challengers are looking into 130mm guns despite being old designs. It won’t be anymore difficult than it will be to put them on a T-14. As in both cases a new turret, new auto-loader and many similar new sub-systems would be required to deal with the recoil and accuracy. The only difference is that Russia may already be trying to develop such a gun.