Every time a new caliber has been introduced on tanks has bee because of this reason: To take advantage over the rivals, to have the tank with bigger hability to destroy the warfare of the enemy in case of war.
The purpose of a tank gun is to penetrate the frontal armour of the enemy tank of the period at battlefield ranges.
A vehicle in the design phase has to be designed to defeat projected tanks the enemy will operate when it enters service.
NATO has been talking about making lighter more mobile tanks because their 70 ton tanks are too difficult to move around easily and like all tanks everywhere do not off 360 degree protection from anti armour weapons. There were several western prototypes of 40 ton tanks in the 1990s.
Against such tanks the 125mm gun would be perfectly adequate.
If Nato went for 90 ton tanks with even heavier armour then a 152mm gun might be critical.
The fact is that they still have tanks from the 1970s in service... albeit in upgraded versions.
If the US started introducing a new tank design with much better armour then the 152mm might need to go into service urgently.... but they aren't so it isn't.
No-one government that impulse new tank designs is waiting until the current caliber can not do the job. Sorry but you are seriously wrong on this, which is a very basical concept on military engineering, used on every type of warfare, that you obviously do not know.
It takes ten years to develop a new system and get it into service in numbers to make it relevant.
Things in design are not developed to combat existing systems but projected future systems the enemy will have in service when the new system is ready.
The first model abrams had a british 105mm rifled main gun.... which was replaced by the German 120mm smoothbore when it was realised that the 105mm probably would not cut it against late model T-72s.... tanks that were already in service.
As example, would you say Russia developed the S-400 because the S-300 can not do the job?
The S-400 was developed because projected western threats were increasing. The S-300 could be improved and was, but new systems are needed periodically.
The point is that there is nothing in service in the west that demands the Russians have a 152mm tank gun in operational service.
The armour of western IFVs means 30mm cannon shells are not longer effective for a BMP so a 57mm gun is being developed and will likely be fielded when it is ready.
The 30mm has been inadequate against Marder IFVs for several decades and newer IFVs make it even worse.
Would you say Russia developed the Su-PAK-FA because the Su-27 can not do the job?
Su-27 can't do the job.
would you say Russia developed the T-14 because the T-90 can not do the job? Far from true.
The T-90 can't do the job... there is not that much room for further improvement past the T-90AM... in comparison the Armata is just getting started and will fill the role of every vehicle in a division.
Russia developed all them finding improvements, trying to take advantage over the rivals.
But the advantage is measured.
You don't install a big gun to have the biggest gun... that is asinine.
You have a gun powerful enough to defeat enemy vehicles and projected enemy vehicles likely to be deployed now or in the near future.
The west has nothing that demands a 152mm gun... now or within the next 5 years.
In 5 years time they might have something, but until then it just makes sense to keep the 125mm standard all their other front line tanks use, and keep developing the 152mm gun and ammo and use the technology developed for that gun and ammo to develop new more effective ammo for the 125mm.
The first requirement is a competent weapon of 152mm that improves the features of the current 125mm weapons. Something that is possible to reach.
It should be fairly easy to create a 152mm gun that offers significant performance advantages over the 125mm gun.
The question is... is that extra performance actually needed at the moment or near future... and I say the answer is currently no.
So keep developing the 152mm gun, but introduce the Armata with a 125mm gun with new ammo.
The T-34 only became useful with the 85 mm gun.
That is not true at all. The only targets the T-34 needed an 85mm gun to deal with were Tiger and Panther tanks... and how few of those were made?
There was a version of the T-34 that had a powerful high velocity 57mm gun mounted... it was a total failure because the vast majority of targets the T-34 fired its gun at could better be dealt with using a moderate velocity but heavy HE shell of 76.2mm calibre than a high velocity 57mm shell made of exotic and expensive metals.
The flexibility of being able to deploy a high
energy 152 mm gun on the Armata platform is worth quite a bit. I am sure that the Armata platform was designed with such flexibility.
The Armata family are expected to be the standard front line heavy vehicle for the next 20-30 years so a 152mm gun will be fitted at some stage as a tank gun... a small calibre EM gun might even be fitted in 20 years time or sooner... but right now they don't need more than a 125mm direct fire tank gun.
The Coalition will have a 152mm gun but not a tank gun.
In terms of the T-80 autoloader... its main fault was exposed combustable propellant stubs... any penetration of the turret sends a shower of hot burning material onto cardboard coated stubs designed to completely combust in the gun chamber.
The T-72 model had an armoured plate a few mms thick over both ammo and propellant stubs so combustion from a penetration was unlikely.
In an unmanned turret as long as there is protection for the stored ammo stubs from hot fragments from a penetration it would not matter whether they were vertical or horizontal.