[Marcello]

"As far as my sources show, the BM-46 was still a couple of yards behind contemporary 120mm rounds, but let's not start this debate again."

I did not mean that the BM-46 was the bleeding edge of world APFSDS design. Merely that it shows that they are not hopelessy behind and do not have a massive amount of catch up to do. They may still have a few thing to learn but not everything.

"by transitioning to bustle autoloader and one-piece ammo with long-rod penetrators. Like they look like doing on the Black Eagle concepts."

A one piece ammo would be incompatible with the existing tanks, without what would amount to a substantially
expensive retrofitting. You might as well change caliber and be done with it. Then I am not sure about why long rod penetrator have anything to do with one piece ammo or separate charge.The british seem to have no issues with having separate charge and decent APFSDS. The limitation with the soviets tanks was the projectile lenght enabled by the autoloader design. There is a workaround which has been incorporated in the T-90/T-72BM.
I have not seen diagrams for the Black Eagle so I do not know what is supposed to be going inside it but there is a possibility that it was done just for flexibility (several weapon configurations were available for it).