[PlasmaKrab]

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. What I meant was that without even having taken the pains of developing and fielding a 135/152mm gun, the Russian engineers could work up the gap and match current western ammo or armor designs (whichever they think of as the target), e.g. by transitioning to bustle autoloader and one-piece ammo with long-rod penetrators. Like they look like doing on the Black Eagle concepts.
Such a solution would be leagues cheaper and could be more easily retrofitted on older tanks, e.g. T-84 Oplot, T-55AGM... There's plenty of life left in the 125mm design, so I don't see the transition as urgent.

And that reflection was regarding real-world only. As far as I'm concerned, I have transitioned the Soviet tanks to 135mm from the late 90s (T-95A) as a direct answer to the first western 140mm tanks (M1A3, Leo2A8...), which is a direct answer to the predicted armor levels of the T-95 generation. Also, this 135mm (chosen over the 152mm as an easier technical solution for transitional designs, as explained above) remains in 1st-class units only for about 10 years, while other theaters get new 125mm rounds able to deal with anything they'd encounter in Central Asia or on the Chinese border.