Russia in considering upgrading future T-14 main battle tnks to use the 2A83 152 mm gun instead of its current 2A82 125 mm gun. The 2A83 gun has a high-speed APFSDS shell with a 1,980 m/s muzzle velocity, only dropping to 1,900 m/s at 2 km.
However, Russian engineers have so far kept the 125 mm-size gun, assessing that improvements in ammunition could be enough to increase effectiveness, while concluding that a larger bore weapon would offer few practical advantage.
Russia is both miniaturizing the nuclear warheads and using sub-kiloton low-yield warheads. Battlefield nuclear weapons could be pared with the larger tank gun.
The 152 mm tank gun could penetrate 1 meter of armor.
For 11 years, China has been testing a 140mm gun on one its Type 98 tanks. The 140mm gun could fire an armor piercing round with twice the penetrating power of one fired from a 120mm gun (about 22 mega joules of energy, versus 11), the amount of ammunition carried was reduced by about a third (to 20-30 rounds, depending on the tank). The 140mm shell was about fifty percent larger than the 120mm one, and could probably knock out an M-1 tank with a frontal shot.
When the Armata (T-14) tank gets the 152mm gun, it will be the most powerful cannon to be mounted on a main battle tank of any country ever.