Sights, stab, and the first-round hit
The fire-control computer is as lethal as the gun.
Stabilization
A two-axis stabilizer keeps the gun on a point while the hull pitches. Without it you halt to shoot (WWII, early Cold War). With it you fire on the move — the defining MBT skill from the 1970s on. The gunner’s sight is independently stabilized; the gun slaved to it.
Rangefinder to ballistic solution
Laser rangefinder, muzzle-reference system, wind sensor, cant sensor, ammo temperature, and a computer that aims off. The commander’s independent thermal sight (CITS / hunter-killer) lets him designate a second target while the gunner finishes the first. Hunter-killer is why Western tanks win short, vicious meetings.
Night is the fight
Image-intensifiers (light amplification) came first. Thermals (FLIR) see heat and work in smoke and dust. An M60A3 TTS in 1980 could kill a T-72 that could not see it. That gap, more than armor, decided 1991. It is closing: T-90M and T-80BVM carry decent thermals. Drones now add a second, overhead sight the tank does not control.