T-12 (anti-tank gun)
In the mid-1950s, a new anti-tank gun was introduced to replace the 100 mm M-1944 (BS3) field/anti-tank gun that had entered service during the final phases of WWII. This gun was designated T-12 (2A19) and has a smooth bore barrel, unlike all other guns of the time, as it uses a new type of armour-piercing fin-stabilized sabot (APFSDS) ammunition, much more effective against the tanks. This gun entered service in 1961 and is still in service in several countries including Russia and Ukraine.
T-12 anti-tank gun gallery and more info