In 1964, the NII-24 research bureau started design work on an improved 3UBM6 anti-tank round. In 1967 the 3BM6 hyper-velocity armour-piercing discarding-sabot round (HVAPDS) entered service: At a range of 2,000 m, it could penetrate 290 mm of flat armour, or 145 mm of armour angled at 60 degrees from the vertical. It was later replaced by the 3BM8 HVAPDS projectile, with a tungsten carbide penetrator. High-explosive anti-tank (HEAT) rounds, which penetrate armour with the focused explosion of a shaped charge, included the 3UBK4 with 3BK5M warhead, later replaced by the 3UBK9 with 3BK17M warhead.