[Marcello]

As far as I have read the D-48 was an attempt at producing a lighter/better antitank gun than the BS-3 100mm. It uses a cartridge derived from that of the BS-3 ammunition but with an 85mm projectile. Higher MV than ordinary 85mm but lighter gun than a full 100mm weapon. Neat solution, as long as kinetic rounds were still the mainstay, as it was at the time it was developed.
Obviously ammunition would not be interchageable with standard 85mm, nor used on normal T-34s.