Autocannon

It was the sound of a Reaper-pattern double-barrelled autocannon powering up.
...
Six mass-reactive explosive shells—each one capable of killing a Rhino transport on its own—smashed into the Atramentar in the space of three seconds. The first destroyed his chestplate and would have seen him dead in moments through the horrendous blood loss from his mangled insides. He was spared this death as the second shell killed him instantly, exploding against his tusked helm and annihilating his head and right shoulder.
The other four shells impacted and tore the remains to pieces. In three seconds, nothing remained of Vraal of the Atramentar beyond shards of broken armour and the wounds carried by First Claw.

Soul Hunter pg. 306