The Hellcannon is not, strictly speaking, a machine; it is an imprisonment. In the fire-lit manufactories of the Dark Lands, the Chaos Dwarfs bind a daemon into a rolling furnace of brass, iron, and rune-etched chain, and sell the result to northern warlords for prices reckoned in slaves. What it fires is not shot but itself — gouts of screaming, half-molten Chaos-stuff that burn through armour, flesh, and the ground beneath, and leave the survivors' courage in worse condition than the crater.
Serving one is a war in miniature. Its Chaos Dwarf handlers feed corpses into the furnace-maw to keep the daemon tractable and haul on its anchor-chains between shots, because a Hellcannon that scents blood will drag its moorings out of the earth and charge, grinding whatever the battle puts in front of it — the enemy, ideally. Northern warlords prize the engines exactly as they would a chained shard of an angry god: an atrocity of tremendous value, best admired from behind.