From Lexicanum

The Hellhammer Cannon is a heavy version of the Battle Cannon mounted on a Hellhammer. It has shorter barrel with heavy-gauge suppressors to fire extremely high calibre shells. This allows it to fire without ripping itself apart. The shells are designed to destroy armour and emplacements with an unstable sub-atomic charge.¹

Sources

• ¹:Warhammer 40,000 Apocalypse