A Grav-cannon, also called a Graviton Cannon, is a large, vehicle-mounted type of Grav-weapon used by the Adeptus Astartes. The power of the Grav-cannon is sufficient to rupture organs and crack bones even inside armour, but its primary use is to counter enemy machinery without the risk of secondary explosions. Grav-cannons have been seen mounted on Rapiers and Centurion heavy infantry. Graviton Weapons like the Grav-cannon are devastating graviton-based weapons employed by the Adeptus Astartes based on ancient and forgotten technology dating back to the time of the Dark Age of Technology. Manipulating the very nature of gravity itself, Grav-weaponry turns a target's mass against it, crushing it to pulp under its own weight. This is especially shocking when used against heavily armoured infantry like Terminators, as the hapless warrior is brutally crushed by the bulk of their ceramite plates until only a crimson trickle remains. Space Marine Chapters are among the few warriors of the Imperium to be gifted with the use of Grav-weaponry, and it is considered a sacred honour to wield such a potent weapon.
All Grav-weaponry fires a stream of graviton particles which affects the local gravitational field of a target area, making the targeted object either far heavier or lighter depending on the weapon's setting. The Grav-weapon also creates a bass rumble as the waves affect the local air pressure, causing the air to vibrate. The effect is generally non-lethal and can be used to incapacitate foes who need to be captured alive, but the power of the Grav-weapon's highest settings is sufficient to rupture organs and crack bones even inside armour. Some living targets will be affected more variably; a very large creature may be killed under excessive weight, but most targets will either be slowed or completely immobilised. A Grav-weapon is very useful when fighting in a voidship or a null-gravity environment, as well as during demolition and siege work. This is because it is particularly effective against massive objects such as bunkers or fortifications, where the building's great mass can be used against it, causing it to collapse.
The secrets of Grav-weaponry construction are known only to a precious few; their design is based upon the graviton weaponry many Space Marine Legions employed during the Horus Heresy era in the early 31st Millennium. Even then, these exotic and rare weapons were little understood by the Tech-priests of Mars. The term "Graviton-gun" refers to a group of gravity projector devices whose sophistication is such that the few that remained were relics of a lost age from before the birth of the Imperium of Man. These weapons proved extremely useful when fighting on a voidship or in a null-gravity environment. The power of the Grav-gun's highest settings was sufficient to rupture organs and crack bones even inside armour, but its primary use was to impede the enemy and damage machinery without the risks of secondary explosions.
But the secrets of Grav-weaponry are still locked away in the deepest vaults on Mars. Only Techmarines who show the greatest promise are entrusted with the sacred binary psalms detailing the assembly and maintenance of such weapons. In battle, all forms of Grav-weaponry affects the local gravity field, using its victim's own mass against them, an ordeal that will stun those it does not kill outright. Heavily armoured targets find themselves crushed as if by the mighty fist of the Emperor Himself, while enemy vehicles are left as crumpled, smoke-belching wrecks.
- Codex: Space Marines (6th Edition), pp. 121, 123
- Dark Heresy: The Inquisitor's Handbook (RPG), pg. 133
- Deathwatch: Rites of Battle (RPG), pp. 137-138
- Imperial Armour - Horus Heresy Book One: Betrayal, pg. 231
- Imperial Armour Volume Nine - The Badab War, Part One, pg. 153
- Rogue Trader: Rulebook (RPG), pg. 128
- Warhammer 40,000: Munitorum - Grav-Guns
- Warhammer 40,000: Wargear (2nd Edition)