Grav-Weaponry

Grav-Weaponry are weapons that employ ancient Graviton technology, selectively altering gravity to turn the target's mass against them. Grav weapons were used by many of the Space Marine Legions during the Horus Heresy, today however the secrets of how to build and maintain them are locked deep within the vaults of the Adeptus Mechanicus on mars and are even rarer then plasma guns, and like Plasma guns this will not stop every marine army and his their primarch from fielding bucketloads of them. Only a select few know these secrets and only the most promising techmarines are trusted with them.

Graviton weapons from forge-world operate completely differently than the grav-weapons shown in the new space marine codex for reasons that will never be explained in universe but from a meta-standpoint can be explained by the design team not having too much contact with it's forgeworld counterpart.

All grav weapons have the Concussive and "Graviton" rule. Instead of comparing Strength to Toughness when rolling to wound you instead use whatever the target's armor save is. If they have a 2+ thats what you wound them on. When rolling to Pen on vehicles 1-5 does nothing, on a 6 the vehicle is immobilized and takes a hull point. Graviton weapons have no affect on buildings.

They come in three varieties similar to the way plasma guns do.

Grav-Pistol: The smallest of the Grav-Weapons. And comparable to Plasma Pistols in terms of how often people use them. Stats: S*|AP2|12'|Pistol

Grav-Gun: The middle sized and of the Grav Weapons.

Stats: S*|AP2|18'|Salvo 2/3

Grav-Cannon: The biggest of the Grav Weapons, Grav Cannons are only carried by Devastator Centurions.

Stats: S*|AP2|24'|Salvo 3/5

Grav-Amp: A piece of Specialist Wargear that allows any Grav-Weapons being used in the same squad as the model holding it is in to re-roll failed wounds and vehicle damage.

Tabletop

Grav-guns are something of an alternative plasma gun in terms of use. It has MEQ and TEQ killing potential comparable to plasma, without the chance of blowing up. However this comes at the cost of a shorter range(which is further shortened by moving due to Salvo) and a weakness to light armor which makes them worthless against Guardsmen, Ork Boyz, and the like. Because of this grav-guns are best used at mid to short range, prioritizing heavy units to get the best use out of them. Also capable of tearing through vehicles when used in large numbers via hull point stripping that can be argued to both ignore a vehicle's cover save and deal two hull points worth of damage for every six after the first.

Essentially, Grav guns are meant for killing the big, tough units in an enemy army while leaving the fodder for the rest of your army to handle.