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 The '''Mortifier Bolter''' was a variant of the [[Bolter]] and the primary individual armament of the [[Nemesis Destroyers]] of the [[Ultramarines]] [[Space Marine Legion|Legion]].{{Fn|1}}

The Mortifier bolter was superficially identical to the tigris[[Boltgun#Tigrus Pattern Bolter|Tigris]], umbra [[Boltgun#Umbra pattern|Umbra]] and phobos [[Boltgun#Phobos pattern|Phobos]] pattern bolters that were in widespread use at the time of the [[Horus Heresy]]. Internally , however, Legion artificers [[artificer]]s reworked the ammunition feed systems and firing mechanisms , allowing the weapons to fire experimental but volatile and often unstable rounds at a reduced velocity. This ammunition was far from standardised, using payloads of chem-agents and toxins concocted to inflict unimaginable pain as it rapidly adapted to the genetic structure of its target, unravelled it them at a molecular level and rendered it them to a grey slurry of base matter. It is purported that these weapons were a further continuation of the development of [[Toxiferran Weapon|toxiferran]] munitions started by the [[Death Guard]] Legion during the [[Great Crusade]]. There are, however, no extant records that chart how the Ultramarines Legion armourers came to devise the resulting technology, and Imperial inquiries on the matter have been silenced by the highest authorities of [[Macragge]] and [[Terra]].{{Fn|1}}