system
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human
[Warhammer 40k] What is the possibility and cost-efficiency of making an all-female Space Marine chapter through 'False' Astartes surgery and operation? Would there be a forseeable benefit of False-Astartes Space Marines over normal Sisters of Battle?

So as it turns out, some time during the Great Crusade, a number of individuals were made Space Marines without having to be picked up as children/adolescents. These adult men would undergo bionics, surgery and gene-forging (players of Rogue Trader will find something about this) to give them 'bodies' that closely mirrored that of an actual Astartes. They did not often receive the traditional gene-seeds of the Primarch, and often, most of the these false Astartes were generally adults while the Primarchs were children of infants.


The most notable are: Kor'Phaeron, the adoptive father of the Primarch Lorgar Aurelian.

The other being Hasik-Noyan Khan, who was one of Primarch Jaghatai Khan's close childhood friends/mentors.

Certain older members of Leman Russ's viking warband, were eventually turned into 'False Astartes'.


Since False Astartes do not need to undergo gene-seed therapy, a majority of their enhancements and operations are not actually gender based, so THEORETICALLY, female 'False Astartes' would be possible.

Before cries of HERESY: Remember that a number of existing Space Marine legions tend to break their own rules all the time, such as the Black Templars going above the 1000 man limit, while others take more extreme philosophies and traditions. Female Space Marines are the extreme end, and it is unlikely they would be formally considered as true Space Marines.
gpt
We don't know how it worked, that was very high tech science to begin with. During the Horus Heresy we lost a lot of technology.

Secondly, why?  
Let's say we can create False-Astartes, why would we priorities females over males? Males do make better fighters in general and Astartes are crafted to be perfect Warriors-Weapons.  
The only reason we even have the Soritias as a large organization is because the Ecclesiarchy is no longer allowed to keep "Men under arms", hence keeping women under arms instead. Having the state church as a sponsor tend to be beneficial to your numbers and equipment.