After the terrible events of the Horus Heresy it was Robute Guilliman, Primarch of the Ultramarines that would have the longest lasting effect on the remaining loyalist Astartes Legions. In his legendary tome the Codex Astartes, he covered all aspects of Astartes warfare. From squad organisation, heraldry and unit markings, tactics and deployment. However the greatest effect he would have was his decision to see the once proud Legions decimated into Chapters of no more than 1000 marines.
Ten thousand years later the majority of the estimated thousand remaining Astartes Chapters claim to adhere to the Codex. Some like the Space Wolves, Black Templars, Dark Angels etc do have minor deviances from the Codex but very few stray very far from the 1000 Battle Brothers commandment.
However this figure of 1000 Battle Brothers is a grossly misleading figure and the number of fully trained Astartes a Chapter could bring to bear could be much much greater.
As an example we will look at the Blood Angles Chapter.
After the fateful events at the end of the Horus Heresy that saw the Blood Angels decimated and leaderless, the Sons of Baal were believed to be one of the first Legions to adopt the directives of the Codex, seeing it as a way to return stability to the now fractured Legion.
Today the Blood Angels are for all intents and purposes a Codex chapter, they have ten companies, each with one hundred Marines organised into the correct number of Tactical, Assault and Devastator Squads. It must be noted that the Blood Angels do have several extra and ad-hoc units such as the Sanguinary Guard and the mythical Death Company, this is not a particularly relevant fact and the reader will soon see that the finding in this text can be applied to any Chapter.
Anyone with access to Codex Blood Angels will note in the Chapter Ordering of the Host that we have a full Order of Battle for the Blood Angels Chapter. It is on this page we see the unravelling of the Codex Astartes.
These hidden Marines can be found in those non Company organisations within the Blood Angles Chapter these will be broken down by element.
1) The Armoury
36 Techmarines, these are ALL fully trained battle brothers.
144 Vehicles including Stormraven Gunships, All of these vehicles need multiple crew to operate and even with a conservative number of one Marine per vehicle we already have an extra 144.
2) The Librarius
31 Librarian Marines. This alone is a staggering figure
3) Fleet Command
64 Space going vessels including Battle Barges as big if not bigger than the larges Ships of the Imperial Fleet. Again with a conservative estimate of three marines per ship we gain another 192 Battle Brothers
5) Sanguinary Priesthood*
22 Sanguinary Priests
*This is a unique organisation to the Blood Angels however in a codex chapter it would be substituted with an apothacarion either as a separate logistical organisation like the Priesthood or integrated into each company at command level.
By just looking at the logistical part of the Chapter we have found an extra fully trained, equipped and deadly 467 Battle Brothers. Not only that but this figure includes specialist formations including trained psykers and veterans. Also this does not include any crew of transportation vehicles assigned to the battle companies themselves. This figure could easily put the number past five hundred.
In conclusion the actual force a Space Marine Chapter can deploy could be upward of 50% of the Codex standard. This could possibly explain the extreme reluctance the Inquisition has in declaring even heavily deviant Chapters Excommunicate Traitoris. As seen with the Astral Claws during the Badab war, a Space Marine Chapter with its back to the wall is a terrible thing to behold.