>Nordic knights.
Yeah, this. Sweden was a military powerhouse (not the greatest power, but A great power) during the later pike+shot periods of european warfare. They had sophisticated western culture just like the British Empire or the HRE.
Also, it's the Storm KingdomS, not the Storm Kingdom. Im thinking they have a very feudal structure, with commanders ruling fiefdoms in the name of the Primarch.
Society is regimented into three classes. You can rise through the classes by merit alone, but birth, political clout, and battle honors help a great deal.
The Herreklassen, or Lords, are high level commanders of fleets, stellar clusters, or other theaters. A Chapter Master and his direct lieutennants are known as Freiherr, and planetary governments, Navy Admirals, etc are known as Grafs. A Freiherr can supercede the orders of a Graf by tradition, but officially the are equal in rank.
The Riddarklassen, or Knights, are the Astartes soldiers and line officers. The sons of Engerand enjoy a special place in the Storm Kingdoms, and even common a Ritter has authority over the common folk. The Riddarklassen are typically chivalrous and respectful to their inferiors, but there are plenty of bad apples who abuse their station.
The Svenneklassen, or Squires, make up the planetary serfs, fleet support staff, auxiliary human soldiers. Some of the Storm Hammers see their squires as nothing but peasants, but others see them as the honored beneficiaries of the Great Crusade, the people for whom wars are fought in the first place. Many Skalds in the Storm Kingdoms sing of the deeds of honored squires, whos bravery in battle sees them promoted to Ritter, giving them the right to join Astartes squads as seconds. Such men are gifted artificer armor by the finest forgesmiths of the kingdoms, that they may endure what a Space Marine can, or near enough.