Kossars
Kossars is a trained militia, and core infantry unit in the armies of Kislev. The kossars originated from a group of Ungols who served as mercenaries for the Gospadars when they first invaded the land that would become Kislev, which means joining the kossars has a bit of a reputation behind it as a way to escape your life for a new one. Kossars are armed with Bows and Great axes, which was how the original Ungol tribe fought and these stuck around as the tactic of whittling down the opposing force with ranged fire, before engaging in melee proved very effective at fighting the Chaos Hordes that wandered into Kislev. Because one 7ft tall murder Viking in full plate is hard enough to kill in combat without having his mates backing him up. Unlike other parts of the Kislevite military like the Winged Lancers and Ungol Horse Archers, the kossars will accept both Ungols and Gospadar applicants as long as they can pass the fitness test and be trained to fight, and while serving together the differences between both groups of people are almost completely put aside.
During his reign Tzar Boris made is to that anyone who joined the kossars would be granted a pardon for any crime committed before they joined the kossars, in exchange for swearing complete loyalty to the Tzar (and later Tzarina). This has been good for bolstering the kossars forces and keeping the loyalty of the army (because we all remember the last time the Russian royalty lost the loyalty of the army), however, it does mean that the kossars are made up of more "undesirables" and are typically less organized and have their fair share of soldiers committing crimes against civilian populations. The exact number of soldiers executed for these crimes is unknown as the Boyars don't keep it written down as it would be too embarrassing for Kislev. Kossars are also pretty slippery when charged with minor crimes, as since the units are made up of both Ungols and Gospadars, kossars will usually site both Ungol and Gospadar laws to keep them from trouble even if they are not part of the ethnicity that the laws are from. It's fairly common for both groups to site the Gospadar law of "finding Ursun", which lets someone take 1 month of leave to find spiritual enlightenment, only for them to leg it for the border and the Empire in this time and become a mercenary down south.
Despite these shenanigans the kossars are an important and relied upon corner stone of the Kislev military to protect the country, and can be fierce warriors comparable to those of both Norsca and the Empire.
Streltsi
A specialized group of kossars from the city of Erengrad, these Kossars have replaced their bows for black powder handguns. These guys are obsessed with their guns, as in they sleep next to the damn things and give them names after their first battle, and using a strelsti's weapon without his permission can result in said streltsi beating you to death with a bag full of stones. Due to their improved firepower over regular kossars, the streltsi think higher of themselves than other Kislevite forces and typically keep themselves apart from the rest of the army and have their own wagons and supplies separate to the rest of the army. They are based very closely to the real world Streltsy who were an elite unit in the armies of the Grand Dutchy of Moscow/Russian Tsardom created by Ivan the Terrible and used them to conquer the Khanate of Khazan. They were armed them with matchlock muskets and would they would use their axes to steady their muskets while they fired.
In Total Warhammer the streltsi have received a bit of a fluff update to their weapons. They used to have both a Handgun and Axe, but newer recruits would get into trouble when fighting as they would always try to get in one last shot and wouldn't have enough time to pull stow their handguns and ready their axes resulting in them fighting off the enemy with the blunt end of the handgun. Some Boyar eventually decided to play into this and ordered the handguns built for the Strelsti in Erengrad to be combined with their axes into one weapon, resulting in the RWBY abomination of a handgun with an axe head replacing the stock and butt of the gun (there is actually some historical precedent for this as flintlock pistols with axe bayonets saw limited use in 17th-18th centuries by German and Polish cavalry).
Gallery
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The Streltsi as they appear in TWW3
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An armoured kossar, introduced in Total Warhammer