I'm planning on running a Black Crusade game where the players are all inmates in a high security Imperial prison colony. The campaign will be humans only (since the Imperium is more likely to straight-up execute Chaos Space Marines rather than take them prisoner) and there will be little starting gear; everyone has an orange jumpsuit, with an extra item such as a makeshift lockpick or a shiv or some smuggled drugs. Everyone will be required to write a backstory for their character detailing what crime they're convicted of, whether or not they actually committed the crime, and how long they've been in the prison. Depending on what they did and how long they've been "on the inside", they will start out with different starting talents and equipment.
For instance. a recently-arrested deserter from the Imperial Guard would have Common Lore (Imperial Guard) and might possess special training like being able to operate military vehicles, and could have some smuggled drugs that they hid in an unsavory place. On the other hand, a con artist who's been in for five years or more would have a knack for getting people to do what they want (Charm/Deceive bonuses) and could have fashioned themselves a shiv out of a sharpened toothbrush handle or something similar. The players may not even be devoted to Chaos at first, but one thing's for sure, the Emperor isn't going to save them now, so it's only a matter of time.
The objective of the campaign arc is to escape from the prison colony, at which point the game will probably proceed like regular Black Crusade. Combat would initially be fought with fists and light melee weapons, such as knives or clubs. Apart from perhaps a scratch built Laslock or Mercy Killer (if they're lucky), the only guns are in the hands of the guards, and they'd have to kill one or knock him out in order to acquire one, a feat easier said than done. They'd also have to deal with sadistic prison gangs who think they can push the new meat around, and that's even before they encounter the mutated creatures lurking in the shadows below the prison depths…
I know there's the example story "Broken Chains", and while I haven't read it, I'm wondering how much use that will be to me here. If anyone has their own ideas or comments, feel free to chime in.