The most amusing thing about writing military and violent fiction is, to be honest, the Amazon referrals you start to get when you're buying books for research purposes. I've been buying a bunch of materials about World War Two and the British Commandos, as well s various aspects of the war. I came across this little pamphlet on the Sten, and picked it up on a lark. As soon as I clicked to buy it, Amazon started recommending all sorts of do-it-yourself gun manufacturing guides, as well as old standbys like The Anarchist's Cookbook and other anti-establishment texts.
The Sten "machine carbine" as it was called, although we would call it a submachine gun, is an extremely simple weapon. There's only forty some-odd parts, and all of them could be manufactured with the equipment found in any semi-professional metal-working shop. If over a million Stens could be manufactured in a couple of years in cottage smithies all over England, making one using the machines found in even a modest shop today would be trivial. Because of this, blueprints and design specifications for the weapon are commonly found among militant "off-the-grid"-ers and the like.
Interestingly enough, because of this, when I was writing Killer Instincts, originally William was going to train and use a black-market Sten, manufactured by some anonymous underground gunsmithing shop and sold to Richard and other clients, who were looking for untraceable automatic weapons. There were a couple of pages where Richard talks to William about illegal arms manufacturing, and the world of "gun runners" who traffic in untraceable weapons. At the time, I thought it would be an interesting way to start introducing William to the "shadow world" that Richard lived in. However, I decided that while this was interesting, I'd rather use a weapon more iconic for the action-adventure genre, which is why I switched to using an Uzi. There's still some of the black market aspect of the story when they talk about the weapon suppressors, but it's not quite as extensive.
The booklet also goes into details such as "official" positions and stances to use when firing the weapon, as well as immediate action drills for clearing jams and so forth. Even if some of the stances or techniques are "dated", it is great information to know as it is how your characters have been trained, and how they would act in certain situations.
While most of the information here might be available in some form or another scattered across the internet, I think for a few bucks it is worth picking up if you have a serious interest in the mechanical workings of this famous weapon.