A firearm is basically a device that contains a controlled explosion and, to build one safely (and that operates safely), you need more than a bit of equipment to do so. The start-up costs are not cheap.
Just to build a Sten (or a crude mock-up of one) from raw materials is a daunting task. Not discouraging you but you may want to do some further research before you think that this is something that you really want to invest a lot of time (and, just as importantly, money) into…
