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So I recently got a few Sten mk2 parts kits shipped to me. They are in short supply and I wanted to get some before they were gone. I have a few posts about the history of the Sten coming soon, but here is a look at how the fire control group works mechanically.

The Basics:

This was an open bolt smg and the operating principle is pretty simple. Viewing it from the rear, you can see 2 things protruding into the inside: The sear (wider), and the disconnector (narrow).

The bolt has a lip on it that would get caught on the sear (marked 2 below) and once the sear is lowered, the bolt will be pushed forward under spring pressure. The disconnector (marked 1) is what allows it to also fire in semiautomatic. Once the bolt goes back after firing, it hits the disconnector down which re-engages the sear so you have to pull the trigger again for another shot. This can be seen below:

With the disconnector in the center, this is in semiautomatic mode. Pressing the external fire selector (simple crosspin) pushes the disconnector out of the way allowing for fully automatic fire as seen below:

Teardown time

Ok first part of disassembly is taking the cover plate off by removing the screws on either side and pulling away. Taking the cover off exposes the entire guts of the firing mechanism:

Next step is unhooking the two linked springs. After that, remove the pin closest to the front and remove the sear (1), sear pin (2), and trigger pawl (3):

Walking through what those parts do is very simple. The sear has already been explained, the sear pin allows the sear pivot, and the sear pawl does multiple functions. It provides a hook for the trigger return spring so the trigger moves back when released, and it converts that lateral pressure into upward force that presses the disconnector to the underside of the sear so it doesn't lower unintentionally. You can then pull out the U shaped pin to remove the trigger (1), trigger link pin (2), disconnector (3), trigger return spring (4), and pivot pin (5):

To reassemble, just reverse the order and do a function check to make sure the sear drops, and depressing the disconnector will pop the sear back up and you are done!

Now you know how a Sten works and how to disassemble the fire control portion. I will be making more of these in the future as I do the build. Thanks for reading!

--@roofcore