The Do-It-Yourself Sten: Part One

Sten Nation

Of all the semiautomatic firearms that one can build, the  STEN has to be the easiest to and requires the least amount of investment in both tools and time to complete.

As an example, the author actually built his STEN using a carpenter’s vise bolted to a Pepper Tree stump (trimmed appropriately with a tomahawk), cut all the slots with with a Rotozip and filed everything to fit with a set of rusty files. The only expensive tool required for the process is a MIG welder. 140 Amps is recommended.

All said though, the author’s build was completed with a lot of mentoring and collaboration from his good friend  bplvr  (true name withheld for privacy reasons), a retired and very talented machinist with a knack for reverse engineering history. It was bplvr that lead the author into the STEN building project and selected Indianapolis Ordinance for the 80% receiver tubes, bolts, spring kits, ejectors and trunnions necessary to complete the project(s) within the scope of existing laws, both federal and state.

For the purpose of illustrating the necessary details of this build, various images from other builds will be shown. The learning process for building these guns has come from a mix of six different STEN projects encompassing multiple MK2, MK3, pistol, German MP3008, and MK2/MK3 hybrid variants. Despite the differences between these STEN variants in overall layout, the basic principles of drilling, dremeling and filing will remain the same.

While it is far quicker and easier to complete the STEN using a milling machine, this tutorial is focused on the budget-conscious builder, or maybe someone that desires to feel somewhat like a backwoods partisan behind Nazi lines. Thus, the only tools required for this build will be a dremel (or other rotary tool), a set of calipers, a set of drill bits (make sure they can cut through 4130 S/S ), thin metal blades for the dremel (.020 thick, not the .060 thick ones), a vise (wood grips inside the vise to prevent marring and crushing the tube preferred), a spring-loaded center punch and files. Try not to break the dremel blades and do not wear the blades down to nothing since as the blades wear down, the smaller diameter remnants will be needed for the smaller slots that need to be cut.

Before starting to cut all the slots, use  your automatic center punch to punch six witness marks down the length of the tube at 12:00/3:00/6:00 and the 9:00 positions. You will need these when the paper is off the tube.

Next, you are going to prepare the corners to be drilled and either squared or rounded off.  To do so, mark the corner diameter center lines with a center punch, drill a pilot hole and then drill a with successively sized drill bits until the slot ends are made. For instance, use a ⅛” bit to start a hole, then a 3/16”, then a 5/16”, then a ⅜” and so on. The pictures below will illustrate this point.

In these photos you will see bplvr’s notes written on the tube. Notice how in photos 1-3 that the corners have been marked with a center punch to allow the pilot hole to be drilled accurately.

Photos 4-9 show how each slot to be cut has a hole in the corner. Each hole was started with a ⅛” drill bit then increased in size so the corners were taken out in a perfectly rounded fashion.

Before you cut anything ,using your automatic center punch, punch 6 witness marks down  the length of the tube at 12:00/3:00/6:00 and the 9:00 positions. You will need these when the paper is off the tube.

Start with the long charging handle slot. Run the entire length .010 deep on one spot and repeat until the whole slot is cut. The goal is to connect the ⅜”diameter drilled hole at each end of the slot and 2 x ⅜” holes for the safety slot to form one continuous slot. At this stage of the game, the slot will not be a perfect ⅜” width, but this is what drill bits and files will be used for at a later stage. The finished slot should be .375 +.010 /-.000. This slot is what determines the angle of the location of the semi-auto bolt to the ejector. (see image 10 and 11 for what the slot looks like before and after)

Next, you will cut the magazine well slot going from front to rear. The same will be done for the ejection slot.  After both slots have been cut,  change out your cutting blade for a fresh one and save the old one for cutting the smaller J-slots and the like. (See image 10-5 to see the J-hooks that are cut with the used, smaller cutting blades)

Next up is the sear lever slot, as seen in image 5. This will be cut in the same way as all the other slots and sized all the way down the channel with a .125-.140 drill bit.

STEN sear lever slot .125-.140 drill hole. Size with drill bit. Square or round end. Ideal width is .015 wider than the sear lever.

The dimension for the location of the sear from the rear of the tube will most likely be shown as 3.750”. It is best to change this dimension to 3.850” and file later to match your sear location.

Most STENS were made  with VERY liberal hole locations, and if you cut the sear slot too short your tube is junk.

Next up are the J-slots to hold in the end cap. Use a big cutter to start the J-slots and finish the corners with a ¼” drill. Use the smaller cutting wheels saved from before to make some of the finer cuts, as there are some tight working spaces in this area. If you don’t have any small cutters, then you will have to take a big cutter and grind it down on a sacrificial piece of metal to get it small enough to get into those small slots and cut them right.

After all the slots have been cut, it is time to file everything (and I mean everything) to fit. For all the channels and slots, you should use the shank (the non-drilling portion) of an appropriately sized drill bit to size the slot from one end to the other. Wherever there is a snag, examine the area and determine which direction one needs to file. Before filing cut some long pieces from a soda bottle and slide them into the tube. This will keep the file from dinging the inside of the tube. All file strokes should be spread out and not concentrated into one part, as this will cause issues with unevenness. The easiest way to tell is to look at the paper glued to the tube and see where the black lines are. The black lines should disappear with the right amount of metal removal, assuming that they don’t catch fire first.

Deburring the inside of the slots is also critical, as the bolt won’t ride in a tube full of obstacles. File down the inside of the tube at an angle to get rid of all those burrs on the inside.

Next article: Trunnions, welding

Good luck and enjoy your build!

– pullnshoot25 and bplvr

bplvr is a long-time manual machinist, tinkerer and collector that is proficient in engineering solutions for adapting firearms to fit into the stringent California legal landscape. While working he was employed as a Process Engineer. He is the California Gun Show Coordinator for and just rescued his 101st Golden Retriever, affectionately named “Major Dick Winters”.

Nathan Wolanyk (generally known as pullnshoot25 on the internet) and is a recent graduate from UCSD with a BS in Human Biology. He was President of the Marksmanship Club @ UCSD and was active in the California Open Carry Community.

*Do not attempt without suitable knowledge and experience. Human Events accepts no responsibility for resulting injury. Firearm laws are subject to change as well as interpretation on federal, state and local levels. Be aware of federal, state and local laws before attempting, and/or consult a licensed local attorney for applicable laws. YOU MUST ABIDE BY ALL LAWS: STATE, FEDERAL AND LOCAL.