Building the Canadian Semi-Auto STEN

[Claven2]

Here in Canada, a semi-auto version of the STEN has finally been RCMP and CFC approved. To get one, you must build one as there is no commercial source. At this time, the only approved semi-auto STEN here in Canada is the SAS3 conversion suitable for the MkII, MkIII and MKV sten.

The FRT Number for the home-made Semi-Auto STEN SAS3 in MkII trim is 126844-1



This thread is intended as a pictorial aid to help those considering a build. It will be updated as progress is made. Additions may be made by any and all builders

[Claven2]

The initial build will be of a MkII based on a Canadian Army demilled Longbranch STEN MkII kit. It was torch-cut by the Canadian Army in the 1970's. The barrel was gouged at both ends with an acetylene torch, the stock base plate was partially torch-cut and in some cases the triggers were cut in half.

This build is actually a pictorial composite of two builds of similar kits being built side-by-side at the same time, one restricted with original length barrel, and one non-restricted with an 18.5" barrel. One kit has an intact trigger, the other will need repair (cut in half). Both stocks were repaired before the build began by filling the torch cut areas using gas welding and they were then filed back into shape.

Kits are available from Milsurps.com advisory panel member Stencollector. Contact him for info. Here is what the kits look like as received:




You will immediately notice that the mag well comes saw cut. This is because the RCMP have ruled that the mag well, bearing the markings and serial number, is the prohibited (automatic) firearm. The kits are shipped with a saw cut mag well so that it can still be used for dimensioning purposes. To get your semi-auto sten registered in Canada, you will need to fabricate a new magazine housing. More on this later.

[Claven2]

The first step is to clean up the kit, that is to say, remove all the parts you won't use for the build such as the old tube, the bolt and spring (if you have them), the old barrel, etc. This can be a challenge on some kits. As a rule, if the bits are arc-welded together, you cut away the welds with a grinding disc, and if they are spot-welded, the easiest thing to do is to drill out hte spot-welds and later on, re-use those hols to plug weld the kit together.

Here is a late production (1944) Longbranch spot-welded kit cleaned up:

[Claven2]

So maybe you want to avoid the hassle of getting a STEN barrel exported from the USA. Sounds reasonable. There are two ways to fabricate a STEN barrel. Both will require a section of 9mm barrel blank. The first is to lathe out the contour of the barrel, the second is to lathe out a cylinder of the proper diameter and then pin on a barrel base collar. Original barrels were made BOTH ways. Use whichever way works for you! In this case, we had a blank with enough diameter to make a one-piece barrel. You will need a lathe to do this.


In this photo we see a raw chunk of barrel blank, a lathed out barrel, the front barrel busing portion of the receiver (salvaged from the kit) and the barrel nut.

Mocked up:

Note the barrel is NOT chambered. This will be done when the bolt is head-spaced.

Now, to finish off the exterior of the barrel, we bead blast it to facilitate parkerizing later on.

[Claven2]

Now we begin fabrication of the magazine well.
It is best to measure the thickness of the steel used on your de-milled mag well. Get a grade of sheet steel that matches so that you dimensions will be the same when you build your box because you will be making all your measurements from the gun you are copying.

This part can be done very simply or very complexly. We chose to fabricate from three pieces so as to get precise bends.
Step 1. make your first two bends. For those interested, this sheet-metal bending rig was bought at Lee Valley in Ottawa (they do mail order). The vise is up to you though!

Step 2. fit the two pieces together, use your original mag well for dimensioning. It is important to note, make your mag well longer than you need so that you can trim to size after you weld it to the receiver sleeve section.

Make sure a magazine goes in BEFORE you weld!!

Just tack weld at first, then completely weld up the seam. A belt sander and file work great for cleaning the outside, the inside must be completely hand-filed. Once done, cut a section of steel to form the back of the box.

Here are the parts up against a receiver tube collar like the one we will be welding to later:

Once you measure and measure and measure and tril fit and measure again, weld on the back plate:

Does it fit? Use your demilled collar to mock it up and check:


Now dress the welds:

The ends of the mag box will need to be squared off. If you have a mill, use it. It works great. If not, carefullt scribe lines to ensure a square mark around the circumference of the mag box and carefully grind it level.
In the mill:

Done!

It should fit a mag at this point:

[Claven2]

The SAS3 kit does not use the whole original cocking handle. Due to the striker-fired design, it uses a truncated handle. You can just trim an original MkV handle like will come with your kit, but a better solution is to use a repro MkII cocking handle to match the fact there will be no cocking handle safety hole on a SAS3 receiver tube. An added advantage to making a (very hard to find and rare in original form) MkII handle is that you can fit it precisely to your bolt so its does not wobble around on you.

You make one on a lathe and will need a knurling tool.


And here's a couple we made up compared to a real mkII handle:

[Claven2]

As Isaid, some kits come with chopped triggers. The top trigger is original, the bottom trigger had the part you pull with your finger cut off. A bottom half was fabricated in sheet steel and mig welded on, then filed flush:

[Claven2]

Now we'll make the mag well locking collar. This holds the locking pin in place to lock the magazine housing into position when firing.

Let's measure the locking pin collar...


Great! This bar stock will fit the bill!

Back to the Mill that Klunk hates, because we're "Fancy"

Looking good...

More measuring...

And sectioned off with the saw...

Drilling out the hole...

Brazed on, an original on a demilled Longbranch mag housing is on the right.

[Claven2]

Now we'll look at building a 18.5" non-restricted barrel with dummy suppressor for the non-restricted Sten we're building on the British Sten MkII kit.

We start with a spare 9mm barrel blank we had lying about. There is some old corrosion on the outside of the blank, but the bore is perfect, and once the tube is bead blasted, parkerized and covered by the dummy suppressor, the cosmetic blemished won't be a factor.

Here we have the barrel retained by a barrel nut that will form the base of the suppressor. The suppressor will thread onto the Sten receiver in place of the ventilated MkII barrel nut/handguard. The serrations on the base of the barrel nut will be done later with a cold chisel.

For scale, note the muzzle bushing at the top:

And here we have the assembly minus the dummy suppressor tube. This suppressor is being built to copy the longer SOE Version of the MkIIS, not the standard tube which is much shorter.

The barrel is threaded into the sleeve that provides the shoulder for the barrel nut, as the original barrel outer diameter was too small to create a shoulder in the lathe. We used 3/4-16 threads, external ones cut with a lathe, internals with a tap. The threads on the collar are lathe cut.

The barrel has been torqued into the sleeve quite tight so we may not install a transverse retaining pin - there would be no way to access a pin without a hole in the barrel casing (dummy suppressor), and its not needed to secure the threaded joint.

To remove the barrel nut from a standard Sten barrel, you would use a punch through one of the rearmost holes.

The body of the suppressor is just made with appropriate diameter steel tubing.

[stencollector]

I usually heat up the old barrel nut to separate the threaded part from the short tube. I then recycle this onto the dummy silencer. On my version of the silencer, I went to about 14" length, and had the barrel stick out about another 4". I left the remaining barrel unfinished so it wouldn't stand out. Also, it reduced the weight at the nose a bit. I think you are going to find this gun to be a bit nose heavy.