The Sten Gun

By Wheats, Group42

Introduction

Enter the 9mm sub-machine gun. The entire Stengun family evolved from a basic design of simplicity. It was designed by Reginald V. Shepperd, directory of the Birmingham Small Arms Company, and Harold J. Turpin, the principal designer, in conjunction with the Royal Small Arms Company at Enfield. The name "Sten" was derived from the first letter of the last name of each of the designers and the first two letters from Enfield. The Stengun Mk I went from the design phase to production in less than three months. Several changes and improvements were made and the famous Mk II design was finalized. The entire family of Stens are noted for their outstanding performance and unrefined appearances. By the end of production, over 2,000,000 Stens had been made at less that $5.00 per copy.

Construction

WARNING!!! Actually constructing a working Sten gun will require the advanced permission of the B.A.T.F.! Constructing a Sten gun with advanced permission is a SEROIUS CRIME!

Of all the many Sten guns that were constructed during the second world war, most of them ended up here in the United States. They were stripped of their receivers ( the part the B.A.T.F. considers the gun) and are now available from many sources as parts kits. These plans will focus on the construction of a new receiver that will work with these parts kits. It is very possible to put a fully functional Sten Mk II or Mk III together for much less than $200

Select a suitable lightwall steel tubing which is commercially available. For example, a fence post pipe (galvanized) is 38.5mm OD and 35.0mm ID, most suitable for use as a receiver. 4130 alloy steel tubing is the preferred tubing to use, referred to as DOM (drawn over mandrel), it is known for it's uniform size, wall thickness and smooth interior surface.

Cut receiver tube to 12.65 inches in length. Then use the enclosed template by either gluing or taping over the blank tube. Make sure at least one end is flush with the tube. Recheck the alignment of the template with the blank tube. It is now a simple matter to remove all the shaded areas on the template. By using this method, layout time is reduced. This feature makes the Sten an ideal project for the handyman with a minimal amount of tools.

The areas to be removed from the blank are as follows:

1. Recoil spring cap locking notches. These are the small “j” or “l” shaped cuts at what will soon be the rear of the receiver.
2. Bolt or cocking handle slot. This is the long slot that allows the bolt handle to reciprocate. This must be straight.
3. Sear notch opening. This is located on the bottom rear of your receiver. It allows the sear to hold the bolt in the open position.
4. Ejection port. This is the large opening on the right side near the front half of the tube.
5. Magazine port. Positioned on the left side opposite the ejection port, this opening is for the insertion of the magazine.
6. Magazine housing alignment slot. By allowing the magazine housing to rotate 90 degrees, this converts the magazine housing into a dust cover. This is located left, in front of the magazine housing itself.
7. Barrel sleeve latch hole. As the magazine rotates upward toward its’ shooting position, this prevents the housing from overtravel.
8. Ejector slot. The ejector in the parts kit will fit here. This is the slot at the rear of the magazine port. Be sure that the installed ejector does not interfere with the travel of the bolt.

Construction Tips

Before making any cuts on the receiver tube, carefully read all of the instructions. The length of your Sten is not very critical. Most wartime copies vary in length to some degree. Do, however, make sure that the ends are square and true. The single most important cut of all is the cocking lever (or both handle slot) be careful to make it straight and square to the ends of your tube. To avoid problems, use the template.

If you are using the template and for whatever reason things don't line up, now is the time to correct it, not after you have cut metal! Cut the sear hole last. Also, make sure that the slots for retaining the butt stock are 180 degrees apart.

If milling machines and/or drill presses are not available, the entire job can be done with hand tools. After laying out the correct cuts and trying on all the parts that fit (without actually cutting), you may start. Simply drill a ¼ inch hole in each area to be cut out. By drilling more than one hole you can connect the holes using files and a hacksaw. It is best to leave these cutouts just a hair on the small side to allow for final fitting.

Once cuts are made on the receiver blank, try the parts on the tube from time to time. This will help guide you along without mistakes. Once the parts are test-fitted and correct you can prepare the tube for welding. Welding can be done by any conventional method available. If conventional methods are not available, you can also use the brazing method. Brass rod can be used but it would be stronger using steel, even a coat hanger will do in place of a brazing rod.

After final assembly and welding is completed, you are ready to hand operate the Sten and prepare to test fire. Make sure the sear will engage the bolt. If it does not, runaway gun will result.

To test fire, use only one round at a time in the magazine. The Sten should fire the single round and lock the bolt reward in the open position. Check to see that the extractor pulled the empty cartridge case free from the barrel. Wear eye and ear protection during this stage.

Diagrams

• Receiver Diagram.
• Trigger Group.
• Receiver Rear End Bushing and Main Spring Cap.
• Whole Gun Parts Layout.

Rate of fire:      550 rounds per minute
Weight:            6.62 pounds
Barrel:            7.75 inches long
                   6 groove
                   Right hand twist
Length:            30 inches long
Operation:         Blowback, selective fire
Cartridge:         9mm Parabellum
                   Bullet weight:      116 grains
                   Powder weight:      6 grains
                   Muzzle velocity:    1400 ft./sec.
Feed:              32 round detachable magazine
Sights:            Fixed rear, driftable front
Recoil Spring:     Wire diameter:      .067 inches
                   Spring OD:          1.00 inches
                   Active coils:       15
                   Free length:        9.40 inches
                   Initial length:     6.80 inches
                   Final length:       3.20 inches
                   Work stroke:        3.60 inches
Bolt:              Weight:             1.327 pounds
                   Cocking handle:     0.077 pounds
                   Recoiling weight:   1.404 pounds
                   Bolt max. dia.:     1.381 inches
                   Bolt overall dia.:  5.75 inches
                   Bolt body length:   4.21 inches

1.  Barrel              2.  Bolt assembly        3.  Bolt handle
4.  Recoil Spring       5.  Recoil spring cup    6.  Recoil spring cap
7.  Front Sight         8.  Barrel sleeve        9.  Barrel bushing
10. Magazine housing    11. Sear Pin             12. Sear
13. Sear spring         14. Selector             15. Selector stop pin
16. Selector Spring     17. Trigger housing      18. Trigger housing cover
19. Stock               20. magazine assembly    21. Trigger pin
22. Trigger             23. Sear lever           24. Trigger spring
25. Ejector