Starting a STEN Mk V SBR in the US

[Vincent]

I never imagined this project would be so informative to me. Rather than a "wow...nice project", you guys are creating a classroom where free discussion rules the day.

Glad you see it that way, 17th. I was feeling bad for rambling on in you build thread.

but the bolt moving forward before firing was always a bit "weird" to me.

Yes, I know the feeling you're talking about. Both the M3 and the Sten are like that. The Sterling has a much better balanced feel.

It looks like you're enjoying building your Sten. Give a Sterling some thought while the kits and semi-auto conversion parts are available. It's a little sweetheart. I am sure you would enjoy it.

Information

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[browningautorifle]

Interesting, Interesting, Interesting...can't wait to watch this develop. Also I'd like to see how it's all arranged parts wise in the end, if you please...for the engineer in all of us...

[17thairborne]

Interesting, Interesting, Interesting...can't wait to watch this develop. Also I'd like to see how it's all arranged parts wise in the end, if you please...for the engineer in all of us...

Will do! I am curious as well. I will dry fit all of the trigger and hammer components just to see how the function is. I can't find anyone whose posted photos of that.

I also have to clean up my ejector tab and size the slot so it fits snugly. Also, the extractor pin in my old bolt is stuck. I do not have a drift pin to knock that out. Was hoping not to have to buy another set. We'll see on that issue.



Fitting the AR firing pin into the bolt with the spring; when the hammer is pressed fully into the recess and held, the pin does not protrude. I suspect the inertia of the hammer striking the pin pushes it through the opening in the block face and onto the base of the primer, and then the spring causes the pin to retract back beneath the face of the block.

[17thairborne]

I have a question for the armorers.

I have fully assembled the STEN, minus the barrel, firing pin and I still need to modify the ejector. When I cock the weapon and let the bolt ride forward, the hammer is visible aft of the block (normal operations for the closed block system). As I pull the trigger, the sear lowers and releases the hammer which is propelled by the tensioned spring behind the hammer causing the hammer to slam forward against the pin. (Still normal operation here).

I simulate the aft forces on the block of a live round by manually pulling the block aft as in recoil. I notice that if I keep rearward pressure on the trigger (simulating a slow release of the trigger) the sear remains in the downward position and does not lock the hammer back. Given the fast block movement under live fire conditions it seems this scenario would result in a block forward, a round in the chamber and an un-cocked hammer. In order to get the hammer to cock, I must release the trigger BEFORE the block moves aft of the sear. Normally the rearward movement of the block would allow the tripping lever and sear to move upward, catching the notch of the hammer and the trigger would (click) once I relaxed rearward pressure on the trigger to fire the next round.

I assumed the rearward movement of the block would allow the vertical arm of the tripping lever to move into the small recess of the block causing the sear to move upward thus catching the notch on the back of the hammer.

If I release the trigger before I move the block aft, it functions normally.

What could be causing this malfunction?

Is the vertical arm missing the grove in the block? Perhaps the narrow slot at the bottom of the receiver tube through which the tripping lever arm passes is off to the felt or right and the arm is not aligned with the grove in the bottom of the block?

[browningautorifle]

I take it that round stock in the pic is the "Hammer" and just the inertia drives it against the firing pic...?

[Peter Laidler]

The inertia firing pin is a mechanical safety feature (I expect that you are all aware of this - hopefully) that means that unless there is the full force of the hammer blow, it won't protrude with sufficient force to detonate the round. We used to do a classroom test with Browning No2 pistols to test this theory which meant dropping the pistol on the hammer to see if the FP would strike a dummy round sufficient to pop the primer. So presumably this is the safety reason for the single shot Sten to prevent an accidental discharge unless it is from a full force of a hammer blow.

This used to raise all sorts of questions such as hard primer material etc...... This eventually caused several modifications to the hammer spring strength due to different ammunition being brought in. This included several uprated striker springs followed by ever greater hammer strength springs and so on and on. Instead of just going back to basics!

[17thairborne]

Here is a video of what's happening. I think i've described the problem using correct terminology. I think I will need to file down the base of the tripping lever so that it protrudes farther into the receiver tube to allow the bolt to push it back down.

YouTube

[tiriaq]

The floating inertial firing pin is a BATF requirement for semi auto Stens made in the US, along with closed bolt operation. If the firing pin isn't a floater, defeating the disconnector function could result in burst fire, as the hammer follows the bolt foreword. If this were not a BATF requirement, a full length firing pin - or even a one piece hammer/firing pin would be an easier way to go. A floating inertial primer is a safety factor, but that is not why it is mandated.
A floating, inertial firing pin will contact the primer when the bolt closes, but should not have enough energy to fire it. An example is the AR-15 type rifle - invariably if a round is ejected, there will be a slight mark on the primer. The US closed bolt semi auto Sten designs use AR-15 firing pins, no doubt because the pins are readily available at low cost.
Some US do-it-yourself designs use a original bolt, altered for closed bolt operation and hammer or striker ignition. This requires extensive alterations, of course.

[17thairborne]

tiriaq and Peter,

I think you've both hit upon the problem and provided some help for me. The thicker tube is preventing contact between the rearward moving breech block and the tripping lever (TL). The TL is staying in the grove, its just not touching the breech. As peter stated I have the Mk V modified TL that has been ground slightly at the rear of the rounded arm. I thought I had some Johnny's Garage modification there until you mentioned it, but I think it's good.

I think my best solution is to modify the tripping lever by filing the base where the notch is. This will allow the arm of the TL to sit deeper into the receiver tube and make contact with the block. I will have to file down the TL both in front of and behind the notch or I will defeat the purpose.

I had thought of filing down the ramp of the sear where the TL sits, but that will change the movement moment from the pin around which the sear rotates.

The ideal solution would be to have a TL that has an arm that is slightly longer and protrudes farther into the receiver tube.

The current state would prohibit FA because the hammer stays in the breech block and would simply ride forward together with the firing pin and spring with insufficient inertia to detonate the primer. I think. That thought had crossed my mind, but there just isn't sufficient force on the firing pin without a fully cocked hammer spring releasing that heavy hammer against the firing pin and its spring. I doubt the primer would even get a dimple. Even if one filed down the TL to create this situation, you still would not have FA. You would have to install a fixed firing pin, which I'm assuming would be illegal.

[tiriaq]

SAS bolt also fouls the ejector. Even prevents the bolt from going all the way into battery. Easiest to just alter the ejector to clear the bolt.
Found this out on my first project, had to alter the ejector after it was welded in place. Much easier to do before welding.

The safety cut in the main casing is sort of awkward to use - to rotate the bolt handle up and into the slot, the guide rods et al also have to rotate. The whole thing has to be dragged around.
There are ways to alter the selector into a rotating or crossbolt safety.