Starting a STEN Mk V SBR in the US

[17thairborne]

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.

Okay...thanks for that comment. I think I will alter the ejector by filing the aft portion somewhat as well as the thickness and work with it until the bolt clears.

The IO instructions indicate a method of modifying the disconnect lever by cutting a semi-circular notch in the arm to accommodate the selector pin. THere is a requirement to weld a piece of metal with a grove to prevent the disconnect lever from moving side to side. These two steps apparently allow you to use the selector pin to put the weapon on safe.

Anyone done this?

Information

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[17thairborne]

Video below showing normal semi-auto functioning

[Peter Laidler]

I don't know anything about these single shot closed bnolt guns but you are clearly on the right lines. I cannot see the usual relieved section for the nose of the tripping lever on your special breech block and so presume that you don't have it because your gun will ONLY work in the semi-auto role. The TRIPPING LEVER is tripped by the breech block on the forward pass of the breech block. If you trip that lever, even if you trip it with your finger to show a class of students, it WILL release the sear upwards, under the tension of the trigger spring (via the pawl of course). If your bolt isn't tripping the tripping lever then something is misaligned. Check this. Occasionally we'd get this with service guns with a slightly bent tripping lever that would foul the edge of the underside of the casing. Occasionally you'd get a knackered change lever slot that might push the lever too far....., or not far enough.

If you are going to bend the lever, be aware that they are bloody hard and springy and occasionally, the buggers would just snap! The nose that contacts the breech block is also hardened and the Mk5 gun lever should have a chamfer on the rear that the bolt face can cam against.

Hope that this has answered the Q's. But if not, it will be a simple alignment question - hopefully.

A question for you........... Can I assume that because your breech block is totally different to the standard ones I'm used to working with, can I assume that you cannot have the Mk5 safety 'push-through-to-lock' cocking handle?

[tiriaq]

It would seem that the bolt assembly is not contacting the tip of the tripping lever enough to depress it, and allow the sear to catch the hammer as it comes forward. If you had a full length firing pin, the gun might fire bursts. As is, it would not recock, and would wind up hammer down with a round chambered.
Make sure that the tripping lever is not being fouled by the sides of its slot in the casing, isn't being twisted to one side, etc. Strip out the bolt assembly, and see the sear will disconnect if you poke the tip of the tripping lever with a rod. It should react positively, with a noticeable click.
Also, the tripping lever was designed to work with a main casing with rather thin walls. Your casing is made with, what, .093" thick walled tube?
It may be that the tripping lever's tip is simply not projecting far enough into the casing to be activated by the passage of the moving parts.
If this is the case, either the tip of the tripping lever needs to be built up, or its notch which engages the sear needs to be deepened, allowing the tip to rise further into the casing.

[tiriaq]

Could you use it with a 16 1/4" barrel in the meantime, or would that muddy the waters?

[17thairborne]

No, that would be perfectly legal as far as I know, I just did not want to spend the dough for something I would not use more than once. The only problem I can foresee aside from the $ would be the tuning of the weapon. The process for tuning the gun involves firing single rounds and evaluating if the hammer and breech block springs need to be shortened or not. I would think the longer barrel would generate some greater blow back forces (could be wrong here) and I would have to have two sets of springs, one for the 16" barrel and one for the shorter one.

DOes that sound right?

[Vincent]

DOes that sound right?

In a way, yes and no.

We're dealing with Newton's third law here. So it's mostly about mass (bolt weight). The longer the barrel, the more mass needed. The force pushing the bolt back hasn't increased. Only the time the bullet is in the barrel has gone up.

Without more mass the casing will be further back in the chamber when the bullet exits the barrel and the pressure drops. Not an issue for your Indy Ord bolt. It has plenty of mass. But with light bolts that relied heavily on advanced primer ignition as full-auto's, the casing can travel too far back in the chamber before pressure drops causing the case to bulge, split or rupture. The PPSh41 and PPS43 are good examples. With long barrels and lighter semi-auto bolts they will split a lot of cases and have very dirty breeches. But who reloads 7.62x25mm?

A stronger recoil spring will have a higher initial closing force, but there comes a point where the weapon becomes hard to charge. That can be overcome to a degree with a double spring system, like the Sterling has. The light spring makes charging easy, while the stronger spring captures most of the energy.

There's an section on blowback gun design in "The Machine Gun" by George M. Chinn. I think it's in volume 4. It also has all the formulas and it's a free download.

For my semi-auto Sten I made a small spring cup. (Same concept as the Sterling spring system). The cup keeps the light spring from reaching its solid length (fully compressed) and getting damaged when the bolt cycles. The cup has a hole in its center and slips over the spring guide rod with about 1 3/4" section of stronger spring behind it.

Thank you for documenting your build here. It's a lot of effort and much appreciated.

BTW, Building is very addictive.

[tiriaq]

Very well could be. I have fired the same carbine with both original length and long barrels, and it certainly seemed snappier with the long barrel.

[17thairborne]

That would certainly get me on the range much earlier than waiting for the ATF!

[Peter Laidler]

Thast's a well thought out piece Vince. You know my views on API of course but that doesn't affect this situation of barrel length here. The Beretta 30's type SMGs have a long 9mm barrels and while it's not comparing like with like here, it's worth pointing out that we regularly used to fire them with standard government Mk2z ammo with no ill effect or signs of bulging.

What is interesting is that in our SMG barrels, they never exceeded the optimum 7" length. Which leads on to another question........... Who decreed that 7" was the optimum length? And optimum length for what? It COULD be the optimum length for mediocrity! Another interesting point was that during manufacture, breech blocks were constantly weighed and before being passed. had to weigh-in within a set of tight criteria. This was important when a different steel, of lighter or heavier density was being used and the weights were controlled by the depth of the central boreing in the rear end.

Oh for a lab where we could all gather after school for some extra after school physics lessons............. You mention Newton...... I used to take a Sten gun (dewat of course) into those lessons knowing that it was one thing that would really hold attention and make it a memorable lesson for its relevance. I used to wrack my brains for a similar small arms related lesson that could be used to realistically illustrate leverage, forces anf fulcrums......