Starting a STEN Mk V SBR in the US

[17thairborne]

Advance primer ignition doesn't really fire outside the chamber. In theory maybe, but the cartridge is actually fully seated and the bolt is the only thing still moving forward. That way it takes more pressure to move it back, keeping things closed longer...in theory. People picture a cartridge firing while on it's forward trip and that's just not what happens. It has to bottom out first. I know what the SAIC instructors taught us, but it just wasn't quite right...

Makes sense what you say regarding the STEN with fixed firing pin firing from an open breech block. In this case there is a point where as the block with round stripped from the mag moves forward into the chamber on the barrel, the pressure of seating the round is sufficient to cause primer ignition. With any slight changes in the chamber caused by dirt or powder residue, or problems with the round, it would seem possible that slightly higher friction between the round and the chamber would be imparted onto the fixed firing pin, and premature primer ignition could occur before the round is sufficiently seated to contain peak pressure caused by powder ignition.

With a closed bolt system, it seems reasonable to conclude that the round would be fully seated before the hammer strikes the firing pin. I would assume premature ignition in the closed bolt system is eliminated if the trigger is not allowed to function until the bolt reaches the closed position. Is that correct?

[browningautorifle]

Both correct. A gritty chamber WILL cause a round to stop and fire out of battery. The resulting minor catastrophic detonation causes shrapnel and smoke to exit the receiver at various angles...and if it's hammer fired, it doesn't go off until you tell it to.

[Peter Laidler]

On any other firearm there would be a mechanical safety that ensures that the weapon cannot be fired while the breech is open or even partially open. It WILL be fully locked. I'm not happy about this notion that crud in the chamber will prevent a 9mm round from being chambered AND FIRED. That might be theory. BUT, in real life, there is another form of mechanical safety with the SMG's. First the practicaliity of what size bit of dirt or crud will prevent a ? weight breech block, under load from the stored enegy of the strong return spring, chambering the round? It might of course......... And secondly, during movement from magazine to chamber, the round is moving over from left to right and then being tipped - or directed - into the open chamber and only in the very last moment of its journey from magazine to full chambering does the base rim of the round finally align with and then sink or drop into the cartridge seating. And even then, it is only THEN that the centre of the primer aligns with the striker situated in the cartridge seating. That's why we can have such a thing with fixed firing pins.

This is clearly visible on skeletonised Stens/Sterlings/Lanchesters etc etc, when you look down through the large opening above the feed way and observe the movement of the round you will see this. And it is this important feature that differentiates the open bolt auto fire UZI from its closed bolt semi auto brothers. Have a look at them when you see them together - unless the rascal has modified his semi............

As for API........ Armourers and production engineers will argue among themselves, pretty well forever, as to whether there is really such a thing as API or Advanced Primer Ignition. Or is it simply a by-product of having such a thing as a fixed firing pin (a FFP)? I say that it's just a by-product of the FFP simply because if you have a fixed firing pin then that gun cannot fire from an open bolt. Well, it COULD if you hand loaded a round into the chamber. The round will have to move across the face of the breech block and it ain't going to is the fixed firing pin is preventing it!!!!! This means that the firing pin is recessed in the cartridge seat and it is THIS final .040" of movement, when the cartridge is 'home', fully aligned and out of harms way that starts the firing sequence.

The benefit is that the bolt is cushioned by the ignition of the cartridge but that dispells another hoary old chestnut. Some old timers will assure you that during single shot or auto fire with a FFP SMG, that the breech block doesn't actually touch the breech face of the barrel. Next time they tell you this, tell them to stop talking a load of old bollo..........., er.......... tell them to stop talking testicles. It certainy DOES touch the breech face of the barrel and a simple smear of engineers blue will very quickly confirm it

Have I veered off course yet........ Just a sign of getting older!

[17thairborne]

And secondly, during movement from magazine to chamber, the round is moving over from left to right and then being tipped - or directed - into the open chamber and only in the very last moment of its journey from magazine to full chambering does the base rim of the round finally align with and then sink or drop into the cartridge seating. And even then, it is only THEN that the centre of the primer aligns with the striker situated in the cartridge seating. That's why we can have such a thing with fixed firing pins.

Peter,
The block is inserting one cylinder (the casing) into another concentric cylinder)the breech). At some point the two cylinders will be aligned along their longitudinal axis (that has to occur well before the casing is fully seated) and the striker will be in contact with the primer before the block is fully forward. Otherwise the breech would have to be somewhat conical except at the very forward part. AM I wrong here? I really need to see the sectioned images to be sure.

[browningautorifle]

crud in the chamber will prevent a 9mm round from being chambered AND FIRED

I had it happen Peter. A sub machine gun C1 9mm that was getting dirty after a few thousand rounds simply had one go off early with that very result. I was standing right there as safety. The gun continued to function after examination and a brush through it. I still have the casing here...in parts...the rest of the engineer's viewpoint on the API, I'm glad you agree with...more or less. I ain't an engineer...

[Peter Laidler]

Not saying that it won't happen BAR. It CAN happen but everything and the circumstances have to be right......... or should that read 'everything has to be WRONG' when it does. I had an out-of-breech cartridge detonate too. But it was caused by a previous ruptured cartridge case and the next few rounds WOULD NOT SEAT PROPERLY OR FIRE. The firing point Sgt took the gun and rammed the breech block home with his left thumb. Infortunately his fingers were over the ejection port and got slightly shredded while his thumb was broken (- or was it badly sprained?) by the reciprocating cocking handle.

17th....... The case actually drops into the cart seating in the breech block at the very last moments.

Someone PM'd me to ask that if I didn't believe in API per-se, what did I think it was then? As I said........ I think it's just a convenient by-product of having a recessed FFP. As to how it works........ I say that blow back is just another form of piston operation. The cartridge case simply acts as a piston. It might be a one shot one-way-trip piston but that's all it is. Mind you, there's views enough for everyone.

But this discussion started much earlier shows that while SMG's might be simple, there's a LOT more to their operation than you think. Good thread.....

[browningautorifle]

the circumstances have to be right......... or should that read 'everything has to be WRONG'

Agreed, it only happened the one time. I never saw it happen with a Thompson for instance. But, I fired vastly more rounds with SMG C1...

[Peter Laidler]

We had multiple bulges in a Thompson 1928 firing captured 'bandit' ammo of which I'll say no more. Rounds backed up in the barrel until the last round went into the chamber, stubbed into the last jammed bullet and wouldn't fire because the last round wasn't fully home and this prevented the hammer operating on the firing pin.

It cost a new barrel (thanks Brian.....) and we slit the old barrel to find 11 (?) rounds in it. It is still on show in the metallurgy lab.

The lab tech was of the opinion that it was the fins that acted as strengthening hoops that prevented the barrel from splitting because it'd already exceeded the limits of its elasticity when it bulged. After that more bullets simply jammed up and this compounded the problem!!!!!!!

[browningautorifle]

Again with the SMG C1, I had a factory IVI round on the range with no powder. It fired and there was the classic failure to operate and resulting "cock and look". Casing ejected, go on...next round had a somewhat louder report and a flash but no other difference. I examined the weapons during cleaning and found the classic ring in the barrel...another training example for the troops, then off to the weapon shop...

[17thairborne]

Fiiiiiiinaly I orders and received the rebuild kit for the STEN. The parts arrived yesterday and I have just unpacked them and laid them out for a quick photo. I have to read all the literature and review some posts here and elsewhere to make sure I follow the proper steps.