This is my new Sten. There are many other like it, but this one is now mine...

[browningautorifle]

it would reduce or eliminate advanced primer ignition. The breech block would be in contact with the barrel and at rest where the fired cartridge starts to move the breech block back

The cartridge is always at rest when it fires...if it wasn't then why would chamber depth be critical? If you run a ream in and deepen a chamber by a few more turns your weapon will no longer function...API is balloon juice...theory...

[Vincent]

The cartridge is always at rest when it fires...

Yes, in this the cartridge is at rest. But we are talking about the breech block, not the cartridge.

if it wasn't then why would chamber depth be critical?

In my limited experience with blowback SMGs the chamber depth is not very critical. There’s no headspacing.

If you run a ream in and deepen a chamber by a few more turns your weapon will no longer function...

Most of the blowback SMGs I am familiar with have short chambers that leave quite a bit of the cartridge case unsupported.

I have made just about every mistake you can think of while making guns. One was to use the spec for a semi-auto that has a much deeper chamber than the full-auto version of the SMG. To my surprise, it shoots fine.

API is balloon juice...theory...

I never thought of it as a theory, just an application of Newton’s laws of motion.

There’s a mathematical analysis in volume 4 of George Chinn’s book “The Machine Gun”. https://www.milsurps.com/content.php...eorge-M.-Chinn)

My first experience converting an open bolt SMG to closed bolt didn’t go well because I didn’t really understand Newton’s laws of motion. I didn’t know a considerable amount of mass needed to be added to the bolt when going from blowback with API to plain blowback. I was lucky. The lugs on the buttstock stopped the endcap from hitting my face. After doing some research I added more mass (we can just call it weight) to the bolt, a stronger return spring and all was good. That experience made me a believer.

I can also attest to the rule of not adding water to a big tank half full of concentrated acid… a TIG welder can give you a nasty electric shock… sneakers will not protect your feet if you’re welding and a big globs of molten steel fall on them… pulling the handle down on a drill press really hard can make the drill bit snap off and it can go a long way into your forearm… if you’re extending the prongs on your wife’s Taser, it’s a good idea to take the batteries out first… a Taser (like the one my wife has) really can make you wet your pants… you don’t need a lot of gasoline to start a bonfire.. don’t fall asleep on the bank of a drainage ditch if you’re hunting hogs at night, big poisons water snakes really will try to stay warm by snuggling in with you…

[browningautorifle]

I follow all of the last paragraph...very closely. I have a couple more, like Black Powder doesn't burn progressively when lit by hand. Never eat ice cream with a knife...

The blowback V?S API blowback, I take it you had a lighter than issue bolt? That could unhinge your receiver. I was wondering what firearm that happened to? I used a stronger than spec spring set on an M3 and it ran like and Ingram...that was spectacular. Harder to hang onto though.

The primer of an SMG has to fire before the bolt hits the breech of the barrel and if the firing pin hasn't done by then you're hoist, so a chamber depth is critical. The ones a bit less supported can burst if you have a dirty chamber, I have the casing remnants from a C1 SMG from the '80s. The cartridge has stopped moving forward, the bolt is still travelling forward. I had problems with an old 1928 Thompson barrel and when another ran a ream in to "freshen" it up, it would swallow the cartridge, and I watched him work. Believe me it was nothing much. It wouldn't fire after. I have other examples too...

Headspacing is the distance from the mouth of the casing to the face of the breechblock where the cartridge is seated against it. So it does exist...but there will be variables. I built up an AR in 9mm recently and chambered the barrel a bit deeper than the original...as long as the case is sandwiched solidly between the bolt face and case mouth it will be fine too...

The sfaety factor in the API guns is the forward momentum of the heavy bolt. That has to be stopped before the bolt can start rearward and during that time the pressures have dropped...

I remember we had a three pager on this one other time. I stayed out of it that time.

[Vincent]

The blowback V?S API blowback, I take it you had a lighter than issue bolt? That could unhinge your receiver. I was wondering what firearm that happened to? I used a stronger than spec spring set on an M3 and it ran like and Ingram...that was spectacular. Harder to hang onto though.

It was a Mk4 Sterling bolt I was converting to semi-auto, like a Mk6 bolt. It was actually a bit heavier than a standard Mk4 bolt. The firing pin spring retainer I made was very light. So the total recoiling mass was considerably less than a Mk6 bolt with its heavy steel spring retainer.

The sfaety factor in the API guns is the forward momentum of the heavy bolt. That has to be stopped before the bolt can start rearward and during that time the pressures have dropped...

My understanding is stopping the forward momentum of the bolt uses energy. So there is less energy available to send the bolt back. This allows a lighter bolt to be used.

I follow all of the last paragraph...very closely. I have a couple more, like Black Powder doesn't burn progressively when lit by hand. Never eat ice cream with a knife...

Ouch!

I like knives. I don’t know why they don’t like me. Especially the ceramic ones.

[Bruce_in_Oz]

The issue of chamber depth is another one of those "mysteries". In a gun like the Sten, the cartridge has to be "restrained" somehow, long enough for the bolt to move forward far enough for the extractor to start engaging at almost the same time the fixed striker is acting on the primer. The catch is that the base of the round has to be "held" until the recessed bolt-face moves forward to surround it and for the fixed striker to engage the primer, unlike in your basic auto pistol. The case head cannot do this until it is effectively fully chambered and thus concentric with the barrel and the bolt-face recess.

The 9mm Parabellum is paradoxically "the latest thing" whilst also being introduced in 1902.

It has a slight, but visible body taper, as one would expect for its age and initial use in pistols; Georg Luger's little gem springs to mind.

The taper was probably employed so that once the toggle "broke" and the case had "cracked away" from the chamber, very little effort was required to fully extract and eject it.

Meanwhile, across the big pond, some bloke named Browning had, in 1902, developed a somewhat similar cartridge, the .38 ACP, not to be confused with the later, smaller, .380 Auto.

The .38ACP, and its "hot-rod" cousin, the .38 Super, have an essentially parallel-sided case.

Browning's "next big thing", apart from sundry rifles and shotguns, was the .45ACP and its "delivery system", the M-1911 Auto pistol. This cartridge has a whole .003" included taper on its body.

It seems that the "case-mouth step" on the chamber of the Browning-designed barrel is more of a "cone" than a "step". The design and specs of the pistol also seem to have allowances for pistol component and ammo "variations".

Modern, commercial barrels all seem to have a "conventional" step. The original .38 Auto / .38 Super, are vaguely "semi-rimmed" and have virtually no "step" in the chamber. The engagement of "rim" and chamber is so small, that the rather beefy extractor is probably doing most of the work. Most modern .38 Super chambers are cut with a "proper" step at the front, not unlike a 9mm Para. See also; 9 x 21 and 9 x 23.

Note that the "classic" blow-back SMGs have a fully "surrounded" bolt-face. If they didn't have this feature, as the case head was sliding across the bolt face during feeding, pistol-style, it would snag on the fixed striker and everything would stop. All those "duly-papered and taxed" manufacturers/armourers who have "jacked-up" an Uzi pistol into a "Micro-Uzi", so beloved of action movie directors, will know what I mean. A LOT of engineering work was involved for something that appeared, in action, on screen for just a few rather visually dramatic seconds. It was “exciting” enough with movie blanks; I would not be keen to try it with 2Z ball..

[Luis Bren]

Congratulation 5 Commando! Your Sten is Beautifull!!

Is my dream, Is my nightmare having or reproducing a sten gun. Specialy the Sten MK V !

Enjoy your Sten!

Regards!

[browningautorifle]

My understanding is stopping the forward momentum of the bolt uses energy.

Agreed...

It was a Mk4 Sterling bolt I was converting to semi-auto

So since I'm understanding it was firing from a closed, forward bolt, therein lay the problem...nothing to lock it. I'm not surprised at the smack, I don't like the 1927A1 Thompsons available in semi auto these days that fire from a closed bolt. It gives .45 an almost unpleasant recoil...for a little cartridge.

That whole API thing implies the cartridge is fired while it's still going forward...yes, the bolt certainly IS still in motion. Hope that isn't what we're going on about...

[Vincent]

That whole API thing implies the cartridge is fired while it's still going forward...yes, the bolt certainly IS still in motion. Hope that isn't what we're going on about...

The cartridge can be fixed, as in a blowback SMG, or moving, as in a Oerlikon cannon. All that matters is the bolt is still in forward motion. The bolt is the mass that has to be stopped before it is sent back. The energy expended in stopping the mass (bolt) leaves less energy to send the bolt back. With less energy available to send the bolt back, the bolt can be lighter.

The .38ACP, and its "hot-rod" cousin, the .38 Super, have an essentially parallel-sided case.

Browning's "next big thing", apart from sundry rifles and shotguns, was the .45ACP and its "delivery system", the M-1911 Auto pistol.

One of my favorite pistols is a blinged out .38 Super 1911. Chrome plated with gold grips. Mexicans love it… the Border Patrol belt buckle not so much.

Some people say all 1911’s are same and all ARs are the same… until they shoot a Les Bear and an LWRC.

[tankhunter]

I wont go into the complexities of the API working principles. It will bore those who understand it, & TBH.
I don't really have the time today!....

BUT!.......There IS a difference in chamber depth for Blowback SMG's/ Carbines that fire from an open bolt.
And closed firing semi only carbines/ look alike SMGs.
It is VERY easily proved!.........Albeit a Dodgy way of showing this!.......(This is Info ONLY. & I am NOT endorsing/ advocating that any of you do this!)

If you have access to a 'standard' sterling SMG of any of the variants in Select fire. Instead of putting a magazine in the housing & firing ONE round.
Insert a live cartridge in the chamber, & squeeze off that ONE round. Locate the empty case & look at it!!!!
You WILL see that it has a large Bulge near the base! This will look like a bottle neck, with a LARGE belted bulge! I have done this in service a few times to demonstrate to service users. The dangers of attempting to fire in this manner!

The round should ALWAYS be loaded & fired from a Magazine! As this is part of the cycle & execution of the complete API Firing system!!!

Closed Bolt firing Semi's operate completely differently. & to that end, the case NEEDS to be supported as completely as possible. During the Ignition Stage of the Pressure curve, during the firing cycle.

WARNING! Service Ammo such as the British MK2.Z 9mm round. Has sufficient case wall thickness to withstand the above situation description. (In most cases!)
COMMERCIAL 9mm Freely available. Will have thinner walled cases, & metallurgical properties that are mostly Variable. To that end, I again URGE you. NOT to attempt to try what I have described above!

**** The information is Technical Advice, & I do NOT advocate anyone on here to attempt to replicate the above procedure!!!!****

[browningautorifle]

The cartridge can be fixed, as in a blowback SMG, or moving, as in a Oerlikon cannon. All that matters is the bolt is still in forward motion. The bolt is the mass that has to be stopped before it is sent back. The energy expended in stopping the mass (bolt) leaves less energy to send the bolt back. With less energy available to send the bolt back, the bolt can be lighter.

I understand all that. What I said is bunk is the actual firing of the cartridge without it being chambered, as in the bolt and cartridge are still moving(both)when the cartridge magically fires. That's an impossibility with a fixed firing pin. I'm not speaking of the Oerlikon. It's a different animal. Just things like Sten, Sterling, M1A1 Thompson... The rest of it I understand... Even the part about the chamber depth. The closed bolt weapons don't have any way for the pressures to drop like the open bolt does. That's why the new 1927A1s have such a different recoil and pressure marks on the case. Yes we too taught not to chamber a single round in the SMG C1 unless emergency (battle). The designer could have left the name of the action at blowback...because it's correct... Then there's retarded blowback as in the Thompsons with the Blish lock...which again didn't do much.