Sten gun question

[Newton]

Is there any possibility that a modern cartridge weapon could be designed in such a way that it lights off a cartridge before that cartridge is fully seated in the chamber.

I have just read this statement in relation to the Sten gun and my instinctive reaction is to call it horse pebbles, but as I'm not sure exactly what does cause the firing pin to impact the round on an open bolt design firing in full auto, I wanted to check first.

Thoughts and opinion please.

 

[Justin]

Slam fires are not unknown. Usually happen in guns with firing pins that aren't under spring tension.

IIRC, the Sten has a fixed firing pin. I may be completely wrong, but it might be possible for the gun to detonate a round if the primer is seated all of the way into the base of the brass.

 

[Newton]

Thanks Justin, but slamfires aside, do you think that the Sten (or any gun) could have been designed with the intention of igniting cartridges prior to complete seating.

I'm thinking case rupture, headspace issues, gas blow by etc etc - it just seems crazy, but I want to be sure, stanger things have been known.

 

[Justin]

Can't speak for the Sten, but no I can't think of a valid reason for setting off a cartridge before it's fully seated for all of the reasons you listed.

 

[Kurush]

Yes, it's called advanced primer ignition. The idea is that the primer is ignited a split second before the chamber is fully closed, by the time the gasses start expanding the cartridge is in place and the inertia of the bolt and the strength of the spring keeps the bolt closed until pressures drop to a safe level. Most open bolt SMGs including the Sten use API.

 

[Cortland]

http://files.uzitalk.com/reference/pages/article_uzihistory2.htm

The advanced ignition principle is quite simple. It requires no extra parts. An advanced ignition open bolt subgun’s chamber depth is made a few thousands of an inch shorter than the cartridge case’s length. This causes the forward moving bolt’s fixed firing pin to ignite the primer a moment before the bolt strikes the chamber face. As the recoiling cartridge case begins pushing rearward, it meets the resistance of the forward-moving bolt. The two forces oppose each other. The bolt’s motion is reversed and the cartridge’s felt recoil is reduced. The use of advanced ignition makes the firing cycle seem smoother to the operator.

Advanced ignition also enhances controllability by reducing the subgun’s muzzle ‘climb.’ The heavy telescoping bolt’s center of mass is moving forward of the subgun’s center of gravity (CG) at the point of cartridge ignition. The inertial action of the telescoping bolt, pushes the advanced ignition subgun’s muzzle forward and down, thereby reducing felt recoil and countering the recoiling cartridge’s attempt to make the muzzle ‘climb.’ If the subgun designer has done his job well, the two opposing forces cancel each other and the subgun is very easy to control in full auto.

Finally, the inertia of an advanced ignition subgun’s forward moving bolt must be overcome by the recoiling cartridge case. This allows the weight of the bolt to be reduced, decreasing the subgun’s weight.

 

[Justin]

Learn something new every day.

 

[Jim Watson]

API has been used clear up to 20mm antiaircraft guns. The famous Oerlikon gun is unlocked blowback operation with API and a 450 lb recoil spring to keep the pressure behind the shell. When the sailors called it "spring operated" they meant it.
http://www.usstexasbb35.com/20mm_gun.htm

 

[Gabe]

I'm not sure if any WWII subguns worked by API. Most post war subguns used this principle. i.e. Sterling, Carl Gustav M45, Uzi, etc.

 

[enfield]

How much is "a few thousandths"? I seem to remember that the common tolerance on case length is +/- .005 inch. if the chamber is designed to be deliberately "a few thousandths" short, it seems to me that the bolt face/case head interference is going to range from, at one extreme "a few thousandths" to, at the opposite extreme, "many thousandths". Am I wrong? What's the nominal dimension (compared to the case nominal length) and common tolerance on chamber depth? Am I missing something? How much interference can you have before the bolt doesn't lock up? Is a Sten a recoil or a blowback design? Does the bolt ever lock up against the chamber face in a blowback design?

Sorry, it's the engineer in me -- just trying to understand this.

 

[Jim Watson]

I don't know the distances involved, maybe somebody has the design specifications. Guns are not made to watchmaker tolerances, certainly not a $9 wartime design like STEN. And the Brits never have been handloaders so they didn't care if the brass was a little beat up. What they started with was captured Italian ammo anyhow. Reason they picked 9mm instead of .45.

A STEN gun - and any other firearm correctly identified as "blowback" - has no lockup. Nothing holding the 35,000 psi in but the inertia of the breechblock and the force of the recoil spring. And the forward momentum of the bolt in a gun running with API.

 

[Newton]

Wow, API - duly noted.

You'll excuse me but I have a sizeable piece of humble pie to consume in front of my work colleagues

Thanks guys - excellent responses as always.

 

[Dave Markowitz]

I'm not sure if any WWII subguns worked by API. Most post war subguns used this principle. i.e. Sterling, Carl Gustav M45, Uzi, etc.

The STEN gun does indeed use API.