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deformed Oerlikon

smle2009

smle2009

Well-Known Member
Hi everyone,
does anyone know what could have caused the necks on these 20mm Oerlikon's to end up as this? both are headstamped RG 43 OeZ,there is a faint line visable were the shoulder once was,one possilbilty that I thought of was that the gun had a damaged/very worn chamber? the round in photo for comparison is also 1943 dated,suppose I now have a pair of cool candle sticks!
Cheers Tony
 

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that 's normal with spent 20x110 RR Oerlikon cases. They take on the shape of the chamber.
 
As far as I know it was meant as a gas seal, although I have heard people say it had to do with bad headspace (breech not fully closed). The last however seems not really plausible to me.
Regards DJH
 
hi smle
It was because of the high cyclic rate with the spring recoil/load characteristic of oerlikons.
the round was chambered but upon firing as soon as the proj left the neck the action started to eject the case. As this was nano secs after firing the pressure on the breach was still very hi as the proj had only just started to travel.
This meant the case started to move out of the breach/chamber therby pulling the neck out into the body space part of the breach thus allowing the neck to expand.

hope that makes sense!!??
 
Hi H,
yes that does make sense,learnt quite a lot about"bigger stuff" this week! many thanks Tony
 
So the 20 mm Oerlikon cannon was a ordinairy "mass lock breach" gun?
 
Oerlikon

That is actully not the strictly correct explanation, Hicky. An Oerlikon works by what is known as advanced primer ignition (API) whereby the round is fired as it is still moving forward and before it is fully chambered. This accounts for the blown out shape after firing.

The advantage of this method is that it allows a simple blowback type mechanism to be used with a powerful cartridge, as the recoil has to overcome the inertia of the bolt and cartridge still moving forward before it can take effect. It is estimated that the bolt needed for this type of operation is only one tenth of the weight that would be required for a straight blow back gun.

The disadvantage is that the gun must have the springs carefully balanced for a particular recoil impulse, so any change in the ammunition needs the gun re-balanced, and all loads must have approximately the same impulse.

The Sten and most SMGs work in exactly the same way.

Regards
TonyE
 
Thanks TonyE,
This bigger stuff is certainly odd at times! also picked up a .303" blank from the same place made from a 1956 Q3! what a bummer! Still got those bits here for you.
Cheers Tony
 
TonyE said:
The Sten and most SMGs work in exactly the same way.
Click to expand...

I've always been dubious about that, Tony. API blowback works in the Oerlikon family of weapons because the bolt is the same diameter as the chamber and follows the cartridge into the extended chamber, ensuring that the case is fully supported (except at the shoulder and neck) just before, during and for a small fraction of a second after firing, as the bolt moves to and fro within the chamber. To make this possible, the cases have a rebated (reduced diameter) rim so that the extractor can hook around the rim while staying within the bolt diameter.

Of course the 9x19 doesn't have a rebated rim, so the bolt can't follow it into the chamber. This means that if the cartridge is fired before it is fully chambered, the sides of the cartridge base will be unsupported. So if there is an API effect I think it must be very limited, taking place only in the last millisecond before the bolt closes.
 
The main player in the case of Stens & Sterlings is Newtons 3rd law.
Not very familiar with Stens but having lugged around the Sterling a few times!! the mechanics of this weapon are very simple.
The round is picked up & chambered by the block & the `fixed` firing pin then fully strikes the primer upon the round stopping in the chamber. The `recoil` or reaction to the bullet travelling forward sends the action rearwards against the spring.
The trigger only releases the block sear it does not release a hammer to drive the firing pin into the round.
whilst the trigger is held rearwards the block will cycle back & forth until the trigger is released or no rounds in mag.
Releasing the trigger alows the sear to lift up & prevent the block moving forward assisted by the spring.
sorry Tony - not meaning to hijack your post!!
 
Sounds to me that the case has started to move rearwards before the pressure in the barrel has dropped to a safe level thus the neck of the case is swelling to match the shape of the conical chamber.
Paul.
 
hicky1300 said:
The main player in the case of Stens & Sterlings is Newtons 3rd law.
Not very familiar with Stens but having lugged around the Sterling a few times!! the mechanics of this weapon are very simple.
The round is picked up & chambered by the block & the `fixed` firing pin then fully strikes the primer upon the round stopping in the chamber. The `recoil` or reaction to the bullet travelling forward sends the action rearwards against the spring.
The trigger only releases the block sear it does not release a hammer to drive the firing pin into the round.
whilst the trigger is held rearwards the block will cycle back & forth until the trigger is released or no rounds in mag.
Releasing the trigger alows the sear to lift up & prevent the block moving forward assisted by the spring.
sorry Tony - not meaning to hijack your post!![/QUOTE

Hi H,
which also explains why stens & sterlings had a habit of going off if dropped!!!
Cheers Tony
Click to expand...
 
hicky1300 said:
The main player in the case of Stens & Sterlings is Newtons 3rd law.
Not very familiar with Stens but having lugged around the Sterling a few times!! the mechanics of this weapon are very simple.
The round is picked up & chambered by the block & the `fixed` firing pin then fully strikes the primer upon the round stopping in the chamber. The `recoil` or reaction to the bullet travelling forward sends the action rearwards against the spring.
The trigger only releases the block sear it does not release a hammer to drive the firing pin into the round.
whilst the trigger is held rearwards the block will cycle back & forth until the trigger is released or no rounds in mag.
Releasing the trigger alows the sear to lift up & prevent the block moving forward assisted by the spring.
sorry Tony - not meaning to hijack your post!!
Click to expand...
Thats OK, I have no argument with that description. But the debate is over whether or not the cartridge fires while the bolt is still moving forwards - which is to say, just before the cartridge is completely chambered. If it does, then the recoiling case first has to stop the bolt's forward movement before starting to push it back again - that's what Advanced Primer Ignition Blowback is all about. Most of the recoil energy is soaked up in stopping the bolt, with only a small part needed to drive it back again, so the bolt can afford to be much lighter.

As TonyE mentioned, in the APIB Oerlikons this effect is so strong that the bolt can be only a tenth of the weight it would need to be if the gun were a simple blowback. Clearly, the effect cannot be so strong in a 9mm SMG for the reasons I gave in my previous post; but just how strong is it?
 
Sten API

That is fair comment Tony. I have seen it often stated that the Sten fires by API and I am guilty of perpetuating that statement.

I must admit that when I typed it, it occured to me that it did not fit in with the theory (from Chinn I think) that the bolt could be a tenth of the weight!

It would be interesting to see a slow motion film of the Sten firing to see exactly when ignition takes place. I wil ask a couple of ex Enfield design people to see what they say.

Regards
Tony
 
From the designers drawings...

Quote: As shown in a series of three historic wartime sketches done in Harold Turpin's own hand, the Sten is designed to fire after the round is fully chambered but while the breech block is still moving foward.

The commencement of primer indentation occurs while the breech block is still .045" away from the breech face of the barrel, and continues until the commencement of ignition, with the breech block a further .017" foward. Full chamber pressure of 15 tons per square inch is reached almost instantaneously, during which time the breech block travels a further .01", at which point it is still 0.18" away from the breech face of the barrel.

The massive build-up of pressure at the instant of firing between the (departing) bullet and the cartridge case slows the forward movement of the breech block, so while the the bullet is still moving through the barrel the breech block is either moving very slowly foward, momentarily at rest, or moving very slowly rearwards. The ratio between bullet movement and breech block movement is in the range of 60:1 in favour of the bullet. The gun timing and the ductility of the parallel-sided brass shell case ensure that the breech is fully obturated throughout this cycle.

Source: The Sten Machine Carbine. Peter Laidler.
 
Thanks indeed - useful information.

It would be interesting to know what the practical benefit of the Sten's API system is, in terms of the percentage reduction in bolt weight compared with a simple blowback.
 
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