Various Question on Locked Breech vs Blowbacks (Question #1)


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Snowdog
January 19, 2005, 01:08 PM
Everyone's under the impression a pistol with a locking breech is inherently stronger than a pistol that's a blowback... but is this true, and if so, do most understand why?

Here's what I know: A locked breech allows for rearward travel of the barrel/chamber which is mechanically locked with the breech face/slide (I believe known as dwell travel) under the inertia of the bullet traveling through the barrel, thus allowing "dwell time" which buys enough time for the pressures in the chamber to decrease. Therefore, by the time the extractor begins to pull the casing from the chamber, the brass is no longer hugging the chamber walls as tenaciously as it was during the dwell travel.

Here's what I don't know: Why exactly do these pressures need to decrease?

Here's what I think: Primarily to aid in extraction.
I'm under the impression the principal advantage of the locked breech design is to ensure easy extraction of spent casings. Certainly it isn't to keep the case from rupturing...or could it? After recognizing the success of various manufacturers that use blowback actions chambered in serious cartridges such as 9mm, .40S&W and .45acp (Highpoint, for one), I’m am cautious of assuming there are dangers of casing failure without the breech being locked.
Sure Highpoint must pay for this by having a massive slide, but they manage to get it done.

So, the only obvious reason I see for having a locked breech is for proper extraction... but I really feel I'm missing something.

I have other questions concerning blowbacks, but I will form a separate thread rather than inundating the thread with multiple questions.

Thanks in advance!

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Jim Watson
January 19, 2005, 05:02 PM
Certainly the main purpose of a locked breech is to get the bullet out of the barrel before the action opens to expose the case wall to chamber pressure. Blowbacks don't have any trouble extracting, if they are balanced properly. Some extreme cases like H&K even though heavily retarded in blowback, or the little AMT .22 Magnum blowback, have gas bleeds between brass and chamber to keep the case from sticking under residual pressure.

A gun that shows this is the the Tanfoglio - Lapua ARS which looks like an ordinary TZ/CZ but is of straight blowback action. The key is in the Lapua cases which have heads solid for about a quarter inch, the distance the slide moves before the bullet exits the barrel.

Jim K
January 26, 2005, 12:10 PM
To begin with, when a cartridge is fired, pressure builds up inside the case, driving the bullet out the barrel and the case back against the breech. If there is no way of keeping the breech sealed until pressure drops, the case will back out too soon, and burst from the internal pressure.

There are two basic ways to keep the breech closed during the high pressure period, which is to say until the bullet has left the barrel. One is to have some sort of mechanical lock. There are many versions, but in pistols, the most common is a method of locking the barrel to the breechblock so that the two pieces recoil together until something unlocks them. The mechanism is made so that the barrel and breechblock stay locked together until the bullet has left the barrel, then the momentum of the breechblock is used to complete the extraction, ejection and reloading cycle. Note that pressure does not itself operate the gun; motion of the bullet forward causes recoil to the rear. If the barrel is blocked so the bullet cannot move, the gun will not operate even though there is pressure in the chamber.

The second way of keeping the breech closed is by the mass of the breechblock. The bullet moves forward at high speed, but it is light weight, so it takes time for the backward pressure to overcome the inertia of the heavy breechblock. This is satisfactory for low-pressure cartridges. But the blowback pistol can have a fixed barrel, obviously an advantage, and there have been many ways developed to make use of the simple blowback action for higher power cartridges. This usually takes the form of a heavy breechblock (Hi-Point), strong spring (not very satisfactory), or some kind of delaying or retarding mechanism.

As for extraction, the simplest extraction is the blowback, because there is no extraction involved - the case is blown back out of the chamber. A blowback pistol does not need an extractor except to remove an unfired or misfired round or to act as a pivot point for the ejector. Some blowback pistols (small Berettas) don't even have an extractor, using a tip-up barrel to remove an unfired round.

Jim

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