Blowback vs Locked-Breach


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samualt
April 6, 2003, 11:32 PM
I'm having some trouble trying to understand the difference in Blowback and Locked Breach automatics.

Locked Breach seems to have a barrel that is seperate from the frame. The breach is on the slide which interlocks with the barrel before being ready to fire. However, the breach is only sealed by the cartridge so I'm not sure I understand the term locked-breach. I guess in a way it is locked with the barrel but it certainly isn't sealed except by the cartridge.

Blowbacks seem to have a fixed barrel. And if I understand correctly the breach/bolt is not interlocked in any way. It's like a solid tube pushing the bullet into the barrel and then firing it, with the bolt being kinda free wheeling. Again, the cartridge seals the chamber. (I don't have one of these so I can't look and see.)

I know I got most of that wrong somehow. Anyone got a better explanation. And why would a Blowback have a worse recoil? It seams to me that which ever you use it's still just going to be the force of the bullet pushing back on the gun. All this came about as I'm looking at the difference in the little .380 and 9mm pistols. Most of the .380's seem to indicate the blowback mechanism and the 9mm has locked-breach. Why not have a 9mm blowback? Isn't the .45 ACP Tommy Gun a blowback? I know people say the locked-breach is better...I'm just trying to understand why?

:confused:

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Handy
April 7, 2003, 12:22 AM
That isn't a bad way to look at it.

In blowback actions, the casing extracts itself under its own gas pressure. It blow itself back. The case and slide act as a single unit until the case hits the ejector.

A locked action is one where some external mechanism unlocks the barrel from the slide. In most pistols, the mechanism is some sort of camming action that takes place after the slide and barrel have both moved a bit. In most auto rifles, the external mechanism is ducted gas that acts on a bolt carrier, unlocking the bolt from the barrel. In both cases, the slide or bolt must then extract the case mechanically.

3 gun
April 7, 2003, 12:58 AM
Weight is the problem with a high power blowback pistol. The slide has to be big enough to keep the chamber closed until the pressure drops. This takes weight.
A locked action delays the case long enough for the pressure to drop. By being locked to the barrel until that drop in pressure, the slide can be much smaller and lighter. Slide weight is why you seldom see blow back pistols larger than 380/9x18. The only exception I can think of is Hi-Point pistols.
Tommy guns got by with the added weight by being shoulder arms, weight wasn't as big an issue as it is in a handgun.

Blackhawk
April 7, 2003, 01:23 AM
Blowback: Nothing except the recoil spring and the weight of the slide prevents the slide from going back once the round ignites. The recoil spring has to be strong enough to prevent excessive slide acceleration and to load the next round into battery.

Locked Breech: The slide cannot move back until the barrel unlocks due to the initial recoil impulse. The recoil spring needs only to be strong enough to cycle the action once the breech is unlocked and the slide has moved to its rearmost postion. A great percentage of the recoil impulse is dissipated in unlocking the breech.

I'm in a hurry now, so I may edit this....

MrAcheson
April 7, 2003, 09:08 AM
Samualt,

I understand your confusion. If the same bullet goes out of both guns, shouldn't they recoil the same? The physics answer is that the impulse required to counteract the pistol recoil is the same (integrate recoil force over time). However the specific feel of the impulse is not set by this equation. Locked breech pistols spread recoil out over a longer time, so the max recoil force is higher and the character of the recoil is softer.

mete
April 7, 2003, 11:51 AM
Guns using 9mm or bigger cartridges require a heavy slide and very strong recoil spring ( Astra 400 , 600) with blowback action. When it comes to recoil ,in the same cartridge blowback is highest, locked breach is second and delayed blowback (HK P7) is the lightest recoil because of differences in the force/time curves.

JohnKSa
April 7, 2003, 09:48 PM
In a locked breech gun, something holds the breech against the chamber until gas pressure has dropped.

Various guns use different methods to accomplish this.

One way to easily identify most locked breech pistols is to retract the slide while watching the barrel.

In a blowback gun, the barrel will not move at all. In a locked breech gun, the barrel and the slide will move together (locked together) for a short distance and then the breech will unlock and the barrel will stop while the slide continues to move. The short period of travel where the slide and barrel are locked together allows the pressure in the chamber to fall to safe levels before the action is opened and the casing is ejected.

There are some guns which use gas pressure to lock the breech but they are usually described as gas-delay blowback guns.

Handy
April 8, 2003, 12:22 AM
Not all locked breech guns have mobile barrels. The Desert Eagle is an obvious example.

PCRCCW
April 8, 2003, 08:50 AM
The DE is a Gas operated locker. A browning design Locked Breech will always travel with the slide.....

A short explanation of recoil....blowback vs browning l/b.....

Due to the "non locking" blow back design..the only moving part is the slide..when the rnd is fired, the bullet going out the barrel starts the "equal and opposite reaction"..the inertia of the slide going rearward....because there is nothing to slow/stop it momentarily it moves very quickly and more quickly.
At the end of the slide travel..the spring compress's fully, the slide stops against the frame and returns into battery.....
Thus the recoil felt is a "rearward force=fast, sharp hit" and its over.......

The recoil on a locked breech gun starts with the same equal/opposite reaction of the rnd....the locked breech is needed because of higher case pressure. The higher pressure takes a nano second to clear the chamber......the slide/barrel stay togethor for a "read very very short but adequate time" traveling backwards togethor...so far same as the locked breech in feel.
At a given point when pressure has dropped from the chamber, the barrel is stopped..usually by unlocking itself from the slide and being pulled down to clear the slides continued movement.
The action of the barrel being pulled down..causes another "equal/opposite reaction"....the muzzle rises...causing a "twisting effect" felt in the recoil. This also in some guns slows the slide travel somewhat.....the slide travels back, spring compress's and returns to battery.....
Thus the recoil felt is a "twisting/rearward force=not as fast" as the blowback design.......

.380 typical guns=lower pressure/but faster more direct felt recoil.
9mm typical guns=higher pressure/but slower/twisting less direct felt recoil.....

In locked breech guns...many things effect recoil....the more bullet weight=more transferred recoil, higher bore axix= more transferred recoil, higher case pressure=faster recoil impulse......

Shoot a +P+ 9mm in a 1911 and then shoot a reg pressure 45 230 gr....you can feel the difference in "sharp/more intense hit/twist" of the 9mm vs the slow/less sharp recoil of the 45...

Shoot well......

45auto
April 8, 2003, 01:03 PM
Good thread.

Where does the Luger fit in? I am not familiar with them, but assumed the bbl was fixed and the "toggle" did all the rest of the work. But, it's a 9mm which is too powerful for a blow back design?

So, to summarize, there are blowback designs (all with non movable bbls), locked breech with various designs to lock the bbl and slide together, and gas operated that allow a fixed bbl for higher power calibers, ie, DE, P7 etc?

Thanks for the input.

SDC
April 8, 2003, 03:39 PM
The Luger is a locked-breech design, but it uses a slightly different method of unlocking the bolt from the barrel; the "knee-joint" in the toggle keeps the bolt forward against the chamber while the pistol is firing, but the whole top half of the pistol moves rearward under recoil, until the "knobs" on the side of the toggle hit the frame-mounted cams; this "breaks" the toggle upward, and lets the bolt continue to the rear to extract and eject the fired case. The barrel IS fixed, but it's fixed to an upper receiver, and that upper receiver moves during feeding, firing, extraction, and ejection.
HTH.

AZ Jeff
April 8, 2003, 03:42 PM
The Luger is a short-recoil operated autoloader. It just happens to use a breach locking system that is different than the traditional Colt-Browning type wherin the barrel tilts to lock/unlock from the slide.

On the Luger, the entire barrel/breachblock assembly recoils on the frame a short distance, at which time the toggle knobs hit ramps in the rear of the frame. At that point, the toggle kicks "over center", and unlocks. The breachblock then recoils the rest of the way rearward, independent of the barrel, extracting the cartridge as to goes.

Handy
April 8, 2003, 05:33 PM
45auto,

It looks like you have 3 categories listed, but there really are only two: Blowback and Locked breech

Blowback has several variations, like straight and delayed. Delayed blowback has 3 or 4 variations itself, including gas delayed, like on the P7.


Locked Breech guns also have several variations. Recoil operated, gas operated, recoil unlocking (the Benelli shotgun system). Each of these, in turn, can be broken down more specifically.

What all locked breech firearms have in common is that breech cannot be opened due to chamber pressure, alone. That function falls souly to the blowback guns. Locked actions, whether semiautomatic or manual are unlocked by a force external to the chamber.

45auto
April 8, 2003, 06:58 PM
Thanks for the responses, very informative.

Take care.

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