What's the difference between recoil and blowback guns


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gunsrfun1
March 7, 2007, 06:11 PM
Hello - Can someone explain to me – in very simple terms - the difference between a “recoil-operated” and a “blowback-operated” pistol. The only thing I can seem to figure out is that a recoil-operated pistol (like a BHP, 1911, Kahr, or Glock), has a barrel that pivots or swivels or moves to some extent when fired, whereas a blowback-operated pistol (like a Walther PP) seems to have the barrel permanently affixed to the frame, with no movement when fired. Am I right, and if so, tell me more as to how they actually work (simply). Thanks

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cuervo
March 7, 2007, 06:21 PM
The other big difference is the locking system.

On something like the 1911, the barrel and slide are locked together by lugs and grooves lining up with each other. On a 1911, these are the raised bars that go across the top of the chamber area on the barrel that mate with corresponding grooves in the top of the slide.

Other manufacturers have modified this idea in different ways, but it is essentially the same on all recoil pistols--the barrel is locked to the slide and then pivots down and the slide moves back.

On a blowback pistol, the only thing holding the slide in place is the recoil spring--there are no locking lugs of any type. Therefore, on a recoil pistol, the spring must be pretty stout. This is why blowbacks usually stop at .380, a .45ACP blowback would have to have a very heavy recoil spring to operate safely. It would be so heavy that trying to manually rack the slide would be very difficult.

A third type of action is to have a hole in the barrel and bleed off some of the gasses to operate a piston to operate the recoil. The Desert Eagle is a good example of this.

Wiki also has a description:
http://en.wikipedia.org/wiki/Semi-automatic_pistol#Actions.2C_blowback_vs._locked_breech
and
http://www.m1911.org/locking.htm

jlbraun
March 7, 2007, 06:26 PM
Therefore, on a recoil pistol, the spring must be pretty stout.

Don't you mean "on a blowback pistol"?

MrAcheson
March 7, 2007, 06:38 PM
You have the basics. On a recoil operated pistol, the slide and barrel recoil together for a set distance before they unlock and the barrel tilts to pick up the next round. That distance allows the bullet to leave the barrel and the chamber pressure to drop to safe levels. On a blowback pistol the barrel is fixed and the slide moves opposed by spring tension alone.

The extra mass of the barrel added to the slide is what allows the recoil design to fire more powerful cartridges than the blowback.

gunsrfun1
March 7, 2007, 06:44 PM
OK all, thanks. And just so I'm clear -- both recoil and blowback operate on the principle of "for every action there's an equal and opposite reaction", right? So basically the bullet leaving the gun forward, basically causes the slide to move backward, correct? (Assuming you don't limp-wrist.)

MrAcheson
March 7, 2007, 06:49 PM
And just so I'm clear -- both recoil and blowback operate on the principle of "for every action there's an equal and opposite reaction", right?It is my understanding that the entire universe operates on this principle, not just firearms. :D

Actually it is the pressure of the expanding propellant gases which does the work that causes both the bullet to leave the barrel one way and the slide to recoil the other. But yeah thats about right.

alucard0822
March 7, 2007, 07:38 PM
More or less when a shell is fired the gas in the case is attempting to push the bullet and anything in frictional contact with it (barrel) forward, the case is pushed backwards, initially a locked breech recoil action holds the breech and barell locked together ,they both slide backwards in opposition to the bullet moving forward. the gun is timed so that as the bullet is leaving the muzzle as the action nears a cam, wedge, link or block release that releases the slide and stops the barrel. Then the extractor pulls the case untill it it kicked free by the ejector, the slide keeps moving rear untill it ends it hits a stop, then the recoil spring pushes it back forward, stripping another round out of the mag, closes the slide and locks the action. A blowback action has the barrel mounted to the frame, and once the shell fires immediately the recoil begins to opens the action and move rearward, the ejector kicks the shell, the action stops and then is pushed back forward stripping another round and chambering it. The big advantage of recoil operation is weight savings and less spring tension needed. The bolts weight speeds or slows the action in a blowback, like the tommy gun with a heavy bolt. There is also a delayed blowback setup that buffers the bolt and slows it as it moves rear, as in the mp-5. a little longwinded, but i hope it helps http://science.howstuffworks.com/machine-gun.htm

Vern Humphrey
March 7, 2007, 08:05 PM
One key point is that in blowback operation, while the recoil spring has some effect, it is the mass of the slide, not the spring, that really governs the speed of the slide as it moves back. If you had a slide of the same weight as the bullet, it would move back at the same speed the bullet moved forward -- and the spring would do little to slow it down!

gunsrfun1
March 7, 2007, 08:17 PM
OK, this is all great info and thanks, but now you have raised another question for me. From what you all and wikipedia are saying, seems like the slide actually starts moving back as soon as the gun is fired -- even before the bullet fully exits the barrel. Is that right? (I always thought everything stayed fully locked up until the bullet left the barrel, keeping the sight picture theoretically undisturbed.) So based on what you're all saying, when you are zeroed in on a target and pull the trigger, the slide is actually moving back even before the bullet leaves the barrel, which would "somewhat" throw off your aim, right? I know its a split-second of course, but am I correct? (Yes, I know it's impossible to hold a perfect sight picture anyway, but hopefully you understand what I'm asking.) thanks

Vern Humphrey
March 7, 2007, 08:31 PM
The slide does move quickly, as soon as the bullet starts moving. It doesn't "disturb" your aim, because once the hammer falls, you're through aiming. Everything that happens from that point on happens according to the laws of physics.

alucard0822
March 7, 2007, 09:08 PM
you be the judge http://www.thehighroad.org/showthread.php?t=259799 This is where "timing" can be important, the case is still locked in the chamber, the gas is sealed from the back as the bullet leaves the muzzle, a lightened slide, light recoil spring and heavy slow bullet could potentially mean that the action could start to unlock before the bullet leaves the barrel. In gas operated guns the breech remains locked in place untill the bullet passes the port in the bore and the gas is diverted. They are also reliant on a powder burn rate close to what the gun was designed for, slow powders can put too much pressure into the cylinder, think bent garand op rod. Accuracy comes from consistent movement before the bullet leaves the barrel, if the slide moves straight back 1/4" each time it will not affect accuracy, by the time the hammer falls aim and sight picture are basically set, this is also the reason heavier bullets hit higher, the muzzle rises more and sooner to oppose the action of the heavier bullet while it is travelling down the barrel

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