difference between recoil and blowback?


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papajack
November 29, 2007, 01:01 AM
in pistols that is. i would have thought a blowback pistol wouldn't need a slide. so what is the difference between the two actions?

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macadore
November 29, 2007, 01:17 AM
In a blowback action, the slide or bolt is not locked and the force of the case moving rearward from firing the bullet forward blows the slide or bolt back. In a recoil operated action, the slide or bolt is locked in place until the recoil unlocks the slide or bolt and pushes the slide or bolt rearward.

jr_roosa
November 29, 2007, 02:00 AM
Well, neither really NEED a slide per se, and the difference between the two actions are more in what the barrel does once the powder ignites.

Generally in a blowback action, the barrel is fixed, the slide is free to move, and once the bullet gets pushed forward the case gets pushed back. Depending on how hard the case pushes on the sides of the chamber (high pressure rounds for example) it might not start moving until the pressure drops. The only function of the slide there is to slow the ejection of the case (so it doesn't act like its own bullet headed the other way and put a big hole in your face), and transfer the energy from the ejected case to the spring where it can be used to reload. Semi auto .22 pistols are a good example of this action...rugers, high standard, etc.

In a recoil action, the slide is locked to the barrel, and the barrel is allowed to travel a bit. When the powder ignites, the barrel AND the slide start moving back together. At some point the barrel hits a stop, the slide is unlocked, and the slide continues back, EXTRACTING the case and doing the other things it did in the blowback model. The idea here is that the bullet is out of the barrel once the slide separates and therefore you don't have to worry about the pressure being wasted, big jets of hot gasses shooting out the breech, and the case needing to move out of the chamber while under pressure. Centerfire semi-auto pistols are good examples of this...1911s, berettas, etc.

Hope that made sense. Get out a 1911 or a .22 pistol and play with the actions any you'll see what I'm talking about.

-J.

brentn
November 29, 2007, 04:46 AM
So a 10/22 is blowback operated and a glock or a 1911 is a recoil operated pistol?

1911Tuner
November 29, 2007, 07:26 AM
The mechanics of the two are the same. The main difference is in how the opening of the breech is delayed. The blowback design uses slide mass and/or high-rate recoil springs. The recoil-operated design keeps the barrel and slide locked together as a unit until the bullet has exited.

Force generated by the pressures of the expanding gasses push the bullet in one direction and the breechblock...slide...in the other. The slide in the blowback moves independently of the barrel, while in the recoil-operated pistol...the barrel is tied to the slide via the upper lugs, and is pulled backward BY the slide for a short distance, until it reaches the unlocking point....typically at 1/10th to 1/8th inch of slide travel for the short recoil design.

At or just before that point, the bullet has exited...the pressures have dropped to or near atmospheric, allowing the breech to open safely....and the barrel disengages from the slide. During the time that the barrel and slide are a unit, and the barrel's mass is part of the slide's mass.

As a side-note...The slide is in motion before the bullet has exited the muzzle, though there are many who mistakenly believe that recoil doesn't start until after the bullet exits. Recoil is simple Action/Reaction, and both events begin at the same instant.

To simplify...You can't push on an object without being pushed, and
whatever measure of force that you impose is immediately returned to you in like measure.

Before anyone throws Kuhnhausen's old "Balanced Thrust Vector" description into the fray...It can't happen, and it's been amended in his later editions. "Balanced Force" indicates that the action/reaction pair are in equilibrium...which means that they're not moving...because the compelling forces and the resistive forces are equal. Only when forces become unbalanced do things start to move.
.

rocinante
November 29, 2007, 07:49 AM
good info. thank you.

MrBorland
November 29, 2007, 09:54 AM
In a recoil action, the slide is locked to the barrel, and the barrel is allowed to travel a bit. When the powder ignites, the barrel AND the slide start moving back together.

So, since the barrel isn't fixed, is a recoil-operated action inherently then less accurate than a blowback ?

As a side-note...The slide is in motion before the bullet has exited the muzzle, though there are many who mistakenly believe that recoil doesn't start until after the bullet exits. Recoil is simple Action/Reaction, and both events begin at the same instant.

To simplify...You can't push on an object without being pushed, and
whatever measure of force that you impose is immediately returned to you in like measure.

I've always understood the bullet was long gone before recoil started for the following reason: While it's true you can't push on an object without being pushed, there's the element of time: The bullet is much lighter than the slide/barrel or entire gun, so it takes much less force to move it. Since force is proportional to pressure, and the pressure wave develops over a period of time, the bullet is put into motion much sooner than the slide/barrel/gun, and is gone by the time the slide/barrel/gun begin to move. If this weren't so, I don't understand how any gun could be shot accurately and would suggest bigger calibers are inherently less accurate than smaller ones. Not trying to hijack the thread, just don't know what I'm missing. Apologies to the OP.

Liko81
November 29, 2007, 10:45 AM
Not necessarily. It's not that the barrel is completely free to move; the barrel and slide TOGETHER are free to move backwards a certain distance before the two parts separate. The barrel and slide are locked together through that movement until the end of that distance, and can only move backwards in line with their original position. Due to Newton's 3rd law, the impulse of expanding gasses propelling the bullet act on the gun during exactly the same time, so the slide is moving backward to some degree while the bullet is still in the barrel. However, it should be gone long before the two parts SEPARATE, meaning all other things being equal, the bullet travels along its original line all the way out of the barrel and to the target, and only afterward does the barrel tilt to eject and feed.

Now, that's the theory. In practice, designers have to cut corners for reliability, durability, fit, cost, reduced recoil and other considerations. This may mean that the barrel and slide do have some play when they're supposed to be locked together, or in the extreme, the parts separate or move relative to each other before the bullet has left in order to guarantee reliable cycling. A blowback, with a fixed barrel and no off-axis motion, would not have these problems.

The "link-less" design of the Ruger P95 for instance allows a very tiny bit of play during backward travel (though with mine there's only play with no chambered round) because the camblock does not stabilize the barrel during travel as much as a linked design might. The advantage is a less jarring, smoother recoil that's better for shooter and gun. In other guns, the breech and barrel may travel backwards, but the chamber end moves downward in preparation for separation, which tilts the barrel up before the bullet has left. This increases durability as the action is less jarring to the barrel at separation, at the cost of higher shots (which can be adjusted for but is a liability in rapid fire). Still others may have a weaker recoil spring than is "ideal", as a result of age, use or design, which guarantees that the weapon will cycle properly. In the extreme, the parts may in fact move or separate before the bullet has left. This seldom happens by design and would cause far more serious problems than inaccuracy (if the bullet's still in the bore when the cartridge is extracted, the gas pressure is relieved out the ejection port, often through the cartridge wall :what:). Then of course there are simply guns where pricepoint is king, where tolerances might be .01" instead of .001" or .0001". It costs far less to make them and the looser tolerances, if done right, can actually increase reliability/durability, but the action will have more play than a more precision-machined weapon.

For all but the most clinical tests and the most highly-skilled shooters, the difference is generally minute; the shooter is generally a far less precise or consistent machine than the weapon. For target shooting at 200 yards, you want a gun that will always shoot exactly where the scope is pointed (allowing you to worry about where the scope is pointed and not how far off the shot will be), and in that case a fixed barrel design is inherently more accurate. But for a man-sized target at 7 yards, inherent inaccuracies of the design simply do not figure in; almost any firearm clamped on a fixed mount will literally drive tacks at that range, and the only difference is that the shooter is not a fixed mount.

1911Tuner
November 29, 2007, 10:54 AM
I've always understood the bullet was long gone before recoil started for the following reason: While it's true you can't push on an object without being pushed, there's the element of time: The bullet is much lighter than the slide/barrel or entire gun, so it takes much less force to move it.

Nope. An experiment on another forum showed that the bullet actually requires more force to get it loose from the case's grip than is needed to start the slide.

The bullet and the breechblock are two sides of an action/reaction pair. If the bullet is gone before the slide moves, one side of the equation is missing.
As far as recoil is concerned, the bullet may as well have never been there.

The slide accelerates at a slower rate than the bullet, due to its greater mass...but it still moves the instant that the bullet moves. If the forces imposed are great enough to move an object...and the object isn't mechanically prevented from moving...it will move. It may not move very much or very fast...but it'll move. It doesn't have a choice.

Let me go find an interesting series of videos and post.

Stand by..

1911Tuner
November 29, 2007, 11:00 AM
http://www.trippresearch.com/media/movement/hispeedgateway.html

Here 'tis. Pay close attention to the very last 1911 clip...the one that so slow that it's almost a freeze-frame sequence. Make a reference point on the end of the spring tunnel, and watch the slide closely. You'll see the blowby gasses escape ahead of the bullet...then the nose of the bullet peeking out...followed closely by the fire and gas plug exiting after the bullet.

Also interesting is the bolt bounce in the AR15 and the jig that the ammunition in the magazine dances in the cutaway pistol.

macadore
November 29, 2007, 12:15 PM
Here is a cut away flash video. Click on the buttons to see an internal view.

http://www.m1911.org/loader.swf

DaveBeal
November 29, 2007, 12:52 PM
In case anyone cares, the physics behind this is Newton's Laws of Motion. The third law is the "equal and opposite action and reaction" one cited above. The second law (force equals mass times acceleration) indicates that when an unbalanced force is exerted on an object, it moves in the direction of the force. Always and immediately. That's why the breech must start moving at the same time that the bullet does. When the powder begins to burn and pressure starts to develop, initially the friction between the bullet and case resist and balance the pressure, so nothing moves yet. However, the pressure quickly overcomes this friction, and the bullet separates from the case. The pressure is equal in all directions, so the bullet gets pushed forward and the case/breech backward at the same time. The only question is how fast they move, or more precisely, how much they accelerate. Their accelerations are inversely proportional to their masses, so the much-less-massive bullet gets accelerated much more than the relatively massive breech.

And thanks to the earlier contributors for the excellent differentiation between blowback and recoil operation. I'm new to guns and had been wondering about that myself.

1911Tuner
November 29, 2007, 01:54 PM
macadore...That animation gives a good view of the internals...but the slide standing still until after the bullet is gone is utterly wrong. Bullet movement and recoil begin at the same instant. Go and watch the videos of actual pistols in operation...especially the last 1911, just before the revolver. It'll help.

To quote an old master:

"For every action, there must be an equal and opposite reaction."
"Equal" being the operative word.

DaveBeal...

The 1911 and other similar pistols are almost...ALMOST...delayed blowback in operation. The only detail that stands in the way is the fact that the slide doesn't move independently of the barrel. Aside from that, the mechanics are the same.

MrBorland
November 29, 2007, 02:54 PM
The only question is how fast they move, or more precisely, how much they accelerate. Their accelerations are inversely proportional to their masses, so the much-less-massive bullet gets accelerated much more than the relatively massive breech.


So, I think this gets to the heart of my confusion, and I think a light bulb just went on. Since the breech, and more so, the entire gun, is much heavier than the bullet, it accelerates much slower. During the time the bullet travels down the barrel, the gun doesn't really move that much, but it does move (if no one is holding the gun and opposing the force).

Working out the math, it appears that to a first approximation, Free Recoil (the distance the unheld gun moves when fired) equals the Barrel length (i.e. the distance the bullet moves while under force) times the bullet-to-gun mass ratio:

R = B (Mb/Mg)

For example, a 40 oz 4" revolver, shooting a 150gr bullet would recoil 0.035" by the time the bullet leaves the barrel if no one is holding the gun. No telling how much it really moves when the gun is held, I guess.

From this simple little equation, I now see there's no free lunch, though. For example, it suggest that those little 120z .357magnum snubbies may be hard to be accurate with, not just because of the short sight radius and the vicious recoil, but because the lighter weight of the gun allows it to move to a larger degree while the bullet's still in the barrel.

Also, barrel length seems a double-edged sword: Longer sight radius helps one be accurate and the added mass mathematically reduces the amount of recoil, but if the barrel is lengthened without increasing the weight of the gun, the longer barrel must also mathematically increase the distance the gun recoils when fired. So, if you're going to add length to the barrel, be sure the total mass of the gun increases appropriately, right? It also says a lot about rifle weight, caliber & barrel length, too. Cool.

Jim Watson
November 29, 2007, 03:02 PM
I do not think your last paragraph follows. A longer barrel would act to increase recoil only by as much as it increased bullet velocity.

I think one of the more interesting approaches to get 'round Mr Browning's patents on the tilting barrel lockup is the Pedersen "hesitation lock" of the Remington PA 51 and 53. It operates on the basis that the separate internal breechblock is allowed to be driven back in straight blowback by the thickness of the casehead, then stopped. During that short travel, it has pushed the slide into motion, so the slide travels back far enough for the bullet to exit, then unlocks and retracts the breechblock. Neat. But Remington couldn't catch up with Colt, or even Savage, and the Navy would not buy because we had gotten into WW I, so they got out of the pistol business.

HammerBite
November 29, 2007, 04:07 PM
Thanks a bunch for the link to that video.

It was interesting to see that when the barrel unlocked and the case was extracted there were only some relatively slow-moving embers and smoke drifting around in the ejection port. I have always wondered how low the pressure really drops before the barrel unlocks and this video shows that it gets pretty close to zero.

Something that I also found very interesting regarding the revolver was the flash that occurs behind the cylinder before the case fully obturates. I have never seen that discussed anywhere. Now I better understand why recoil shields get so dirty.

DaveBeal
November 29, 2007, 04:41 PM
So, I think this gets to the heart of my confusion, and I think a light bulb just went on. Since the breech, and more so, the entire gun, is much heavier than the bullet, it accelerates much slower. During the time the bullet travels down the barrel, the gun doesn't really move that much, but it does move (if no one is holding the gun and opposing the force).

If someone is holding the gun, they get moved backward with it. In fact, if the gun was rigidly attached to the earth, the entire earth would recoil, but not very quickly. ;) The rules don't change.

Working out the math, it appears that to a first approximation, Free Recoil (the distance the unheld gun moves when fired) equals the Barrel length (i.e. the distance the bullet moves while under force) times the bullet-to-gun mass ratio:

R = B (Mb/Mg)

It's actually the velocities that are determined by the masses. The mass of the bullet times its velocity will always equal the recoiling mass times its velocity. The greater the recoiling mass, the less velocity it needs to balance that of the bullet.

Also, barrel length seems a double-edged sword: Longer sight radius helps one be accurate and the added mass mathematically reduces the amount of recoil, but if the barrel is lengthened without increasing the weight of the gun, the longer barrel must also mathematically increase the distance the gun recoils when fired. So, if you're going to add length to the barrel, be sure the total mass of the gun increases appropriately, right? It also says a lot about rifle weight, caliber & barrel length, too. Cool.

Barrel length doesn't explicitly enter into the calculation, but a longer barrel means that the bullet spends more time getting accelerated by the propellant gases, so its muzzle velocity will be greater. So yes, the bottom line is that lengthening the barrel without adding mass to the gun will increase recoil.

1911Tuner
November 29, 2007, 04:43 PM
A longer barrel would act to increase recoil only by as much as it increased bullet velocity.

Not by much, I'm afraid...and it would only amount to a push instead of a punch.

The highest recoil impulse comes at peak pressure, and peak pressure in a pistol occurs within a very short distance. After a lot of experimentation, the best estimate is roughly the first half-inch of bullet movement, and in a non-magnum rifle caliber...say .308 or .30-06...using medium-slow powder burn rates...like IMR 4064 or the like...probably within the first two inches.

This all boils down to:

In a pistol caliber...About 90% of the total velocity and probably 95% of the recoil impulse occurs in the first half-inch of bullet travel. Peak drops off quickly with little area under the curve because of the light powder charges and fast burn rates. Slower rifle powders will, or course generate a high impulse over a longer period of time because of the area under the curve...but it still falls off fairly rapidly because of the increasing volume of the cylinder behind the bullet.

Think of this:

If you gain or lose 20 fps per inch of barrel...and the barrel has 5 inches of rifling...and the muzzle velocity is 1,000 fps, you get 100 fps from the barrel length. When do you get the other 900 fps?

macadore
November 29, 2007, 04:54 PM
1911Turner,

You’re right. I hadn’t noticed that.

MrBorland
November 29, 2007, 05:21 PM
It's actually the velocities that are determined by the masses. The mass of the bullet times its velocity will always equal the recoiling mass times its velocity.


I think we may be saying the same thing in the end. For example, I get the same result using the above as a starting point. I'm solving for Dg, the distance the free gun moves under recoil, that is, the moment the bullet leaves the barrel.

Let:
Mg = Mass of the gun
Vg = Velocity of the gun during recoil

Mb = Mass of the bullet
Vb = Velocity of the bullet

D = distance & t = time

As stated above:
Mg x Vg = Mb x Vb

or:
Vg = Vb x (Mb/Mg)

where V = D/t

Therefore, Dg = tg x (Mb/Mg) x (Db/tb)

Since tg = tb, Dg = Db x (Mb/Mg)

where Dg = R = distance the gun moves under recoil
Db = B = barrel length

So that R = B x (Mb/Mg)

jr_roosa
November 29, 2007, 05:55 PM
So, since the barrel isn't fixed, is a recoil-operated action inherently then less accurate than a blowback?

No. Accuracy is dependent on how consistently things happen from shot to shot.

With a blowback, the fixed barrel aids consistency, but there are a lot of things that can happen inconsistently when the unlocked breech and case start moving, like how much gas gets spit out the breech and how consistencly the case is able to start moving back after ignition (is the chamber or case more or less sticky from shot to shot?). If you could keep the breech closed until the bullet left the gun, then a lot of the inconsistency would be eliminated...unfortunately then you'd have something like a bolt-action because there wouldn't be any energy stored to cycle the action. Depending on the consistency of the ammo and how well engineered the gun is you can have a very accurate or a very inaccurate blowback gun.

With a recoil action, more stuff is moving, but it's possible to engineer the action so that the movement happens very consistently each time. For example, an "accurized" 1911 pistol one that has been modified so that everything happens very consistently from shot to shot by tightening up various moving parts and making sure everything fits perfectly. What you sacrifice doing that is cost (it takes a skilled smith and well-made parts) and reliability in adverse conditions (tightly fit parts can get bound up when they get dirty). So the same design of recoil pistol can be very accurate or very inaccurate.

So a 10/22 is blowback operated and a glock or a 1911 is a recoil operated pistol?


Yes.

-J.

Keith Wheeler
November 29, 2007, 06:42 PM
In a "less theoretical, more design practice" sense, typically the dividing line between the two is that .380 and 9mm Makarov on down tend to be blowback (with some notably exceptions like the Colt Gov't .380 models) while anything bigger tends to be recoil, because of the size of the slide/bolt and springs necessary. This only holds true with traditional slide type automatics. Semi versions derived from subguns (MAC and Uzi pistols, for instance) tend to be pure blowback, like their full auto cousins, with their massive bolts.

papajack
December 3, 2007, 04:00 AM
okay, i think i got it. i was under the impression that a blowback pistol would be somehow like a SMG with an exposed bolt that you have to pull back. this site (http://science.howstuffworks.com/machine-gun5.htm) also helped explain that recoil, blowback, and gas are basically the same thing - just differing in how the parts moved.

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