At what point in the firing process does the slide start to cycle ?

[teros135]

Not my video, but may contain the answer to the OP's question.

Interesting. Unfortunately, we can't tell at what point the barrel links down, which would change the actual trajectory of the bullet. According to Jerry Kuhnhausen's 1911 shop manual, the barrel and slide move together for about the first 1/8" or so before the barrel link rotates past vertical and starts to draw the rear end of the barrel downwards, which starts the process of unlocking the barrel/slide lugs. It's not until about 1/4" of travel that the unlocking process is finished; until then the barrel and slide are still moving together.

A quarter inch is actually a fair amount, and it doesn't appear in the video that the slide moved all that much. My guess would be that the gun was designed to stay locked until after the bullet left, otherwise the bullet would leave at any old angle in the barrel's vertical shifting and accuracy would be nil. And 1911s, set up properly, are known for accuracy.

[teros135]

Let's try this:

("Pistol shot recorded at 73,000 frames per second" - done by Mythbusters)

On a quick viewing it seems that the bullet was looooong gone before the slide moved far back enough to unlock the barrel.

It appears to be a 9mm, since they said the MV is 1200 fps .

(Note the comment that "That 45 acp is NOT going 1200 fps, it's going more like 900 fps, at least get some facts right" by a guy who apparently can't tell the difference. Nobody said it was a 45.)

Edited May 12, 2016 by teros135

[busdriver02]

The slide starts traveling back as soon at the charge ignites. As the bullet accelerates one way the slide/barrel starts in the opposite direction, equal and opposite reaction and all.

Given bullet travel down the barrel time is around .5ms (quickload estimate) the slide will have traveled something less than .15 inches (.15 is 25fps for .5ms).

No, it doesn't. There is a rearward force exerted by the case on the breach face, and a forwards force exerted on the barrel by the bullet. Those forces are equal until the bullet leaves, at which point recoil begins.

In practice, a little gas blows by the bullet and adds net rearwards force but the practical impact is minimal.

The second pic is most likely embers / flash as others have noted.

So let's get silly micro on this. If the chamber pressure is ~30k psi, that same pressure is exerting force on the breach face and the base of the bullet. If the friction between the bullet and rifling was sufficient to do what you're asserting, it would also be enough to prevent the bullet from traveling down the barrel at all. Since we are obeying the laws of physics, we're conserving momentum. If the mass of the bullet goes one direction, the mass of the slide (and the barrel that is locked to it at this point) goes the other direction. There's absolutely friction loses and they reduce muzzle velocity, but nothing close to enough to stop the slide. The maximum average SAAMI pressure for a .40 is 35k psi, which means at peak, the base of the bullet is seeing 4400 lbs of force. Bore friction isn't stopping or balancing that.

Here's another way to think about it, if the slide and barrel didn't move at all until after the bullet left the barrel then all the slide momentum would have to come from gas escaping the muzzle. If that were the case, bullet mass would have zero impact on recoil force and a 165g .40 projectile sitting on top of 6g of powder would have the same recoil as a 180g bullet on top of the same charge.

The video MikeRush posted shows just what I'm saying, here's another:

[teros135]

Good clip. Here it appears that the barrel unlocks and links down long after the bullet is gone. Guess we're just trying to figure out the physics of that.

[bountyhunter]

A large part of the recoil is actually the jet of gasses exiting the front of the barrel. Without them, action of the slide would be anemic.

But if you fire blanks, the recoil is anemic. In fact, semi autos which are firing blanks can only operate (cycle the action) if some kind of gas restrictor is placed in the barrel to give push back. On the old .50 cal maching guns on TV, you could see the restrictors on the end of the barrel. It had a hole in it about 0.1" diameter.

I think most of the recoil action applied to the slide (breech face) is push back result from equal/opposite forces as mass of bullet is launched. Don't think the gas alone does much, the gas behind the bullet does propel it.

Edited May 13, 2016 by bountyhunter

[zzt]

bounty, the gas propels the bullet forward and propels the case back against the breech face equally. The propellant force of the gas continues even after the bullet leaves the muzzle (for a while). If it did not, a compensator would have no effect on the velocity of the slide, nor any on the strength of the recoil spring needed. That effect is measurable.

You can actually prove this to yourself. I chrono my Open load with the comp on, because I want to be sure I make major. If I take the comp off and chrono the same load in the same barrel, the muzzle velocity is higher. For my steel load using a much faster powder, the difference is not as great. Only 19fps.

[teros135]

bounty, the gas propels the bullet forward and propels the case back against the breech face equally. The propellant force of the gas continues even after the bullet leaves the muzzle (for a while). If it did not, a compensator would have no effect on the velocity of the slide, nor any on the strength of the recoil spring needed. That effect is measurable.

You can actually prove this to yourself. I chrono my Open load with the comp on, because I want to be sure I make major. If I take the comp off and chrono the same load in the same barrel, the muzzle velocity is higher. For my steel load using a much faster powder, the difference is not as great. Only 19fps.

How great is the difference with the Open Major load (comp on/off)? Could you provide some figures?

I wonder, if there's a lower velocity with "comp on" is that because of disruption of the smooth flow of gases after the bullet leaves the muzzle?

[GrumpyOne]

bounty, the gas propels the bullet forward and propels the case back against the breech face equally. The propellant force of the gas continues even after the bullet leaves the muzzle (for a while). If it did not, a compensator would have no effect on the velocity of the slide, nor any on the strength of the recoil spring needed. That effect is measurable.

You can actually prove this to yourself. I chrono my Open load with the comp on, because I want to be sure I make major. If I take the comp off and chrono the same load in the same barrel, the muzzle velocity is higher. For my steel load using a much faster powder, the difference is not as great. Only 19fps.

How great is the difference with the Open Major load (comp on/off)? Could you provide some figures?

I wonder, if there's a lower velocity with "comp on" is that because of disruption of the smooth flow of gases after the bullet leaves the muzzle?

I've never chronoed it without the comp (with the comp is was around 1240 avg I think), but when I first got my 38 super, I bought a box of the hottest (besides CorBon) ammo I could find...Magtech 130 grain FMJ's...It wouldn't even cycle the slide, with an 8lb recoil spring...

[zzt]

teros, for the few I've tested that way, between 20 and 40fps. FWIW, that testing was not done in the same chrono session. So the delta may be a little more or less. Bullet weight and powder used make a big difference, as does comp design. The more gas jetting out the front of the comp, the less the velocity difference. While I still have the chance I think I'll run that test again with my two main open loads, and in the same chrono session. I also have a new to me gun coming that has two popples in addition to the comp. It will be interesting to see the differences.

[teros135]

Indeed. Let us know how it turns out.

[CZinZA]

CZ, you are quite wrong. 5% is not nearly enough to account for it being necessary to drop 7lb in recoil spring strength to get the slide to operate in the example I gave above.

If you look at the formula for recoil, you will find that the powder weight is multiplied by 4700 before being squared. So it isn't 4 gr, or 0.0006 lb that is added to muzzle velocity times bullet weight squared. It is 0.0006 * 4700 =2.6857 squared, or 7.2131 lbs that is added in that section of the formula.

So what you are considering as the effect of powder weight (0.0006 lb) is actually 1202.1769 times too small.

I don't know this formula for recoil that you're referring to.

Recoil is caused by conservation of momentum. The momentum of the gun moving back is equal to the momentum of the gas and the bullet moving forward. If both gas and bullet leave the barrel at the same velocity then their contribution to momentum is in the same proportion as their masses. Since the gas weighs less than 5% what the bullet weighs, its contribution is small.

Sent by Jedi mind control

[PatJones]

CZ, you are quite wrong. 5% is not nearly enough to account for it being necessary to drop 7lb in recoil spring strength to get the slide to operate in the example I gave above.

If you look at the formula for recoil, you will find that the powder weight is multiplied by 4700 before being squared. So it isn't 4 gr, or 0.0006 lb that is added to muzzle velocity times bullet weight squared. It is 0.0006 * 4700 =2.6857 squared, or 7.2131 lbs that is added in that section of the formula.

So what you are considering as the effect of powder weight (0.0006 lb) is actually 1202.1769 times too small.

I don't know this formula for recoil that you're referring to.

Recoil is caused by conservation of momentum. The momentum of the gun moving back is equal to the momentum of the gas and the bullet moving forward. If both gas and bullet leave the barrel at the same velocity then their contribution to momentum is in the same proportion as their masses. Since the gas weighs less than 5% what the bullet weighs, its contribution is small.

Sent by Jedi mind control

I believe the powder weight is calculated at a higher velocity. I don't currently own a copy, but I think I remember seeing formulas for this in Hatcher's Notebook.

[zzt]

The formula for recoil E in pistols and rifles is as follows:

E =(Bw x Mv +4700 Pw)² / 64.348 Gw

Where:

Bw = weight of the ejecta in pounds (bullet, in this case; shot and wad in shotshells)

Mv =muzzle velocity (or 3' velocity for shotshells)

Pw = powder weight in pounds

Gw = gun weight in pounds

64.348 = twice the acceleration of gravity

4700 = speed of sound in the propellant gasses

[bountyhunter]

bounty, the gas propels the bullet forward and propels the case back against the breech face equally. The propellant force of the gas continues even after the bullet leaves the muzzle (for a while). If it did not, a compensator would have no effect on the velocity of the slide, nor any on the strength of the recoil spring needed. That effect is measurable.

Michael Plaxco covers compensators in his book Shooting From Within (been many years since I read it). The recoil offset effect of a compensator is due to the fact that the forward moving gas expands in the comp chamber and slams into the front face providing a forward directed force opposite to the recoil force. He did an experiment where he gradually opened up the hole at the front of the compensator where the round goes through and the effect of the comp went away as the hole size increased since the gas was no longer as constricted to hit the wall face. My point was that the force of the gas alone is miniscule unless it is constricted in some way. When it is trapped behind a projectile and forcing it forward, you certainly get an effect. But when the bullet leaves the barrel (as when the comp's front hole is too big) the effect is drastically reduced because all you have is gas and that gas is no longer restricted by the presence of the bullet blocking the exit forward. That is why semi autos have to have some kind of BFA (blank firing adapter) installed to generate enough recoil to cycle the slide with blank ammo.

https://en.wikipedia.org/wiki/Blank-firing_adaptor

Recoil operated firearms

Since blank cartridges generate very little recoil, far less than that produced by a live round, the recoil operation mechanism is not suitable for use with blanks. BFAs used with recoil operated firearms typically replace the locked breech of a recoil operated firearm with a simple blowback system using a restricted barrel, similar to a gas operated BFA. Short recoil operated pistols, the most common type used for self-defense and by police, are typically converted with a simple barrel replacement; the replacement barrel will lack the locking lugs to lock the slide to the frame, and will be built with an adjustable restrictor to control the chamber pressure.

Edited May 14, 2016 by bountyhunter

[Jcalero]

First pic the slide has already cycled and is at the end of loading next round. Second pic has the bullet in the barrel while slide is cycling. Would it be further out, the slide would be on the way back.

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[Kevin G.]

In reference to the argument that the case exerts force on the beech face equal to the force exerted on the bullet...

I know in rifles, maybe not as much in the lower pressure of pistols, but the case expands and locks in the chamber until the pressure drops as the bullet leaves the bbl.

Based on this, the bullet has to leave the barrel before the link starts pulling the barrel down. Further, the case doesn't push the slide back. The extractor pulls the case from the barrel. This is why timing is so important. All the kinetic energy that causes the slide to recoil is generated while the bullet is in the barrel. Once pressures drop, momentum handles the rest. However minuscule, the barrel and slide have to begin movement before the bullet leaves the barrel. The friction inside the barrel from the bullet is a non factor when compared to the primary force pushing the bullet forward and the slide/bbl assembly rearward. Definitely not equal.

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[StealthyBlagga]

Oh brother.

In a locked breach pistol (which is everything we run in USPSA), the barrel and slide remain LOCKED together via the locking lugs until the bullet leaves the barrel. The recoil forces (equal and opposite reaction, conservation of momentum etc.) cause the barrel-slide unit to start moving backwards. Only after the bullet has left the barrel will the link cause the rear of the barrel to drop, allowing the locking lugs to disengage from the slide. The rearward impetus imparted on the slide during this brief rearward motion is sufficient to cause it to cycle (extract, eject, cock the hammer, chamber the next round etc.).

If the barrel and slide unlocked before the bullet left the barrel, the chamber pressure would easily blow out the case web... this is how Glocks work .

Blowback, gas operated, inertia driven and other actions all have their own variations, but all must be designed to prevent the case being extracted before the bullet leaves the barrel.

Edited May 23, 2016 by StealthyBlagga