Recoil Operated. Is it?

[John Travis]

Most people who are into autopistols have at least a working understanding of short recoil operation.

But, is it really recoil that causes the whole thing to function...or is it actually something else? Is ballistic recoil...the recoil generated by firing a cartridge...actually what causes the gun to function, or does recoil only get it started and something else finishes it?

First, a few things to understand.

Recoil is only in play for a very brief instant. Assuming 230 grains at 830 fps and a 5-inch barrel, it takes about 2/1000ths of a second for the bullet to make the 4.2 inch trip through the rifling to the muzzle. During that nearly immeasurble instant, the slide moves 1/10th of an inch. This will vary +/- a couple thousandths depending on several factors, but for our purposes here, 1/10th inch will do.

Once the bullet has broken free, ballistic recoil ends. All forces that set the system in motion are gone. Neither bullet nor slide can accelerate or even maintain their respective velocities that existed at the instant of bullet exit. They can only decelerate.

So...speaking to the slide...everything after that is on the momentum that was generated by that very short time and distance during which it was being accelerated.

And, then the slide hits the impact abutment in the frame and everything stops. The action spring then starts to unload its stored energy and gets the slide moving back toward battery. While the slide is accelerating...because momentum is a function of Mass X Velocity...it's gaining momentum, and momentum is more necessary than the spring in getting the slide back into battery. I proved that once to a guy by removing the little weights from the buffer assembly in his AR15 and firing it. He was curious as to whether he could get it to cycle faster by removing weight from the reciprocating assembly. It failed to go to battery about one time in 3 tries, even though the spring was still fully in play. I put the weights back.

So, would it be more accurate to describe it as "Spring and Momentum Operated" since momentum gets the slide to the impact abutment and spring and momentum get it back into battery?

Or, am I just overthinking this?

 

[GymB]

You don’t have a way to get the energy, potential energy, into the re-action spring without firing the cartridge and having the mass of the slide start to move. That’s recoil, so I think it’s properly named.

 

[John Travis]

You don’t have a way to get the energy, potential energy, into the re-action spring without firing the cartridge and having the mass of the slide start to move. That’s recoil, so I think it’s properly named

No argument there. Recoil gets the movement started, but momentum is what carries it through when recoil ends. The starter on your car spins the engine, but it doesn't make it run.

 

[DrPhudd]

A friend of mine was running my .40 STI limited gun. There was a squib. It had to have been some powder, not just a primer, as it lodged about halfway down. It fully cycled the gun, no one even noticed the sound being different. The next round split the barrel in two.

 

[some_kid6]

No argument there. Recoil gets the movement started, but momentum is what carries it through when recoil ends. The starter on your car spins the engine, but it doesn't make it run.

Keeping with the engine theme, it'd be closer to describe it with "gasoline is to car engine as recoil is to tilt lock system". Recoil is the fuel for the system and tilt lock being what's making that energy useful in the same way as a gasoline engine using gas as the fuel and rotating pistons for the engine. You can use other systems to extract the energy and convert it into something useful, such as a rotary engine, but it's still a gasoline engine at the high level.

 

[JRV]

This is a fun one!

How are we defining “operation?” Is the “operating system” the means by which we disturb the device from “rest,” or is it how we return the device to “rest?”

It seems like the language of guns is all over the place. Matchlock, wheellock, flintlock, caplock, recoil-operated, inertia-operated, gas-operated, hammer-fired, striker-fired… all focused on the way the party gets started. But, we describe bolts and slides in terms of how they lock up at rest (breechlocked, roller-locked, toggle-locked, flapper-locked, etc.). Unlocked breeches are “blowback,” which just describes that they fly open freely—except if they’re “delayed” by a retarding design feature.

For a pistol, it seems to make the most sense to define “operation” based on disturbance of the rest condition.

The gun in battery, with lugs engaged and the recoil spring “stuck” trying to expand between the guide rod and the plug, is at “rest.” The springs always want to expand, but for practical purposes, it’s a static object. Maybe the real question here is why we call “breechlocked” pistols something other than “spring-and-lug-locked” pistols?

The explosion (which also fulfills the purpose of the pistol’s existence) starts the cycle of operation by disturbing the “rest” condition—dumping the only significant source of energy into the gun besides the springs.

To me, just thinking about it, saying “springs and momentum” operate a pistol is a lot like shoving someone off a roof and blaming gravity for their death. The push is the important part!

 

[GymB]

No argument there. Recoil gets the movement started, but momentum is what carries it through when recoil ends. The starter on your car spins the engine, but it doesn't make it run.

Yes, and we call it a gas engine or internal combustion engine. The starter consumes energy and isn’t actually a part of the engine IMHO.

 

[John Travis]

This is a fun one!

I thought it might be.

For a pistol, it seems to make the most sense to define “operation” based on disturbance of the rest condition.

For the win!

I don't see how anybody is gonna top that.

But...isn't it the starter that disturbs an engine's at rest condition?

To me, just thinking about it, saying “springs and momentum” operate a pistol is a lot like shoving someone off a roof and blaming gravity for their death. The push is the important part!

For some reason, this is my favorite part. Does that mean I've got tendencies toward the sinister?

 

[John Travis]

It seems like the language of guns is all over the place.

It is. The slidestop on a 1911 pistol actually performs five functions if we don't count takedown. If if had been named for everything it does it would have a long, awkward name.

 

[John Travis]

. Recoil is the fuel for the system...making that energy useful in the same way as a gasoline engine using gas as the fuel and rotating pistons for the engine.

The next best response. Excellent!

One point...

Pistons don't rotate. Crankshafts and cam shafts rotate. Pistons reciprocate.

I know that you know. I just wanted to use it as an example of how SOME people like to use a small misstatement to try and discount an otherwise proper and factual explanation.

 

[John Travis]

A friend of mine was running my .40 STI limited gun. There was a squib. It had to have been some powder, not just a primer, as it lodged about halfway down. It fully cycled the gun, no one even noticed the sound being different. The next round split the barrel in two.

I've seen that very thing happen. Was actually looking at the gun. I noticed the lack of report, and yelled to stop the lady, but she was on the trigger too quick. There are people who insist that it can't happen with a squib in a locked breech pistol...the recoil cycle...but it can and does.

The reason that it doesn't stop a shooter from pulling the trigger is because the moving slide compressing the spring provides felt recoil. If they're on the first round of an intended quick double...they pull the trigger almost without exception.

As an interesting aside...As a rule, only stainless barrels bulge and split. Carbon steel barrels bulge, but rarely split.

 

[RetiredUSNChief]

Yes, this is fun! And yes, you are over thinking it!

Momentum is nothing more than mass in motion.

Recoil is really two slightly different things in the gun world. Recoil, in the physical sense, is the result of good ole Newton's Third Law about "equal and opposite reaction". This says that for every action (force), there is an equal and opposite reaction (again, force). Force, in this case, being something that changes an objects state of rest or its straight line motion.

(The other recoil is what the operator FEELS. Felt recoil may vary from person to person, for example, due to a variety of factors. But we won't go into this.)

So...in Newtonian physics, momentum is simply mass times velocity. And in a closed system, momentum is ALWAYS conserved.

The classic example is that of two objects colliding, such as billiard balls. In a straight on collision between two billiard balls of equal mass, one at rest the other moving, the end result is that all the momentum of the moving ball is transferred to the stationary ball. The end result is the moving ball stops as its momentum is transferred to the stationary ball, which in turn takes off at a velocity equal to that of the ball which impacted it.

SO...when you fire a bullet, you impart (fairly) linear acceleration to the bullet as its velocity increases from zero to some number of fps by the time it exits the barrel. The expanding gases also impart a rearward movement of the entire gun at some lower velocity because the mass of the gun (which is experiencing the same forces as the bullet).

BUT...the gun isn't actually a solid piece of steel. It's actually composed of many components, some of which are allowed to move within the limited framework of the gun design.

This means that while the whole gun may start to move rearward in reaction, some parts of the gun may end up moving faster than other parts because their relative mass is somewhat lower than that of the rest of the gun.

This means that within the total framework (the "closed system"), the slide and associated movable parts will have their own momentum, equal to their mass times their velocity. The movement of these components will in turn transfer some of their momentum to the slide spring, which will cause the slide spring to move in a compressive manner. This, in turn, transfers some of the applied force into stored mechanical energy within the slide spring as it compresses.

The slide continues all the way back until its full range of motion is arrested by the slide stop...at which point the rest of its momentum is transferred to the frame of the gun.

Then the stored force within the slide spring acts to accelerate the slide back to the battery position, adding momentum back to the slide as its velocity now increases in the opposite direction.


So, while you are correct in that this whole thing is really momentum transfer, the average layman's understanding of "recoil operation" is accurate enough for what it is.

 

[RetiredUSNChief]

As an interesting aside...As a rule, only stainless barrels bulge and split. Carbon steel barrels bulge, but rarely split.

Heh! Now, don't get us going down the path of fracture mechanics now!

 

[John Travis]

So, while you are correct in that this whole thing is really momentum transfer, the average layman's understanding of "recoil operation" is accurate enough for what it is.

Precisely. The rest of your response is excellent and spot on

One point, if I may.

And in a closed system, momentum is ALWAYS conserved.

Correct again, but...speaking strictly to the slide and bullet...momentum isn't always equal as is often stated/insisted upon.

I'm not talking about momentum transfer to the frame via springs, but only the momentum in the slide.

Because the slide is subject to outside forces that the bullet isn't...springs, hammer mass, rail friction, etc...the momentum is never perfectly, truly equal. As a physics teacher explained to me so many years ago...Momentums between two moving objects in a closed system of action-reaction are only perfectly equal in the absence of outside forces, or in the presence of equal outside forces.

A small point, but a point nonetheless.

 

[nc-by-the-c]

Precisely. The rest of your response is excellent and spot on

One point, if I may.


Correct again, but...speaking strictly to the slide and bullet...momentum isn't always equal as is often stated/insisted upon.

I'm not talking about momentum transfer to the frame via springs, but only the momentum in the slide.

Because the slide is subject to outside forces that the bullet isn't...springs, hammer mass, rail friction, etc...the momentum is never perfectly, truly equal. As a physics teacher explained to me so many years ago...Momentums between two moving objects in a closed system of action-reaction are only perfectly equal in the absence of outside forces, or in the presence of equal outside forces.

A small point, but a point nonetheless.

Don’t forget deformation and sound sucking up some of your energy as well

 

[RetiredUSNChief]

Precisely. The rest of your response is excellent and spot on

One point, if I may.


Correct again, but...speaking strictly to the slide and bullet...momentum isn't always equal as is often stated/insisted upon.

I'm not talking about momentum transfer to the frame via springs, but only the momentum in the slide.

Because the slide is subject to outside forces that the bullet isn't...springs, hammer mass, rail friction, etc...the momentum is never perfectly, truly equal. As a physics teacher explained to me so many years ago...Momentums between two moving objects in a closed system of action-reaction are only perfectly equal in the absence of outside forces, or in the presence of equal outside forces.

A small point, but a point nonetheless.

Well...it's not truly equal because of friction losses in the real world.

The gun in its resting state has no momentum at all because its relative velocity is zero. The same for all its components as well as any ammunition loaded into it.

Lots of people also get confused by the various terms and their definitions, too. Force, for example, is mass times acceleration. It's the push (or pull) on an object which causes it to change its velocity. Force has both a magnitude and a direction. Often people confuse "pressure" with "force". The two are related, but are not the same. Pressure is force being applied per unit area.

For an object at rest, its net force is zero. Case in point the slide assembly on a semiautomatic pistol. The spring IS applying a force, but since the slide isn't moving, the net force of the system is zero. If you apply an external force (cue the firing of a round), then the slide will increase its relative velocity from zero to some nominal value proportional to the force applied and the slide will start to move, thereby gaining momentum.

This entire system is a closed system in and of itself. However, it's a more complex closed system than the billiard ball example. The springs, hammer mass, and all that are not "external forces" when taken as a whole because they are a part of the entire mechanical structure.

However, most people's schooling on the subject involves gross simplifications in order to learn the general principles. So they only consider, for example, the motions of the bullet and the slide while making the assumption that nothing else moves (like the frame or the hammer) and that their input is insignificant to the end result. If the frame is held entirely stationary, then this calculation becomes much simpler. But in the real world, it's not held stationary at all, because the hand/arm the gun is in will move...and not in a simplistic linear direction 180 degrees from the direction of the bullet. There are lots of angles to be considered because of this.

 

[cold1]

But ihe real world, it's not held stationary at all, because the hand/arm the gun is in will move...and not in a simplistic linear direction 180 degrees from the direction of the bullet. There are lots of angles to be considered because of this.

A friend of mine had an AMT 30Cal , I never could shoot it. It would always jam on me. I can't remember if it was a stove pipe or FTF. The only thing we could figure out was that I was probably limp wristing it.

 

[RetiredUSNChief]

A friend of mine had an AMT 30Cal , I never could shoot it. It would always jam on me. I can't remember if it was a stove pipe or FTF. The only thing we could figure out was that I was probably limp wristing it.

I wanted one of those when they first came out. I saw one at a gun show and I ended up not liking it because of the way it fit my hand. Just didn't work in my grip.

Beautiful gun, though.