Calculating rifle movement as bullet reaches muzzle

[davidjoe]

I did use my junior high math in post #54 and posted how close it was.

Show me where the kid’s velocity had anything to do with center of mass as he is being propelled down the board by his legs:

It doesn’t have anything to do with velocity. In my mind, this visualization is the best evidence proving a point that is deeper below the surface of gun shooting than most people care to look. The boy’s mass, just it, determined how far the boat went. An ant instead of a boy, would barely move the boat, the ant would do nearly all the moving. A man instead of boy would move less than the boy and the boat would move further.

And as we can intuit from looking at this, nothing more than thin kite string tied to each end would be required to keep that boat from moving at all. This video also demonstrates there is a limit to how far that boat is capable of ever going, when the body motivating it is still attached. It is not proportional and unlimited. If an Olympic long jumper stood at its edge and leapt, then and only then, could it be flung backward with conceivably ever increasing “recoil”. The absolute limit to how far it could ever move otherwise, with the body still attached, is well inside its length, and that means the magnitude of the recoil vector pre-exit is very, very limited. Fantastic video!

To me, TaperPin, this video is hard, visual, irrefutable evidence. I admit I hadn’t played it until now. Thank you for posting it. It supports you, it supports me, and I honestly think it is stunningly on point.

I saw you are a more recent member, whatever you do, don’t inadvertently, (or intentionally), delete this thread. The subject matter of these threads is in the top 1% of what makes the site unique.

 

[Brians356]

shit, any talk about moving something down a barrel to hit something else is interesting to me.

Yeah, I still miss the little peashooter dad made me, short length of thick-walled plastic pipe he brought home from the paper mill. Of course cigarette filters bound around needles (aka darts) were more fun to launch. More trouble, too.
-

 

[Varminterror]

Does a completely plugged barrel, where the bullet never exists, still recoil, such that if judiciously controlled, this method could be used as a ventless type of engine, in zero gravity?

As I mentioned in response to the other poster:

The center of mass would not return to the same position, because the center of mass for the resulting system would no longer be the same as the original state. Unless the bullet perfectly rebounds to its original position, the original center of mass will not be regained.

If the bullet MOVES within the barrel, then the center of mass moves, so yes, there will be a couplet of matched actions.

So let's say we have a muzzle brake on the end of the barrel with a plug that will 1) allow the bullet to move forward in the barrel as if it were firing, then 2) the bullet is "caught" by the plug, so the bullet doesn't actually leave the rifle. Even if we neglect the mass of the expelled gas which WAS in the bore, which we'll pretend for the sake of simplification flows back into the bore so we have the same final mass as we originally had - then the center of mass has moved, which means we must have had, at least temporarily, a net force acting WITHIN the system....

So let's say the rifle is laying on a frictionless table and there's a mark on the table where was the end of the muzzle, we fire the rifle, the bullet begins moving down the bore and the rifle begins sliding in recoil towards the buttstock. Then the bullet is caught by the muzzle brake, the impact which then aborts PART of the rifle's rearward movement in recoil, because now we have transferred momentum from the bullet to the plug, pulling the rifle out of recoil. Relative to the original mark on the table, the muzzle is STILL farther rearward than its original position, because the bullet is still forward of the center of mass (take note of the shift in center of mass in the canoe photo posted a few pages ago, or the video of the kid walking down the paddleboard). Only if the bullet were to IDEALLY bounce back to its original position relative to the barrel would the muzzle come back to its original position on the table, restoring the original position of the center of mass in space (on the frictionless table).

Only posting on a forum a few times every few years doesn't take away my degrees in engineering and physics... It only means I haven't posted here much...

 

[davidjoe]

V, I could discern there were credentials. The “horns” of the dilemma as I’d see it are:

If it is possible to manipulate a free floating rifle with dimensions or traits such that it does not recoil when a bullet is fired, (capped off, too long, etc.; at most, merely recenters itself) that proves that exit is necessary for recoil, which a great many people, I think you as well, say happens before exit.

And the other horn is this: if you manage to create a rifle that when free floated, does recoil when nothing whatsoever has left it, because the capped barrel is so long that movement is contained and stops, well, you prove exit is not necessary and recoil starts first, but then you have created a ventless engine, which is not possible.

If these “horns” are escapable, I am incredibly eager to hear how. The way I see it, if it recoils without anything exiting, it breaks multiple laws, if it recoils normally, but then not at all with certain modifications, then exit IS necessary. NOT trying to sound harsh, but that would have to be squarely refuted logically.

Watching that video, the answer that reconciles everything is recentering, and it’s nothing more and nothing less.

 

[TaperPin]

Closed but not static.

If the center of mass of the system is accelerated by the internal force, you HAVE to understand that this means the bullet moves one direction and the rifle moves the other, due to the nature of the system and inherent motion of the components.

The disagreement I have with most of the explanations provided here revolves around the effect of acceleration within in a closed system on the center of mass of the system. Everyone wants to insist the center of mass of the rifle/bullet/powder closed system moves. According to the physics examples of universities all over, like the comment below, accelleration of internal parts of the system DO NOT change the overall system center of mass or cause it to accelerate. The rifle moves an amount to equal the bullet mass movement and maintain the center of mass’s lack of movement. Internal forces cannot accelerate the overall closed system. Either someone agrees with the physics principle or not.

If a bullet accelerated to 100,000 fps it would not change or accelerate the overall center of mass of the system until the bullet leaves the barrel and becomes an external force.

The rifle/bullet/powder system starts with a velocity of zero and has to remain zero until the bullet interacts external of the system. Bullet goes one way, rifle goes the other, but the overall center of mass hasn't moved at all.

 

[Varminterror]

If it possible to manipulate a free floating rifle with dimensions or traits such that it does not recoil when a bullet is fired, (merely recenters itself) that proves that exit is necessary for recoil,

I just described above that this isn’t the case. Exit is NOT necessary for recoil. Period.

Movement is necessary for recoil. The boy didn’t jump off of the paddle board (bullet exiting the gun), he simply walked forward and his feet pressed the board rearward (rifle recoiling rearward while bullet is in the bore).

Exit is NOT necessary for recoil. Recoil is absolutely REQUIRED to happen if the bullet moves forward in the bore.

 

[Varminterror]

The rifle moves an amount to equal the bullet mass movement

This isn’t true.

If a bullet accelerated to 100,000 fps it would not change or accelerate the overall center of mass of the system until the bullet leaves the barrel and becomes an external force.

And ALL of this is absolutely terrible science.

1) Once the bullet exits, it - an object with mass - does not become an external force. Once the bullet exits, the bore pressure is vented and NEARLY immediately the motive force completely dissipates, and there is absolutely no external force on the rifle or bullet except gravity and air resistance.

2) It seems you’re misapplying that because the center of mass doesn’t move implies something remotely relevant. The rifle has to move for the center of mass to be retained, because the mass of the bullet is moving relative to the original center of mass.

Bullets move, rifles move. It’s really that simple.

 

[davidjoe]

Exit is NOT necessary for recoil. Recoil is absolutely REQUIRED to happen if the bullet moves forward in the bore.

I don’t think you have as big a problem with the idea of a ventless engine breaking the laws of physics, as I do.

If I put a 300 foot long capped barrel on a rifle, nothing is going to escape. Let’s build the margin of safety, - 1,000 feet.

That bullet and all the gas are going to move. None will escape. This gun in space, and it would need to be zero gravity, will recoil under your premise, and there is logically no limit to how many bullets you could put through it doing so, and nothing comes out the end. You have made a ventless rocket engine, if it recoils, then adds to that, again and again and again.

This is the same idea as an actual rocket engine whose base is cast in a concrete block preventing escape of gas, floating in space. How is it going to generate thrust?

 

[Varminterror]

I don’t think you have as big a problem with the idea of a ventless engine breaking the laws of physics, as I do.

If I put a 300 foot long capped barrel on a rifle, nothing is going to escape. Let’s build the margin of safety, - 1,000 feet.

That bullet and all the gas are going to move. None will escape. This gun in space, and it would need to be zero gravity, will recoil under your premise, and there is logically no limit to how many bullets you could put through it doing so, and nothing comes out the end. You have made a ventless rocket engine, if it recoils, then adds to that, again and again and again.

This is the same idea as an actual rocket engine whose base is cast in a concrete block preventing escape of gas, floating in space. How is it going to generate thrust?

Dude, you watched the video of the kid on the paddle board, and you’re reading the science. You are out of your depth here. The board “recoiled” under his feet.

When the bullet moves relative to the rifle, the center of mass is retained, and because part of the mass moves, the bullet, the OTHER mobile part of the mass has to move. It’s the exact same principle which allows a figure skater to control how fast they are spinning by spreading or contracting their arms and legs - it’s conservation of momentum, LITERALLY one of the 3 most basic principles of physics.

A rifle barrel and bullet isn’t a rocket engine, and we’re not trying to generate thrust. It’s a rifle barrel and bullet. But as literally as it gets, when the bullet pushes itself away from the boltface, the boltface WILL move away from the bullet - this the ONLY way the system can remain balanced.

It is relatively clear to me that you can’t understand that the rifle does not have infinite displacement, just because it moves. The boy walked to the end of the board, shifting the center of mass of their system relative to the board, and the board respectively moved in the opposite direction from the boy. The center of mass didn’t move. So no, you can’t fire round after round in the plugged barrel and create propagating movement beyond the simple calculation of the displacement of the center of mass caused by moving the mass of the bullets from one end of the barrel to the other.

But we have described the physics, and you’ve even seen a video of the principle in action. If you can’t get it at this point, then you can’t. But don’t spout pseudoscience to other folks, please. It’s not a sin to be wrong, but to knowingly mislead others… well, yeah…

 

[TaperPin]

This isn’t true.

What you quoted is out of context.
The mass x distance of the rifle has to equal the mass x distance of the bullet while it is still internal to the bullet/rifle system, is what I was describing.
If you don’t believe this basic premise, that is repeated dozens of times in physics departments all over the country, then we are definitely never going to agree.

 

[davidjoe]

Dude, you watched the video of the kid on the paddle board, and you’re reading the science. You are out of your depth here. The board “recoiled” under his feet.

When the bullet moves relative to the rifle, the center of mass is retained, and because part of the mass moves, the bullet, the OTHER mobile part of the mass has to move. It’s the exact same principle which allows a figure skater to control how fast they are spinning by spreading or contracting their arms and legs - it’s conservation of momentum, LITERALLY one of the 3 most basic principles of physics.

A rifle barrel and bullet isn’t a rocket engine, and we’re not trying to generate thrust. It’s a rifle barrel and bullet. But as literally as it gets, when the bullet pushes itself away from the boltface, the boltface WILL move away from the bullet - this the ONLY way the system can remain balanced.

But we have described the physics, and you’ve even seen a video of the principle in action. If you can’t get it at this point, then you can’t. But don’t spout pseudoscience to other folks, please. It’s not a sin to be wrong, but to knowingly mislead others… well, yeah…

In what relevant way to this discussion, besides name, is a rifle not a rocket? Just because? Expelling mass at high speed is exactly what a rocket does. If you put that plugged contraption in a sealed cardboard tube with a nose and fins, that starts moving faster and faster, with absolutely nothing external acting on it and nothing escaping, who is going to say, ah ha, that’s not an impossible rocket, it’s a giant plugged gun inside because that’s the only thing that can accomplish such a movement? Is it of any concern to you that literally nothing else devised by man you can probably think of, would accomplish this???

I watched it. You aren’t drawing any distinction between recentering that happens while bodies are still connected, and the recoil when two bodies separate, pushing off against each other.

Recentering isn’t velocity dependent. Recentering is inevitable.

The amount of that significant thrust at exit is velocity dependent. Exit is not inevitable. I consider that separation effect at the crown to be something more specific than what happens to literally everything, but fine, call it all recoil, the gun recoils because it’s “out of balance” and then recoils 900% more for an entirely different reason that is unique to guns.

Video: I suppose it recoiled under his feet walking, where he remained attached and would end up in the same place no matter how slowly he moved, and also would have “recoiled” when they separated had he long jumped off the edge, sending it back at what is a determinable speed.

I don’t know why we are using the same word when recentering has only to do with the boy’s mass and recoil at exit absolutely has to do with velocity, is there a reason?

Yes, any shift of even a body in a wheeled office chair creates “recoil” then, and guns are a sliver of everything.

However, recentering is exceedingly easy to control. How much effort would it have taken to overcome that raft moving such that he walked back and forth on an unmoving board? A very finite, capped amount of resistance, probably in guns solely determined by powder and bullet weight. The resistance provided by a gun rest or heavier gun would likely do the trick and we have to step back and bear and mind that this “will start moving” phenomenon is no unstoppable force.

Muzzle recoil though, is not capped, and can always increase with a bigger cartridge or heavier bullet, another conceptual reason not to call recentering, recoil.

 

[TaperPin]

The boy walked to the end of the board, shifting the center of mass of their system relative to the board, and the board respectively moved in the opposite direction from the boy. The center of mass didn’t move.

EXACTLY! You have just agreed with the whole argument I’m making. Mass x distance of the boy is offset by mass x distance of the board. If the boy stepped off the end, we all agree the board would accelerate away from him, but that is not the question we’re trying to answer - the question is how far the board recoils just prior to the boy stepping off. How far does the rifle recoil just prior to the bullet leaving the barrel.

I’ve purposely limited the question to something that is easy to compute. The behavior of a closed system with few parts moving away from each other.

I agree with everyone that after the bullet has left, any other questions do require calculus and the forces and calculations are much more complicated, and what I’ve suggested prior to the bullet’s exit no longer applies.

 

[TaperPin]

It’s been suggested that I bring up what happens to a cannon ball in space that explodes due to internal forces. The original center of mass has a speed of zero before the explosion. After the explosion the center of mass of all the parts and their displacement will sum to the original location. The original origin (center of mass) is not accelerated because no external forces are at work on the system.

If the cannon ball exploded into two identical pieces the original center of mass does not move and the sum of the displacement and mass of the parts is equal to the initial un exploded center of mass.

If the cannon ball exploded and one piece was the weight of the rifle and the other piece was the weight of the bullet, the displacement of the parts and their masses would sum to the original center of mass and no acceleration of the initial center of mass would occur.

 

[RegionRat]

After the explosion the center of mass of all the parts and their displacement will sum to the original location.

Sorry, but it doesn't really.

Just because we simplify concepts to begin the teaching process, doesn't make them complete.
We have to simplify basic concepts just to start young minds on their path.

The difference between a 100 level physics course, versus being able to solve real world dynamics is not an easy gap to close.

 

[davidjoe]

So no, you can’t fire round after round in the plugged barrel and create propagating movement beyond the simple calculation of the displacement of the center of mass caused by moving the mass of the bullets from one end of the barrel to the other.

I see your new edit above. Does this mean I am kind of right, but of course still not actually right, and you still are?

Does that “simple calculation of the displacement of the center of mass” mean that the gun just gets going more slowly, or doesn’t move at all, after all, even though the bullets and gas are still exerting full pressure against the bolt face, which means the gun absolutely moves. How about just a yes, you can make a sealed gun that recoils, or, a No, if that gun is truly sealed it sits still, and doesn’t recoil?

From what you said, the gun will move slightly, reload at that exact place, move back slightly more, reload and keep repeating. Unlike the board that is only a few feet long, there’s really no limit to bullets, is there? If I just restated what your thought actually was, that cardboard tube walks itself across space with no influence, instead of steadily accelerating, does that seem more plausible to you? Here’s another one to mull, if an object in space starts moving, it won’t stop. So indeed, that small impulse will build on itself each and every time, yes? Yes.

Things get lost in the written word. I think you could tell I immediately saw experience and knowledge in your posts and even said you could shed light on this.

I not trying to antagonize you or mislead anyone. There is a sister thread I started years ago trying to get to the bottom of intuition that was creating tension between experience what I read.

I know that you know, that there are gaps identified in the application of principles we believe describe properties. There’s as much valid retrenching as revealing, now. I’m not trying to be contrarian at all and I am also not incapable of grasping concepts that I wasn’t trained in, though it may take me a minute.

 

[RegionRat]

If you attempt to describe a gun, cannon, bomb, combustion engine, etc., with simplifications, but then want to jump to more realistic or useful solutions or actual results, you have to be prepared to deal with messy stuff called thermodynamics and that includes concepts like entropy and enthalpy, friction, damping, etc.

Some folks want to work in time domain, others work in terms of energy, others in frequency domain, etc. all of them have shortcomings. You can't cover this detail in a forum thread.

The "gun" for a 100 level physics course is an open cycle adiabatic engine with no friction or heat loss, but in reality to get the dynamic models to match real world results those missing terms have their say.

The vast majority of practical ballistics work is done by coming at the problem from both directions at the same time. That is, a blend of theoretical exact solutions on one side to guide the principals, and with simplified parametric fudge factor curve fitting on the other.

We can pretend there is conservation of momentum, but in the lab the actual numbers don't match. Those gaps don't mean we don't start the students off with basic physics, but there is an ocean of things to close the knowledge gaps for the rest.

 

[TaperPin]

Sorry, but it doesn't really.

Just because we simplify concepts to begin the teaching process, doesn't make them complete.
We have to simplify basic concepts just to start young minds on their path.

The difference between a 100 level physics course, versus being able to solve real world dynamics is not an easy gap to close.

I’m all ears - from the first post I had hoped someone really on top of physics and engineering would explain the calculations to obtain a rough answer to only a few decimal places. Honestly, I assumed this would be a no brainer for someone smart, and I’ll be the first person to accept a good calculation and give that person credit for figuring it out. We have a simple example from Quickloads and my elementary understanding of physics and junior high algebra gets an answer with 3% error of Quickloads. Does anyone have a calculation within 2%?

Show me any evidence or passage out of an advanced engineering or physics book that says in a closed system with two bodies moving away from each other due to internal forces, that basic physics doesn’t apply.

This has to be one of the simplest problems, a simple linear displacement in one direction of the rifle of known weight and a bullet of known mass and displacement due to internal forces going the opposite direction for a known distance. Powder has an initial location and expands to fill the barrel behind the bullet - a known mass and known location of the center of mass at the point in time the bullet is ready to leave.

So far the engineers have said:
- It can’t be computed because the pressure curve can’t be known.
- It can’t be computed because of gasses escaping out of the barrel.
- It can’t be computed because the calculus Involved would cause our brains to explode.
- It can’t be computed because time can’t be factored out of the mass x velocity = mass x velocity conservation of momentum equation even though junior high algebra shows it can for a fixed moment in time.
- It can’t be computed because I don’t have an advanced engineering degree.
- It can’t be computed because the center of mass of a closed system does’t stay constant with only internal forces.
- It can’t be computed because simple problems are really much more involved in the real world.
- It can’t be computed because copy and paste quotes from various universities is bad science.
- It can’t be computed with simple equations.
- It can’t be computed because what is shown on the boy and paddle board doesn’t really occur.
- It can’t be computed because we have to use the formula for total recoil after the bullet leaves the barrel.

Sweet baby Jesus I hope somebody can eventually produce any evidence or formula that applies to the moment the bullet is about to leave the barrel, and actually work the Quickload problem.

Almost 150 posts and it seems like a disagreement with basic physics, that nobody can back up.

I’m all ears from the first post until now.

 

[TaperPin]

If you attempt to describe a gun, cannon, bomb, combustion engine, etc., with simplifications, but then want to jump to more realistic or useful solutions or actual results, you have to be prepared to deal with messy stuff called thermodynamics and that includes concepts like entropy and enthalpy, friction, damping, etc.

Some folks want to work in time domain, others work in terms of energy, others in frequency domain, etc. all of them have shortcomings. You can't cover this detail in a forum thread.

The "gun" for a 100 level physics course is an open cycle adiabatic engine with no friction or heat loss, but in reality to get the dynamic models to match real world results those missing terms have their say.

The vast majority of practical ballistics work is done by coming at the problem from both directions at the same time. That is, a blend of theoretical exact solutions on one side to guide the principals, and with simplified parametric fudge factor curve fitting on the other.

We can pretend there is conservation of momentum, but in the lab the actual numbers don't match. Those gaps don't mean we don't start the students off with basic physics, but there is an ocean of things to close the knowledge gaps for the rest.

Simplified to ignore friction on the bags, free recoil, ignore mass of air in the barrel that is compressed, and pressure on the trigger.

The question is the simplest measurement humanly possible of rifle recoil as the bullet is about to leave. It’s a real world question every shooter works with, but doesn’t know how to compute. We smooth out bags and do a dozen things to allow for smooth consistent recoil while the bullet is in the barrel.

I believe you’re the first to bring up some of the thermodynamic issues, and that this can’t be fully understood in a forum post. Noted.

If in the laboratory, the calculations and measurements are so complex, how do you explain a simple algebraic exercise with a result within .003”? I’m not even claiming to be correct, I’ve made two wrong calculations and come up with the correct answer on problems in the past. I’m just saying so far nobody has been able to produce anything else - nadda, zip, zilch.

 

[RegionRat]

TaperPin,

Even if I gave you a rigid lumped mass model of a rifle with all the inertia values in 6 degrees of freedom, you wouldn't be happy. You would still require damping coefficients and boundary constraints to begin to get to within less than 10% error for solving the dynamics of the barrel line and the recoil action.

If we were to hang a gun on strings and fire it, the boundary supports will affect the answer.

Those web based recoil estimators are basics for putting a perspective on various rifles and cartridges, not ones for solving where a bullet will land on a target due to the displacement of the gun.

Take a peak over at VarmintAl's old web site and look at those simulations. They are crude and rough but come closer to the work involved to run dynamics on a weapon system.

https://www.varmintal.com/ashot.htm#Recoil

Please try to have a little respect for folks who have done this sort of work. If you don't understand the difference between those recoil estimates and the question you posed at the beginning of the thread, then none of us can help you.

 

[wwbrown]

At risk of wanting to pull more of my hair out I will try to add something to the discussion:

If what you want to know is how much the barrel moves AND you want a solution that does not rely on some simplifications of the physics at hand that may cause large errors in the final, measure it.

With high speed computers and sensors this is a category of problem that is much easier to measure than probably is to calculate. It could be measured in a variety of ways:

1. High speed video photography where the camera is perpendicular to linear scale.
2. High speed linear recorders and associated ADCs
3. Lasers reflected off of linear scales read by a photocell.

Are just the 3 that come to mind.

What would not work is a using a high speed accelerometer as Physics and Math rear their ugly head and the errors from the accelerometer grow huge by the second integral.

Now to possibly rain on some parades.

If we realize the that the motion that impacts the impact on the target is not the motion parallel to the barrel or bullet's path recoil ... but the motion transverse to the bore axis, We then realize that the conservation of momentum while is a tool that can help us quantify recoil but will tell diddley squat about the impact of the accuracy of the system. Accuracy as I understand it is measured at the target because the measurements at the target are all transverse to those quantities that conservation of momentum helped us understand.

The transverse motion largest contributor is probably barrel whip, and to state for the last time conservation of momentum as discussed will not get any information about barrel whip.

To bring it to boys and boats system which seem so popular the barrel whip is analogous to the motion caused by the boy if he jumps up and down and has nothing to do with a leisurely stroll down the boat.

Added in the Edit: Modeling the barrel whip makes the recoil calculations trivial and the solution is not in modeling and sim but measuring it as I started this post off with.

Like I said this was against my better judgement,
wade