Why Bullets are Spun for Stability

[Michael D. Sewell]

Clausius2,
I enjoy chatting with you and the other guys too! I have only been online for a few weeks so this is all new to me. I have learned a lot, and I have been having a lot of fun at the same time.

I'm not very good at spanish, I can say a few phrases, but I can't spell them. My spanish dictionary is not very good, I guess I better up-grade it.
Best wishes amigo,
-Mike

 

[Michael D. Sewell]

Excentric bullets are semistable, but as soon as they touch anything, that excentricity causes very sharp trajectory changes.

wimms,
I've never heard of this. It seems to me that these bullets wouldn't be very practical, but I would be interested in seeing some links, because I have never seen any information on these bullets from any of the manufacturers.

I seems to me that accuracy is generally the most important thing in a projectile. Bullet wounds are absolutely horrible, I can't imagine why we would need to "improve" on them.
-Mike

 

[wimms]

I have no links. I've heard about them from russian military dudes long ago. Supposedly had been used in Afghanistan. Not even sure what terms to google for. Although I'm quite sure they are real, there is possibly some element of legend in this. Well, there are older russian people on this site, they should know better.

Practical? Of course not, they were never primary ammunition, only replacement for specific conditions.

Accuracy is important when you care to aim. In automatic fire against enemy while being outnumbered most efficient is to get maximum damage with minimum aiming accuracy. Nasty, but that's what they were designed for - to assure kills instead of just wounding or scratching.

Their property of changing trajectory without loosing too much momentum could also be used to fire at enemy hiding behind mounds.

I never really understood how they work, imho was something to do with interaction between moment of rotational inertia and moment of linear inertia after being disturbed by brushing something.

 

[Mr. Robin Parsons]

Wonder if Wimms is thinking of those *starfire* (tradename?) type of bullet, the ones that 'shatter' upon impact...never heard of any that changed trajectory due to slight contact though, other then the normal realities of physics upon any normal bullet...

 

[wimms]

no, no shatter. Perhaps if I describe how they are made it would become clearer. Take ordinary concentric projectile, it has center of mass in geometric center of rotation. Drill out core slightly off the center and replace it with either heavier of lighter metal. Result will be that projectile will have new center of mass around which it will be rotating. Geometric shape will 'wobble' around it while in flight. Thats basically excentric (eccentric?) bullet. Nothing else. Normally, its considered bad bullet, but in this case its intentional and controlled eccentricity. New center of mass is supposed to be well aligned with flight path.

If I understand it right, then whenever such bullet brushes anything, it touches it with either front or rear edge and due to force from rotation goes precessing. After that its center of mass stops being aligned with flight path, its tip follows spiral around the flight path while gyro effects try to stabilize it again. Because of wobbling, force acted on it during brush is heavily sideways with little longitudal forces. Due to very high rpm, the sideways force can be pretty high, causing trajectory change. I don't know, maybe air drag is major factor here, thinking of why tree leaves might have such impact on it. Anyway, its not an ordinary ricochet.

When entering tissue, every density change is causing trajectory change, due to higher drag sharper changes, and bullet is messing around in body like mincer quite a lot before it stops, causing very nasty damage beyond any hope of survival. Thats what I've heard.

I really don't know, but I'd be interested if someone could explain what behaviour is expected from heavily eccentric bullet.

 

[Cliff_J]

I seems to me that accuracy is generally the most important thing in a projectile. Bullet wounds are absolutely horrible, I can't imagine why we would need to "improve" on them.

I've always been amazed at how many brilliant minds have created so many destructive weapons. Not like I'm some pacifist, and I'm glad that our society can benefit from technologies as they trickle down from miltary applications, much less the umbrella of 'safety' that having superior weapons provides as a deterrent. But just the thought of 'maximum damage' as a design goal seems to require a shift of context from the weapon to that of the greater good of an army or nation. Well, at least for me in terms of how I've viewed science as being a tool for 'good', but I guess its only a tool that can be used as the person wielding control sees fit.

Cliff

 

[Michael D. Sewell]

I have no links. I've heard about them from russian military dudes long ago.

wimms,
Please don't take this wrong, but I think the Russians with whom you spoke may have been "bullshevics", if you'll pardon the pun. Maybe someone will show up here with some links or info that prove me wrong. It certainly wouldn't be the first time.
-Mike

 

[Mr. Robin Parsons]

Sounds like an intrigueing design Wimms, not that I am really into "things that kill" as 'thinking' of newer, or better, manners of execution (Pardon the pun?) but without further, or more, evidence, (Of this bullet) well, we will await you finding a reference to it...perhaps?

 

[wimms]

perhaps not. I'm not into spending too much time on finding refs. If you need refs, search for them. I'm only interested in physical analysis of if and how it could behave.

Quick search revealed no mention of such bullet type, so Michael could be right in that's an urban legend, though I've heard that from several unrelated no-bullshet types, so I'm confused.

I recalled this only because of Drags mention of his weapons instructor in relation to spinning bullets. Some guys of similar rank had told this to me. If weapons professionals tell bs, then well, anything may be.

So, if you are not interested in approaching it from physics side, let the subthread die.

 

[Mr. Robin Parsons]

From the physic's side, is fine, for an already made product, ergo referable, but I ain't a joinin' in on no conversation that may assist in giving rise to something like that!

 

[Michael D. Sewell]

Mr. Robin Parsons,
I agree.
-Mike

 

[Clausius2]

Well, sit down guys and not fight each other . I have never imagined a disagreement about guns and bullets in USA. I think you were more closed each other than here. Ok, it's a joke.

If anyone can prove the trajectory stablity by means of a theoretical analisys (ie by Newton Motion Law or Lagrange Equations) , say it now or shut up forever!. (I feel like a prayer in a wedding).

 

[Mr. Robin Parsons]

Mr. Robin Parsons,
I agree.
-Mike

Thanks!......last thing I think we need is another new way to kill each other...better "No nuke'm all"...as in let's get rid of the Nuclear Bombs before we have proven the need to...get rid of them...

(Only One persons 'opinion')

 

[Michael D. Sewell]

Amen brothers. Well said.
Best wishes,
-Mike

 

[Clausius2]

Rockets and such all have fins, thrust control and control surfaces, so they don't need passive gyroscopic control.

I've been thinking in what you said. As bullet spins around its axis, an aerodynamic friction torque is exerted in the opposite direction. Due to the kinetic momentum conservation, dL/dt=-M(aerodynamic), so dL has the same direction of M. Thus L is confined on the plane perpendicular to the axis or spinning. This appears to be the reason for the stability of the trajectory, isn't it?.

And what do you mean with passive gyroscopic control?

 

[Mr. Robin Parsons]

I suspect what you just explained is the 'passive control', rockets (and missles) use active controls...

 

[DRice.72]

Hello, I came across this thread after trying to find information relating to bullet dispersion. Some lonmg range shooters talk of a phemonon of having a better MOA at longer ranges than at closer ones. I was wondering if anyone has seen any articles on actual expirments conducted to see if this is possible. It is said by some that as a bullet leaves the barrel. It is inherently unstable, until some of the initial velocity decreases. During this time the bullet is said to travel in a helix around the true trajectory. Then the bullet "goes to sleep". Flying in a truer, more even trajectory.

 

[Bob S]

I don't think that clockwise - counterclockwise makes a difference. What does make a difference is how the moment of inertias line up. You can see the effect by tring to spin a thin book. Two of the ways you'll have no problem. The third it'll topple out of control.

I believe a long range shot will pull to the right with right-hand rifling, and to the left with left-hand rifling. I also recall that the coriolis force pushes the bullet to the right in the northern hemisphere. Some guns and cannons have left-hand rifling to improve accuracy in the northern hemisphere. Google " coriolis force left-hand-twist rifling"

The polhode rolls without slipping on the herpolhode lying in the invariable plane.

Bob S

 

[FredGarvin]

Not that this is an answer, but I just recently saw a show on the Military Channel about Marine snipers in Iraq. They had made a mention of almost the same thing. This particular sniper mentioned (IIRC) that his weapon had a preferred range of around 500-800 m for the best accuracy. They didn't state why.

 

[Brian_C]

Hello, I came across this thread after trying to find information relating to bullet dispersion. Some lonmg range shooters talk of a phemonon of having a better MOA at longer ranges than at closer ones. I was wondering if anyone has seen any articles on actual expirments conducted to see if this is possible. It is said by some that as a bullet leaves the barrel. It is inherently unstable, until some of the initial velocity decreases. During this time the bullet is said to travel in a helix around the true trajectory. Then the bullet "goes to sleep". Flying in a truer, more even trajectory.

I know a little bit about rifle ballistics because I'm into hunting and reloading. The bullet is not shot "straight" out of the barrel, but instead has an initial yaw that can be as large as 5 degrees. The bullet then experiences yaw nutations that decay downrange. These nutations can be observed by placing paper targets at regular intervals and looking at the shape of the holes. According to U.S. Army tests, some bullets may not stabilize for 250 yards or more.

There is a lot of fascinating physics involved. I would recommend the following book if you want to learn more:
https://www.amazon.com/dp/0964559846/?tag=pfamazon01-20

 

[Kentucky Wind]

After several years of this thread being started, I find interesting to reply.

It DOES matter the direction of the bullet spin it, however, does not affect the aerodynamics.
The reason it DOES matter the direction of the torque applied to the bullet, has to do with the resulting vector inherent to an free-flying object.
The bullet spins clock-wise so the torque applied to a free-flying object results in a deviation of the flight path, in this case to the right.
You can of course see this effect in LONG distance shots, which have a lot of other factors that modify the bullet path.
Snipers compensate for this effect, as for gravity, using Kentucky Windage, which is the easy way for the "click" compensation of sights.

 

[Ranger Mike]

Cliff_J,
A click is used in that way is military jargon for 1 kilometer. The Army seems to like clicks very much because everywhere we went on foot was always several clicks away.

The meaning of a click in shooting has to do with the sight, or scope on a rifle or pistol. As the sight is adjusted, it "clicks". Each click represents a given amount of elevation or windage at a given range. One click on a military rifles often corresponds to one inch at 100 meters(used to be Yards).

actually a click , more like several to be exact(for the 11B gravel agitators) is that nebulous point on a map some staff puke decides to send you...,,where just before you arrive ( usually at night carrying 90 pounds of toys) you are then notified that the mission is call off and you do an about face and return to base by another route,,,

 

[Mr_Firelock]

In terms of a bullet's trajectory and why it spins, the secret lies in the interior of the bore - it has a spiralling cut through it.

If you ever look at a cross section of a barrel, you will see a series of spiralling cuts inside the bore. This is called rifling (where the term Rifle comes from.) Basically you make the windage (the gap between bullet and bore inside the barrel) as small as possible to focus the bullet's trajectory in a certain direction, and the bullet will engage itself on the spiralling cuts, causing it to spin. By spinning, it practically drills through the air, allowing it to break the sound barrier easier, accelerate faster, cut through the air easier. The result is that once it exits the muzzle it is as fast as possible. Furthermore, by cutting through the air easier, it travels a better trajectory and travels faster (thus more accurate and straighter, and higher penetrating,) and further (due to increased velocity.)

Prior to rifling becoming widespread (though it has existed since the 1500's) all firearms were smoothbore - there is no rifling inside the bore and it's pretty much just a tube. Whilst it was cheap to make and quick to load, firearms were inaccurate. Rifles did exist, though they weren't used by the military extensively for many centuries to come because rifles were expensive to make and slow to load (this is the time of muzzle loaders were you load down the barrel of the gun, and only get one shot per barrel, Rifles were hard to load because the windage is very small and hard to ram a ball down into the powder charge.) If you've ever seen movies like, "The Patriot," there is a scene where the Continental Army and Redcoats engage each other out in a field - they march in shoulder to shoulder to close range, then start firing. This is how armies fought each other from the 1500's till about the early 1800's, because their smoothbore muskets were only accurate to ranges of about 50-70m and they would all fire together at once, to increase their chances of hitting a target.

This idea worked for hundreds of years, but Rifles still saw occasional use. A good example is in the American Revolutionary War in which American Minutemen would use their Rifles (which could be accurate out to 200-300m compared to Smoothbore Musket's 50-70m) to just shoot the Redcoats before they were in range, then run away and reload to repeat again later on. Once Rifling became able to machined easily rather than requiring skilled gunsmiths (which were few) they began to see more common use, until finally in the mid 1800's the Smoothbore Musket disappeared in favour of the Rifle.

Rifles proved more accurate, longer ranged and more deadly. The ability to make a projectile spin is why they are so effective.

 

[M James]

I don't think that's completely accurate; the bullet doesn't 'screw itself into the air' because it is smooth. The spin imparted by the rifling gives the bullet gyroscopic stability, it is this that improves the range and accuracy over an un-spun projectile.

If you imagine a bullet in flight, that isn't spinning, the leading edge of the round will have slight abnormalities on one side relative to the opposite side, there may also be differences in air pressure on either side of the bullet. Due to the extreme speed of the round, these minute differences conspire to rotate the round (around it's pitch and/or yaw axes if you like) in a particular direction, until it is no longer stable and begins to tumble. By spinning the bullet, these forces act in a spiral, thus cancelling themselves out.

Also, for a given charge, the muzzle velocity would be lower for a rifled barrel because some of the force used to propel the round is used up in making the bullet spin.

Also also, someone mentioned (a few years ago, *cough*) that the spin of the bullet has nothing to do with the damage caused on impact. Soft and hollow point rounds are designed to mushroom on impact, imparting more or all of their energy in a shorter distance. If this is coupled with a high enough spin rate the round may fragment as it deforms.



That's my take on it anyway!


I was wondering if anybody knew if the rifling's twist rate was constant along the barrel length, or if it got tighter towards the end?

I'm trying to work out if it's possible to induce a spin magnetically, thus reducing the charge needed for a given muzzle velocity and reducing the mechanical stress on the barrel. I've seen a couple of posts here and there about it, but my googlefu can't find much recent discussion about it.

 

[nsaspook]

Also also, someone mentioned (a few years ago, *cough*) that the spin of the bullet has nothing to do with the damage caused on impact. Soft and hollow point rounds are designed to mushroom on impact, imparting more or all of their energy in a shorter distance. If this is coupled with a high enough spin rate the round may fragment as it deforms.

Bullet ballistic gelatin block videos. http://www.brassfetcher.com/index_files/HighSpeedVideoSlowMotion.htm

50 BMG ballistic gelatin block video http://www.youtube.com/watch?v=xYSGuiko6Gg&feature=related

Most of the damage is caused by the shock-wave from the deceleration of the bullet in the media. Long thin bullets (The .223 FMJ is a good example) tend to flip and break causing a massive cavity from a small but high speed bullet. I would always chose a M14 (.308) over a M16/M4 in most cases during a security watch because it has more stopping power at long range with much better ballistics (second shot on target) during bad weather conditions.


http://upload.wikimedia.org/wikiped..._protection_watch_on_board_USS_Blue_Ridge.jpg