Why Bullets are Spun for Stability

[Clausius2]

I was wondering me why a bullet is shoot spinning around it axis of symmetry. The cannon of the gun has usually small channels in spiral shaped, to exert a torque on the bullet in order it can rotate about it axis (i suppose).
I've been thinking about this, and i heard that it has to do with bullet trajectory stability, but i can't demostrate that.

What is the true reason?.
Missiles are shoot spinning too?.

 

[Mr. Robin Parsons]

A spinning bullet breaks the air better, and missles have guidance systems so they don't ususally need to spin...I suspect the spinning helps trajectory a little like a bicycle wheel spinning help to keep the bicycle upright, angular momentum..something like that (mentor, correct me please, if I am wrong)

 

[one_raven]

So a spinning bullet is something of a mini gyroscope?
I never thought about that before, but I guess it must have some gyroscopic effects.

 

[Chen]

I think, and of course I may be wrong, that when the barrel of a gun is built in a way that forces bullets to spin as they are shot (using helical slices along the barrel) the bullet's vertical displacement is lower. So if you shoot something 100ft away, a spinning bullet will hit 2" below the target whereas a non-spinning bullet will hit 5" below (I made those numbers up, they are not necessarily true).

 

[Averagesupernova]

Spinning the bullet is called "rifling". Hence the name: Rifle. What happens to a football when you throw it if you don't add some spin? The same thing would happen to a bullet. How far the bullet drops has nothing to do with spinning other than that it keeps it in the most favorable aerodynamic position.

 

[Michael D. Sewell]

Just so you know, a 30-06 with a 150 grain pointed projectile fired (level) at a muzzle velocity of 2910 ft/sec will drop a total vertical distance of 470 inches at 1000 yds after a flight time of 1.824 seconds.

If you use the equation $$\Delta x = 1/2at^2$$ for the given time you will find that delta x = 642 inches.

This difference is not directly related to spin. The aerodynamic drag is a vector quantity. The dynamic pressure is exerted on the projectile from an angle below the horizontal as the bullet falls through the air during flight. This is also true of the wind drift correction, and must be taken into account to make any sort of accurate prediction of flight path. Wind drift is directly proportional to wind velocity. A wind velocity of 10 miles/hour perpendicular to the gun-target line will give a drift of 140 inches at 1000 yds.

I realize that the units I have used here are a pain(not metric, not consistant), but they are the units that are in common use for ballistic calculations.

Without spin, the bullet would tumble in flight. Modern rifles are only capable of such amazing accuracy because the bullet is stable in flight.

Even spherical projectiles must have spin to achieve any sort of acceptable accuracy.

The rate of spin can be very critical. If the bullet is "over stabilized" the bullet will retain a nose up attitude after reaching it's highest point in the trajectory giving an unpredictable point of impact and very, very poor accuracy. Also, bullets often leave the muzzle of the gun with quite a bit of yaw; "Overstabilization" can cause accuracy problems with that as well.

There are formulas that are used with very good success to predict the proper spin rate for a bullet of a given shape fired at a given velocity. The old formula(1879) was called the Greenhill formula but has been changed to take into account the different shapes of modern bullets. If anyone is interested I will give you some links.
-Mike

 

[Mr. Robin Parsons]

Not to be questioning the authority of the knowledge as you present it, but I had a 'hunter', and another, look at the figure, a 40 FOOT drop in 1000 yards seems rather lots...but I can accept it is the sense of the knowledge of what you do to the gunsighting, to adjust to that kind of range, essentially 'shooting it uphill' so to speak...still seems like a lot to me too...your sure of it, aren't you?

 

[Michael D. Sewell]

Not to be questioning the authority of the knowledge as you present it, but I had a 'hunter', and another, look at the figure, a 40 FOOT drop in 1000 yards seems rather lots...but I can accept it is the sense of the knowledge of what you do to the gunsighting, to adjust to that kind of range, essentially 'shooting it uphill' so to speak...still seems like a lot to me too...your sure of it, aren't you?

Mr. Robin Parsons,
For someone who doubts my data, the tone of your post was very nice. Thank you.

I assure you that the information that I gave you is correct. I am not asking you to believe me alone. I will provide some links. I do want to say that I know that the information given in these links is correct, not just because it is correct mathematically, or agrees with Newton's laws, but also because I have used it myself on the range many times. The equations given on these sites are mathematically correct, and are derived from Newton's laws. I have gone over them very carefully to be sure that I understood exactly how they were derived. They are also in agreement with data from Remington Arms, Winchester, Sierra, and the Army Ballistics Research Laboratory. Also, I have a personal shooting journal that confirms the data on the range. I assure you, this information is rock solid(in one of the links there are a couple of typos in the charts, I have the corrected numbers if you need them).

M-1 rifles were sighted in for 200 yds, which is up 5 clicks of elevation from level. At the 1000 yd line you put on 41 clicks of elevation for a total of 46 clicks. From there you have two sighters to fine-tune your wind calculation and temperature correction, and center your zero on the target. If you are old enough to remember, on an M-1 rifle, a click is 1 inch at 100 yds, or about 1 minute of angle. 46 clicks is 460 inches at 1000 yds.

I hope these links will help your hunter friend,
-Mike

www.aeroballisticsonline.com
www.ballistics-software.com
www.ballistics.org
www.eskimo.com/~jbm[/URL]
[url]www.remington.com[/url]
[url]www.shootingsoftware.com[/url]
[url]www.sierrabullets.com[/url]
[url]www.snipercountry.com[/url]
[url]www.winchester.com[/url]
[url]www.arl.army.mil[/url]
US Army Research Lab, Aberdeen Proving Grounds Md.
[url]www.usamu.com[/url]
US Army Marksmanship Unit, Ft Benning Ga.

 

[Mr. Robin Parsons]

(YIKES!) THANKS! (sorry I questioned it, then again, look at all the new information you gave, Thanks again!)

 

[Mr. Robin Parsons]

If you would forgive me Mr. Sewell, you would then know the answer to the 'philosophical' question, "Would A bullet, hitting a snowflake, it gain in accuracy of flight, from the wetting?

(Clearly Sir, your are way past my 'league')

 

[Cliff_J]

Mr. Sewell, very informative posts!

One question: What's a click? Isn't that what the SF guys used in Vietnam used as a distance measurement or something to that effect as in "...the target is 3 clicks past that hill..." or have I been led astray by pop culture?

Cliff

P.S. Mr. Parsons - have you ever seen the classroom experiment with a dart gun fired horizontally at a stuffed animal dropped at the same moment? Very effective teaching tool, trick is getting the timing correct but once sorted very simple to repeat and a regular video camera offers enough resolution to see the effect.

 

[Averagesupernova]

Guys, just remember this: A speeding bullet doesn't drop any slower than something that is dropped stationary.

 

[Michael D. Sewell]

Guys, just remember this: A speeding bullet doesn't drop any slower than something that is dropped stationary.

This is not correct. Re-read the posts, and check out the links.

 

[Clausius2]

Well, I was not imagining such a conversation due to bullet spinning. If this spinning provides stablity to bullet trajectory, as I have undertood in this posts, it would not work for space rockets, spacecraft s, missiles and bombs?.
I have seen a little spinning when the space shuttle takes off. Does it do with that?. I have not cleared myself one thing: the bullet runs more yards with an spin around its axis?. I mean, is it possible that for the same initial force, the spinning bullet goes faster than another that has no spinning?. To answer this question I will be grateful if you consider the aerodynamic drag. Does the spinning reduces this drag?.

 

[Michael D. Sewell]

Mr. Sewell, very informative posts!

One question: What's a click? "...the target is 3 clicks past that hill..."

P.S. Mr. Parsons - have you ever seen the classroom experiment with a dart gun fired horizontally at a stuffed animal dropped at the same moment?

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).

The stuffed animal demonstration is a great one, especially if it is set up ahead of time. It is a logical place to start when teaching parabolic motion.
With hi-speed projectiles it doesn't always hold, for the reason I mentioned in my first post.(it could work depending on the individual teddy bear's coefficient of aerodynamic drag in free-fall). Aerodynamic forces in the classroom experiment are not a significant factor because of the short range and low velocity. The aerodynamic forces involved with hi-speed projectiles are tremendous. If you use the charts and graphs in the links I gave, and do the math to check out the acceleration due to aerodynamic drag, the results will probably come as quite a surprise to you.

I Hope you have fun with this stuff,
-Mike

 

[Michael D. Sewell]

Well, I was not imagining such a conversation due to bullet spinning. If this spinning provides stablity to bullet trajectory, as I have undertood in this posts, it would not work for space rockets, spacecraft s, missiles and bombs?.
I have seen a little spinning when the space shuttle takes off. Does it do with that?. I have not cleared myself one thing: the bullet runs more yards with an spin around its axis?. I mean, is it possible that for the same initial force, the spinning bullet goes faster than another that has no spinning?. To answer this question I will be grateful if you consider the aerodynamic drag. Does the spinning reduces this drag?.

Clausius2,
I was not imagining such a conversation either, but each answer seems to bring more questions. So I'll do my best to help.

I am not in any way, shape, or form an expert on rockets or spacecraft but I'll answer your questions the best I can.

Once a bullet leaves the muzzle of a gun, the shooter's influence on it is over. Nothing can be done to change the path of the projectile once it is in flight. This is not the case with many of the other types of projectiles you mentioned, most are guided, some have fins for stability(like the fletching on an arrow).

The space shuttle's roll maneuver is not like the spin of a rifle bullet at all. I'll leave any further explanation of this to someone who knows what they're talking about.

The spin of a rifle bullet is for stability in flight only, which is in turn for accuracy.

As for the questions on maximum range and velocity...well...If the bullet were to tumble in flight it would lose velocity quickly and would also have a much shorter maximum range. The spin on a bullet does not in any way violate the the law of conservation of momentum. It doesn't give you a free lunch. As a matter of fact, you have to "pay" for the spin with velocity, a study of rotational motion will show you why.

Spinning increases the total aerodynamic drag.

The art and science of precision shooting is very complex, and I have left out a lot of information, and simplified things as much as possible. If you want to go deeper into the subject start with the links. If I can be of any help, just let me know.

I hope this helps,
-Mike

 

[enigma]

Well, I was not imagining such a conversation due to bullet spinning. If this spinning provides stablity to bullet trajectory, as I have undertood in this posts, it would not work for space rockets, spacecraft s, missiles and bombs?.

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

If you take a look at the video for the Mars landers, you'll see that it spins up before it does its trans-Mars injection burn. That is done to utilize gyroscopic effects for control during the burn.

 

[Clausius2]

Hey, and what is the correct spinning way? clock or anticlockwise?. Or is it the same thing for the bullet?.

All of us has experienced how we can introduce an object inside a dense fluid by moving it like an screw going forward. Is it does to do with this bullet spinning?.

And I was wondering how i can prove the trajectory stability, Do you know if Lagrange formulae would be appropriate for this problem?.

 

[enigma]

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.

Read up on gyroscopic motion to get the equations of motion. I have packed my class notes away somewhere, and don't remember the details...

 

[Michael D. Sewell]

Hey, and what is the correct spinning way? clock or anticlockwise?. Or is it the same thing for the bullet?.

All of us has experienced how we can introduce an object inside a dense fluid by moving it like an screw going forward. Is it does to do with this bullet spinning?.

And I was wondering how i can prove the trajectory stability, Do you know if Lagrange formulae would be appropriate for this problem?.

Clausius2,
enigma is absolutely correct in saying that the direction of spin doesn't matter as far as stability is concerned. This is no suprise; enigma has a much better understanding of gyroscopic motion than I do. He's the guy to talk to.

The bullet does not in any way "screw itself into the air". The spin introduced by the rifle's bore is for stability only, and not for any other reason. You are trying too hard to find other uses for spin here. There just aren't any.

I am assuming that english is a second language for you. Are you finding that the links I gave you are helpful, or do you need help from me. There is a huge amount of information in the links, and you may be finding it to be difficult to read. If you are, let me know, and I will help you if I can. You can also send me a private message if you want. I can probably give you quite a bit of assistance in questions involving the practical application of shooting, but your physics questions should be addressed to people like enigma. Also, your questions are better answered in the public forum where many people can see them and respond.

The are some equations on stability in the links, but I don't know if they are exactly what you are looking for. Bear in mind that the velocity of these projectiles changes rapidly during flight, and bullets may be more stable at some times during the flight than others. Shooters disagree about twist rates in rifles, and this is often a "hot" topic of discussion. The answer is that no one twist rate is the perfect one for a given bullet or muzzle velocity. Twist rate is a compromise. The question is: at what range do you want accuracy?
Hope this helps,
Mike

 

[drag]

Greetings !

Well, frankly I don't know anything about how spinning
effects the flight of a bullet if in any way at all.
As far as I'm aware the pupropse is completely different -
a spinning bullet causes considrably greater damage to
the human body because it scrambles the internal organs
in its path. That's also why it's banned by the Geneva
Convention, if I'm not mistaken, despite the fact that
M16s, AK47s and many other weapons are built
that way.

Live long and prosper.

 

[Michael D. Sewell]

Greetings !

Well, frankly I don't know anything about how spinning
effects the flight of a bullet if in any way at all.
As far as I'm aware the pupropse is completely different -
a spinning bullet causes considrably greater damage to
the human body because it scrambles the internal organs
in its path. That's also why it's banned by the Geneva
Convention, if I'm not mistaken, despite the fact that
M16s, AK47s and many other weapons are built
that way.

Live long and prosper.

Drag,
The primary damage done to internal organs at impact has nothing to do with spin. Bullets that are used in combat are non-expanding.

Spinning bullets are not banned by the Geneva convention, and never have been.

M16s, AK-47s "and many other weapons" have rifled bores and do indeed impart spin to their bullets.

-Mike

 

[HallsofIvy]

Mike, while I agree with your physics, your e-manners can use some work. Drag did not "make a fool" of himself by being mistaken. One only does that by being rude.

 

[Mr. Robin Parsons]

I would supect that it is simply that Drag has erred in the bullet type as a crosscut front will cause an entering bullet to fragment as to increase bodily damage, especially if it is spinning...is that perhaps the difference in what has been read, and what is meant?

 

[Michael D. Sewell]

...is that perhaps the difference in what has been read, and what is meant?

No. Since Drag seemed to have no interest in the physics discussed here, I took his post as a political statement, and I wrote a very sarcastic post in reply. I have since edited it.

We were not discussing the damage that bullets do to people, we were discussing gyroscopic motion and parabolic motion. But since the question was raised I'll do my best to answer it.

Biology is not my forte. However, after seeing probably a hundred or so large animals that were killed by bullets, I have never seen anything that leads me to believe that a bullet's spin contributes much to tissue damage. Soft point bullets expand to about twice their unfired diameter and fragment to some degree, and hollow point bullets fired at high velocity virtually explode on impact, but these two things would happen whether or not the bullet had any spin. Most of the damage appears to me to be the result of bone fragmentation, and hydrostatic pressure causing burst blood vessels. Further, I have seen several animals that were killed by non-expanding bullets that were spinning. In these cases the damage was a hole that passed completely through the animal with only a small amount of burst blood vessels surrounding the hole. The organs were not "scrambled". I must conclude from what I have seen that the spin has very little to do with the damage caused by a bullet.

-Mike

 

[Mr. Robin Parsons]

Your right, the spin would simply change the trajectory, within the carcass, that it is passing through...me too, to much media hype sometime, not enough time for simple thought...like questioning you about the drop, you have stated the velocity of the bullet, and the distance, I did it in my head, roughly, yesterday, and couldn't figure out why, after that, I had even raised the question...happens to us all I guess...

Thanks though, prefer accuracy in information...

 

[drag]

Greetings !

Mr. Sewell, hmm... you're slightly "heavy", wouldn't you agree ?
If I may so suggest, lighten up a bit dude.

As for my post, I could of course be mistaken. It's just
what one of our weapons instructors said once and I
remmembered, maybe I misunderstood him or he was bull****ing us.

Kin'na makes some sense though, if a bullet is spinning
it can transfer some torque to what's in its way, don't know.
Maybe he was talking about those sprawling bullets.
Got'ta wait a couple more semesters 'till I can comment on
weather it helps fix the trijectory or something, I guess.
(Can see no reason why that should happen for now.)

Peace and long life.

 

[Michael D. Sewell]

Drag,
I took your first post to be a criticism, and a left wing political statement. I wrote a scathing post in response. I would like to apologize to you and to anyone else who read that post, for my harsh words.
-Mike

 

[Clausius2]

Clausius2,




I am assuming that english is a second language for you. Are you finding that the links I gave you are helpful, or do you need help from me. There is a huge amount of information in the links, and you may be finding it to be difficult to read. If you are, let me know, and I will help you if I can. You can also send me a private message if you want. I can probably give you quite a bit of assistance in questions involving the practical application of shooting, but your physics questions should be addressed to people like enigma. Also, your questions are better answered in the public forum where many people can see them and respond.


Mike

Very politeness, Mike. So how can you guessed english is my second language?. My english is perfect!
Well, I am able to understand all your links. All the same, I am too lazy for viewing a lot of text and formulae, probably I could find this web-pages on my own. I enjoy further talking or discussing the topics with all of you, and have a "buen rollito" as we say in spanish (spanish of Spain, not of SouthAmerica, of course).

 

[wimms]

Banned by the Geneva Convention are maybe excentric bullets, not anything about how any riffle is built. Excentric bullets are semistable, but as soon as they touch anything, that excentricity causes very sharp trajectory changes. It makes awful damage to body because it doesn't simply go though but takes quite some complex trajectory inside body, making it a total mess. I've heard that even by brushing a person it would turn right into the direction of contact and thus invade body even though normal bullet would have been harmless.

These bullets are short range and pretty imprecise in flight, but very lethal. One interesting detail is also that they are so sensitive to any contact that change trajectory even by brushing tree leaves. This makes it almost impossible to use them in forest combats - precision approaches zero due to leaves, person behind a bush is almost "safe". They are also very unpredictable so that in close combats lots of own people can die. Aswell these bullets are extreme hazard in places where civilian people should be avoided.

So, Drags thoughts about spin is almost right, he just missed the excentricity that makes it a whole different story.

 

[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.