Spinning bullet, why?

  1. Clausius2

    Clausius2 1,479
    Science Advisor
    Gold Member

    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?.
     
  2. jcsd
  3. 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)
     
  4. 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.
     
  5. 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).
     
  6. 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.
     
  7. 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 [tex]\Delta x = 1/2at^2[/tex] 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 acheive 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
     
    Last edited by a moderator: Apr 14, 2004
  8. 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 alot to me too....your sure of it, aren't you?
     
  9. 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
    www.remington.com
    www.shootingsoftware.com
    www.sierrabullets.com
    www.snipercountry.com
    www.winchester.com
    www.arl.army.mil
    US Army Research Lab, Aberdeen Proving Grounds Md.
    www.usamu.com
    US Army Marksmanship Unit, Ft Benning Ga.
     
    Last edited by a moderator: Apr 14, 2004
  10. (YIKES!) THANKS! (sorry I questioned it, then again, look at all the new information you gave, Thanks again!)
     
  11. 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')
     
  12. Cliff_J

    Cliff_J 789
    Science Advisor

    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.
     
  13. Guys, just remember this: A speeding bullet doesn't drop any slower than something that is dropped stationary.
     
  14. This is not correct. Re-read the posts, and check out the links.
     
  15. Clausius2

    Clausius2 1,479
    Science Advisor
    Gold Member

    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, spacecrafts, 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?. :confused:
     
  16. 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
     
  17. 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
     
    Last edited by a moderator: Apr 4, 2004
  18. enigma

    enigma 1,815
    Staff Emeritus
    Science Advisor
    Gold Member

    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.
     
    Last edited: Apr 4, 2004
  19. Clausius2

    Clausius2 1,479
    Science Advisor
    Gold Member

    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?.
     
  20. enigma

    enigma 1,815
    Staff Emeritus
    Science Advisor
    Gold Member

    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....
     
  21. 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
     
Know someone interested in this topic? Share a link to this question via email, Google+, Twitter, or Facebook

Have something to add?