Quantcast
  1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

Bullet trajectory

Discussion in 'Rifle Country' started by blackops, Jul 10, 2009.

Thread Status:
Not open for further replies.
  1. blackops

    blackops Member

    Joined:
    Jun 6, 2009
    Messages:
    1,238
    Correct me if I'm wrong please. I have a 270 and at 100yds my bullet should be 1.5 inches high, this in turn means at 200 yds I'm dead on. So the bullet out of the barrel actually rises before it starts to drop correct? With the rifeling of the barrel it gives the bullet that tight rotation lifting the bullet right or wrong?
     
  2. Tim the Enchanter

    Tim the Enchanter Member

    Joined:
    Feb 17, 2008
    Messages:
    52
    Location:
    Phoenix, AZ
    Gravity acts on the bullet as soon as it leaves the muzzle, and the bullet begins to drop. Your scope is tilted down and looking further down than the bullet will actually drop at a given range, creating the illusion of lift.

    See:
    10_19_07_Figure_2.jpg
    and
    10_19_07_Figure_3.jpg
     
  3. MTMilitiaman

    MTMilitiaman Member

    Joined:
    Apr 28, 2005
    Messages:
    2,629
    Location:
    Missoula, Montana
    No. As mentioned previously, gravity begins acting on the bullet as soon as it leaves the muzzle. Therefore, if the muzzle is level, the bullet will never go above the line of sight.

    Sighting in a rifle is done by inclining the muzzle such that the bullet rises above, then drops back down to, the line of sight at a predetermined range. The arc of the trajectory then brings the bullet highest above the line of sight around midway between the shooter and the sight-in distance. This is referred to as the 'midrange trajectory.' Once the bullet reaches this point, it will drop continuously throughout the rest of its flight-dropping to the line of sight at the sight-in distance, and then exponentially below the line of sight from there on. Because of this, the bullet crosses the line of sight twice--once rising above the line of sight due to inclination of the bore, and again coming down to the line of sight at the sight in distance. The first time the bullet crosses the line of sight is usually between 20 and 30 yards for most centerfire rifles, depending on velocity, and the height of the optic above the bore. This is why when sighting in our centerfire rifles, it is standard practice for us to sight in for windage and elevation at 25 yards, then make corrections as necessary at the desired sight in distance. This works quite well with most hunting rifles and optics sitting ~2.5" above the bore (center of objective lens to center of bore). The higher the optic is above the bore, the further down range the bullet will initially cross the line of sight. For example, the 4x Trijicon RCO mounted on the M16A4 sits higher above the bore than most rifle/optic combination. This combined with a desired zero range of 300 yards means that the optics are initially sighted in at 36 yards. The BZO (Battle ZerO) is then confirmed on pop-up targets at 300 yards.

    Rifling puts a spin on the bullet because modern spitzer (pointed) style projectiles are heavier towards the base than the nose. Therefore, they naturally want to fly base forward. Rifling imparts a spin on the projectile to overcome this tendency. The longer a projectile is, or the more dense the medium is that it is traveling through (air, tissue, water, ect) the faster it must be spun to be stable. Think of it like a top. The longer a top is, the faster it must be spun to remain stable. Once it drops below the necessary rotational velocity, it quickly loses stability. The same thing can happen to a bullet when it drops below the rotational velocity necessary for it to remain stable in a given medium, at which point it tumbles or "keyholes." While rifling can have a slight effect on linear velocity as well, its effect on external trajectory is minimal.

    Think of it as throwing a football down field. Few people possess the arm strength and coordination to throw a laser pass dead-on much past 15 yards. To get the ball further down the field, it must be thrown upwards at an arc so that it falls to the desired target at a specific range. Imparting a spin on the football isn't quiet as necessary as the football is more balanced than most rifle bullets, but it still helps the ball fly more aerodynamically.
     
    Last edited: Jul 10, 2009
  4. waterhouse

    waterhouse Member

    Joined:
    Jan 7, 2005
    Messages:
    4,285
    Location:
    Round Rock, TX
    here is another picture which may help. The bullet start out below the line of sight, crosses the line of sight at the first zero, and then continues along its arc until it falls to the line of sight again, which is generally where people say the rifle is "zeroed."

    02-trajectory.jpg
     
  5. rcmodel

    rcmodel Member in memoriam

    Joined:
    Sep 17, 2007
    Messages:
    59,082
    Location:
    Eastern KS
    +1 to all the posts above.

    The only way a bullet could generate lift is if it were airfoil shaped like a wing, and not spinning.
    Thus presenting a constantly changing round profile to the air that cannot generate lift in an upward direction.

    If the bullet yaws during flight, and all bullets do to a certain extent, it would generate lift in the direction of the yaw.

    But again the bullet is spinning thousands of times per second, and the yaw angle is constantly changing around the 360 degree rotation.

    rc
     
  6. iiranger

    iiranger Member

    Joined:
    Aug 10, 2007
    Messages:
    109
    Old Jack O'Connor did

    an article that Weaver (or TX) used to include with scopes...

    What has "stuck in my head" now these 5 decades:

    You survey 100 yards exactly level. Put the .22 rimfire rifle on the bench at 48 inches, say. Bore sight. Target center also at 48 inches. Load one long rifle and fire. Bullet will strike 12 inches below the center. Move back to 125 yards and the bullet will strike 36 inches low.

    While the bullet starts to fall as it leaves the muzzle, you point the "line of bore" so high above the target that the bullet will fall into point of aim. And you use reference points on the barrel, called sights, since you cannot look through the barrel while firing... !!
     
  7. Uncle Mike

    Uncle Mike Member

    Joined:
    Nov 5, 2008
    Messages:
    3,129
    Location:
    never never land...never land here!
    Huh?......:scrutiny:
     
  8. rcmodel

    rcmodel Member in memoriam

    Joined:
    Sep 17, 2007
    Messages:
    59,082
    Location:
    Eastern KS
    Like sticking your hand out the car window going 70 MPH.

    If you yaw, or tilt your hand up, it goes up.
    If you tilt it down, it goes down.
    If your hand is spinning 300,000 RPM like a bullet, it pretty much doesn't go anywhere though.

    See this about yaw:
    http://www.exteriorballistics.com/ebexplained/5th/40.cfm

    rc
     
  9. Uncle Mike

    Uncle Mike Member

    Joined:
    Nov 5, 2008
    Messages:
    3,129
    Location:
    never never land...never land here!
    Does your hand drill try to 'fly' out of your hand whilst it's spinning?....
    Does your drill press scoot across the floor when in use.

    The 'spinning' of the projectile does not create any lift, the 'spin' only offers the gyroscopic forces that stabilize the said projectile.

    As the knuckle ball thrown with no spin rises and falls unpredictably the two finger fast ball that has spin, stabilizes and offers the pitcher control ability...accuracy if you will.

    All the above explanations in the other posts are spot on... do you understand.....y? or n?:)

    The next time your out with the water hose, hold the nozzle parallel to the ground and squirt away... notice where the water hits the ground... now angle the nozzle slightly upwards.... notice how much further away the water is hitting the ground?

    Same principle with the projectile.;)

    :D
     
  10. cbrgator

    cbrgator Member

    Joined:
    Jan 23, 2008
    Messages:
    2,525
    Bullets really spin at 300,000 rpm? Hmm... guess you learn something everyday! That's very cool.
     
  11. Uncle Mike

    Uncle Mike Member

    Joined:
    Nov 5, 2008
    Messages:
    3,129
    Location:
    never never land...never land here!
    Right... but your hand is flat and isn't creating any lift, per sey, but is being deflected perpendicular to the direction of the oncoming wind... kind of like a sheet of plywood in a wind storm.

    If you do not tilt the hand, or plywood from the parallel position in relation to the direction of the oncoming wind, it will remain in that position because there is no lift being created, an foil on the other hand will create lift in all but a 'stalled' position.

    To create lift your hand would have to exhibit the shape of an airfoil, creating pressure differentials which allow for increased pressure on the bottom and decreased pressure on the top of the foil, respectively.

    :D
     
  12. rcmodel

    rcmodel Member in memoriam

    Joined:
    Sep 17, 2007
    Messages:
    59,082
    Location:
    Eastern KS
    Some spin even faster then that.

    Example One: In a 1:12″ twist barrel the bullet will make one complete revolution for every 12″ (or 1 foot) it travels through the bore. This makes the RPM calculation very easy. With a velocity of 3000 feet per second (FPS), in a 1:12″ twist barrel, the bullet will spin 3000 revolutions per SECOND (because it is traveling exactly one foot, and thereby making one complete revolution, in 1/3000 of a second). To convert to RPM, simply multiply by 60 since there are 60 seconds in a minute. Thus, at 3000 FPS, a bullet will be spinning at 3000 x 60, or 180,000 RPM, when it leaves the barrel.

    Example Two: What about a faster twist rate, say a 1:8″ twist? We know the bullet will be spinning faster than in Example One, but how much faster? Using the formula, this is simple to calculate. Assuming the same MV of 3000 FPS, the bullet makes 12/8 or 1.5 revolutions for each 12″ or one foot it travels in the bore. Accordingly, the RPM is 3000 x (12/8) x 60, or 270,000 RPM.


    Then, consider a .223 WSM, with a 1/10 twist, and a 45 grain bullet going 4,100 FPS.
    I'll let you do the math. :D

    rc
     
  13. John Parker

    John Parker Member

    Joined:
    May 14, 2009
    Messages:
    679
    Well RC, it's not quite that simple. Not only does the bullet begin to drop when it leaves the muzzle, it also begins to slow down...neither trajectory nor acceleration are static with objects moving due to an external, non-constant power source.

    Neener neener!
     
  14. Uncle Mike

    Uncle Mike Member

    Joined:
    Nov 5, 2008
    Messages:
    3,129
    Location:
    never never land...never land here!
    What is... 295,200 RPM? I dunno! It's damn fast! I know that hehehe

    :D
     
  15. rcmodel

    rcmodel Member in memoriam

    Joined:
    Sep 17, 2007
    Messages:
    59,082
    Location:
    Eastern KS
    I know how simple it is.
    I was shooting 1,000 yard high-power for 5th. Inf AMU in 1968.

    We pretty much had to understand exterior ballistics to get on paper the first day.

    Short of writing a book, it's a little much to explain to someone who ask about bullet lift being caused by rifling spin in the OP.

    Neener neener to you too!

    rc
     
  16. Uncle Mike

    Uncle Mike Member

    Joined:
    Nov 5, 2008
    Messages:
    3,129
    Location:
    never never land...never land here!
    hehehe.... bottom line is... the bullet starts to head for the center of the earth AS SOON AS it leaves the barrel.;)

    We angle our barrels upward to get that magical extra distance.... water hose thing again, so that is why the projectile crosses your line o' sight(which is as straight as a laser from our eye to infinity) twice. Once on the way up, and once on the way down.;)

    :D
     
  17. gunnie

    gunnie Member

    Joined:
    Mar 23, 2007
    Messages:
    787
    back to the OP's question...

    ...I have a 270 and at 100yds my bullet should be 1.5 inches high"...

    this will vary in relation to the scope center line distance from the bbl centerline.

    the bullet path will be higher @150 yards in relation to POA, and then start to slow/drop enough to cross the line again.

    as above, it starts to drop upon leaving the bbl, it is the downward angle of the scope crosshairs, not the bullet rising.

    gunnie
     
  18. Uncle Mike

    Uncle Mike Member

    Joined:
    Nov 5, 2008
    Messages:
    3,129
    Location:
    never never land...never land here!
    Provide us with the ammo you are using, bullet weight and ballistic coefficient also, the length of the barrel which you will fire this load from and the zero distance and we can provide you with various POI's and trajectories.

    You can have some fun at this site:http://www.norma.cc/default.asp?Lang=2#

    go to ballistics US. Play with this computer, it is a good visual trajectory form.

    :D
     
  19. John Parker

    John Parker Member

    Joined:
    May 14, 2009
    Messages:
    679
    Ah, the ol' Army Marksmanship Units. You know, I learned more about the 240B from those guys than anyone else. Pretty amazing!
     
  20. Whitman31

    Whitman31 Member

    Joined:
    Jul 2, 2007
    Messages:
    413
    Location:
    Minnesota
    This may be a stupid question, humor me...

    I've hear rumor that a bullet fired while the muzzle is very close to the ground will actually rise some. Suggesting that the muzzle blast reflects off the ground and provides a little upward push. Is this just a myth?
     
  21. tactikel

    tactikel Member

    Joined:
    Apr 28, 2009
    Messages:
    1,026
    Location:
    Northeastern Illinois
    A bullet would be infront of any muzzleblast, and long gone before any gas/shockwave could bounce and deflect it- IMHO

    RC has forgotten more than most of us will ever know about rifles and shooting ;) 200k-300k rpms are why some varmint bullets will fly apart when fired from a barrel with too fast a twist.
     
  22. iamkris

    iamkris Member

    Joined:
    Jan 6, 2003
    Messages:
    2,839
    Location:
    My own little slice of Purgatory
    Technically, a cylindrical object that is spinning counterclockwise into a relative wind will generate a very small amount of lift. Most first year college physics classes will have an experiment with a paper towel roll that demonstrates this.

    The "top" of the roll is moving faster than the bottom of the roll into the relative wind...that means you'll have lower pressure on the top vs the bottom...thus lift.

    That said, it is minute for a bullet and of little consequence.

    See...that bachelors in mechanical and masters in aerospace engineering came in handy after all! (I'm not an engineer anymore).
     
  23. Hud

    Hud Member

    Joined:
    Nov 3, 2006
    Messages:
    370
    Location:
    Central California
  24. gunnie

    gunnie Member

    Joined:
    Mar 23, 2007
    Messages:
    787
    Hud,

    good stuff there!!!

    gunnie
     
  25. Quilbilly

    Quilbilly Member

    Joined:
    Dec 25, 2007
    Messages:
    162
    Location:
    Quilcene, WA
    Is there anything RC does not know?
     
Thread Status:
Not open for further replies.

Share This Page