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Difference Between Musket and Rifle Trajectories?

  1. May 23, 2017 #1
    Wikipedia's article on rifles in the American Civil War mentions this:

    Is it true that smoothbore guns have a flatter trajectory than rifles? Can someone explain the physics of why that would happen?
     
  2. jcsd
  3. May 23, 2017 #2

    phinds

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    Sounds backwards to me. What are we missing? Hope someone jumps in who can explain it.
     
  4. May 24, 2017 #3

    anorlunda

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    Speculation on my part.
    1. Given the same powder charge, a rifled barrel takes some energy to spin the bullet, to expand the minne ball, and to carve rifling marks in the ball. All that means a lower muzzle velocity.
    2. The civil war generals were wrong. I just read an article that said that because black powder smoke obscured vision, all tactics were designed for max 40-50 yards anyhow. It said soldiers were trained to raise to horizontal and fire without aiming. I find it hard to believe that in 40 yards, there is a significant difference in the smooth vs rifled trajectories. Also firing at 40 yards without aiming, how could observers make accurate comments about accuracy? The article said that the generals were very slow to embrace the new technology, and they may have perpetuated myths.
     
  5. May 24, 2017 #4
    No. Smooth bore guns do not have flatter trajectories than rifles. Smooth bores do not impart transverse spin to the projectile as it travels down the barrel (internal ballistics). Therefore, it is not spin-stabilized when it exits the barrel (external ballistics). This means that oblong projectiles will tumble erratically through the air if they were launched from smooth-bore firearms. Therefore, they have to use mostly spherical projectiles.

    The reason that this is important is because of the cross-sectional area that the oblong bullet has is smaller than that of a spherical projectile of equal weight. Having the same momentum, a 200 grain rifle bullet with a diameter of .308" will encounter far less wind resistance than a spherical musket ball with a diameter of about .68".

    Rifles have significantly flatter trajectories than smooth bore muskets, as long as they have the same weight bullet at the same velocity. The reason that the soldiers thought the rifles had more bullet drop is because the smooth bore firearms were not generally able to be used at ranges farther than a hundred meters, or so. This means that the projectile was not airborne long enough for significant drop or decrease in speed to occur. They were not accustomed to using firearms endowed with effective ranges at which bullet drop could become an actual factor in combat accuracy.

    They also could have been decreasing the amount of powder that they used to propel the bullet in the rifles. This would have been done to reduce velocity. Reducing velocity would have been done to reduce heat/friction that the bullet had with the barrel. Using the non-jacketed lead projectiles that they used back then, high velocities can cause the lead to melt and therefore clog the rifling.

    Smooth bore firearms do not have flatter trajectories than rifles, unless the smooth bore's spherical projectile is launched with significantly higher velocities or is endowed with greater cross-sectional density than the oblong spin-stabilized rifle bullet. Smooth bore firearms launch spherical projectiles that decelerate more quickly due to their low ballistic coefficient.
     
    Last edited: May 24, 2017
  6. May 24, 2017 #5

    anorlunda

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    It is easy to visualize how that fact could have been misstated and/or misunderstood in the telling leading to false myths. Especially among those opposed to the new technology. In modern times, we call that "fake news" :wink:
     
  7. May 24, 2017 #6
    I believe that the word "transverse" is the wrong choice here. The spin imparted by rifling is along the axis of the bullet, an axial spin. The word "transverse," IIRC, means "across" and in this case that would refer to tumbling of the bullet. Would you agree, RogueOne?
     
  8. May 24, 2017 #7
    I know it's not quite about trajectories, but how does this work for smoothbore and rifled anti-tank cannons? Smoothbore cannons are mostly preferred now and fire fin stabilized discarding sabot rounds and are best for firing HEAT rounds (although the shells can be modified for rifled cannons), but rifled cannons are required to fire HESH rounds.
     
  9. May 25, 2017 #8
    The tumbling end-over-end spin that you describe is referred to as longitudinal rotation, and is influenced by longitudinal stability. The axis for longitudinal rotation (tumbling end-over-end) is transverse. Transverse rotation happens about the axis that is longitudinal to the projectile. So bullet spin is transverse. The motion of the spin is transverse. The axis that the spin happens about is longitudinal.

    https://en.wikipedia.org/wiki/Miller_twist_rule

    upload_2017-5-25_7-39-55.png
    = upload_2017-5-25_7-43-10.png

    • S = gyroscopic stability
    • s = spin rate in radians per second squared
    • m = polar moment of inertia
    • a = angle of attack
    • t = transverse moment of inertia
    • d = air density
    • v = velocity
    https://en.wikipedia.org/wiki/Stability_derivatives
     
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