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Thread: The trajectory

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    Default The trajectory

    Just curious, but how is it that a rifle sighted in at say a 100 yards shoots low at 20 yards rises and then drops to hit its mark at a 100 yards. Is it simply the fact that the chosen sights are above the barrel and thus the rifle is actually pointed at a slight upward angle when you shoot?
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    Premium Member MarineHawk's Avatar
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    Quote Originally Posted by Carnivore View Post
    Just curious, but how is it that a rifle sighted in at say a 100 yards shoots low at 20 yards rises and then drops to hit its mark at a 100 yards. Is it simply the fact that the chosen sights are above the barrel and thus the rifle is actually pointed at a slight upward angle when you shoot?
    Yes. Correct.

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    Default Yep

    The barrel IS pointing up at some ANGLE.

    The "line of sight" is STRAIGHT to the target.

    Since the bullet starts to drop, as soon as it comes outta the barrel, the path of the bullet is a CURVE.

    With a Scope Sighted rifle, the bullet crosses the "line of sight" TWICE. Once at 25 or 35 yards, and again at whatever distance the rifle is sighted in for.

    The 25 or 35 yard crossing might be half that with iron sights. It depends on how high above the bore the sights are.

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    Default One exception

    Quote Originally Posted by Smitty of the North View Post
    The barrel IS pointing up at some ANGLE.

    The "line of sight" is STRAIGHT to the target.

    Since the bullet starts to drop, as soon as it comes outta the barrel, the path of the bullet is a CURVE.

    With a Scope Sighted rifle, the bullet crosses the "line of sight" TWICE. Once at 25 or 35 yards, and again at whatever distance the rifle is sighted in for.

    The 25 or 35 yard crossing might be half that with iron sights. It depends on how high above the bore the sights are.

    Wunnerful, Wunnerful.

    Smitty of the North
    If you raise the line of sight (through the scope), the bullet crosses the line of sight for the first time furthur out, and the second time closer in. If you keep raising the line of sight through the scope enough, the two points meet. At that setting, the line of flight of the bullet and the line of sight through the scope are tangent and the flight path of the bullet coincides to the line of sight only once.

    Raise the scope's line of sight any furthur and they never cross.

    Lost sheep.

    Just an academic exercise, but hopefully it helps to visualize what Smitty so well described.

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    Default And NOT so Academic

    A higher mounted scope, effectively, flattens trajectory.

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    Premium Member MarineHawk's Avatar
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    A couple of explanations:
    http://www.rifleshootermag.com/shoot...303/index.html

    http://www.chuckhawks.com/bullet_trajectory.htm

  7. #7

    Question

    Quote Originally Posted by Smitty of the North View Post
    A higher mounted scope, effectively, flattens trajectory.

    Smitty of the North
    Define trajectory? Because the trajectory or path of the bullet never changes only its referance points for aiming. It has the same parobalic(?) ark.

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    This is getting a little nuts. The answer to the first question from the original poster is yes.

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    Default 'Ceptin' this here one time...

    out around a thousand yards at a local match, one feller's bullet chose to go through the line of sight three times. Once on the rise from the muzzle, the second when the bullet came back through, and yet again when it ricocheted off the berm where the shot fell short, and from there sideways into the target. (along with a bit of dirt and gravel it kicked up, but they didn't score those hits).......

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    This is a great little chart and for what it's worth, or what I think is an important point; The mid range trajectory is not the highest point of bullet travel. This would vary with velocity and BC but will always be a bit farther down range than the mid range and must be taken into consideration if you follow the point-blank-range system. This high point is called maximum ordinate or max-ord. This is the point where Lost Sheep mentioned where the near and far crossings are the same. It seems so many think the mid-range point is the high point of bullet travel. This is because the trajectory is not a curve (sorry Smitty) but a parabola, having a steeper declining slope at the far end. Calculus is used to plot this bullet travel using the natural log function, (e, epsilon) to a negative exponent. e^-T/t (How far do you want to go with this?)

    Anyway I think it is a great little chart.
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    Quote Originally Posted by Darreld Walton View Post
    out around a thousand yards at a local match, one feller's bullet chose to go through the line of sight three times. Once on the rise from the muzzle, the second when the bullet came back through, and yet again when it ricocheted off the berm where the shot fell short, and from there sideways into the target. (along with a bit of dirt and gravel it kicked up, but they didn't score those hits).......
    Ha just once? you should in the butts when the rattle battle folks are shooting at 600 as many rounds as they can for one minute...we have to hide under pieces of carboard to keep the dirt off!!!!

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    Quote Originally Posted by tjen View Post
    Define trajectory? Because the trajectory or path of the bullet never changes only its referance points for aiming. It has the same parobalic(?) ark.
    That's true. I said, "effectively".

    (Sorry, I called a "parabola" a curve, Murphy.) I betcha Highway 137 has some of those "Parabolas" mixed in with them "Curves", don't you think?

    With a higher mounted scope, the first time the bullet crosses the line of sight it is farther away from the barrel. (Read Lost Sheep's post, and look at MarineHawk's diagram.)

    Here's are quotes from the NRA Fact Book, so's I don't get this RONG.

    "Nevertheless, the scope height does have an effect on the trajectory measurement from the sight line, and thus it makes a practical difference to the shooter".

    "Trajectory height has most often been tabulated as measured from a line from rifle muzzle to target. As taken from the line of sight, the trajectory height is seen to be less."

    I thought this was interesting when I read it, long ago, and that's why I remembered it.

    Smitty of the North
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    Actually if air resistance where not an issue then the path would be a parabola h(t)=h+v*t+g/2*t^2. However, in taking air resistance into account the curve becomes something else as determined by the drag function as modified by the bullets ballistic coefficient. Since a curve is any none straight line calling the bullet path a curve is correct. A parabola is just a specific type of curve.

    I am in the middle of playing with trying to make my own ballistic calculator in the form of a spread sheet. So far I have had limited success. I have back figured the drag function from the velocity for a set BC provided by Win Ballistics and checked with my reloading manual. I can input any mussel velocity and BC I want and it uses the data to predict the velocity at ranges out to 500 yards. So far it has matched other calculators within 10 fps over the entire range. I am still having some problems predicting bullet drops from line of departure but I think I figured out what is wrong. my hope is to have a calculator that will not predict bullet path based on the range that bullet was zeroed on, but instead on the angle between line of sight and line of departure. The idea being to predict (ignoring bending of the barrel) the location of different bullets of different velocities in a rifle zeroed for a different load. for example to predict the location of a pot load with a BC of .20 and a velocity of 1000 fps given that the rifle is sighted in for a BC of .48 and a velocity of 2500 fps.

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    Quote Originally Posted by bandhmo View Post
    I am in the middle of playing with trying to make my own ballistic calculator in the form of a spread sheet. So far I have had limited success. I have back figured the drag function from the velocity for a set BC provided by Win Ballistics and checked with my reloading manual.

    You guys must be bored.

    Don't forget the altitude, temperature and humidity when figuring the drag. These will have an impact on air density and your arc. Or you could just go hunt

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    Default YEA

    Quote Originally Posted by marshall View Post
    You guys must be bored.

    Don't forget the altitude, temperature and humidity when figuring the drag. These will have an impact on air density and your arc. Or you could just go hunt
    Put a little Kentucky windage and elevation on it, then just go out and kill something.
    THAT's what people did,BS (before scopes), many years ago.
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    Just samantics (another word I don't how to spell) I was just be a smarty a pants.

    I did remenber a post some where that a guy wanted to raise his scope a hafe inch by switching to extra high monts because it would make the gun shoot flatter. Like useing HE or LM ammo. I said picking a more aproperate point of zero was a better thing to do.

    This is MI's first 70+ degree day hope there is more to follow.

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    Bullets really don't rise, they begin to drop as soon as they leave the barrel. It's the line of sight that effects the bullet impact. I believe the first response was fairly correct, your line of sitght is a straight line to the target, and that line of sight is higher than the end of the barrel. If the barrel and line of sight were parallel, even if the bullet did not drop, the bullet would alwasy hit below the point of aim. But as stated, the barrel actually cants slightly upward and crosses the line of sight at two points, the second is usually your long range point of aim, ie:100yds. If you can determine the first point you can actually sight your rifle in at that closer range. The attached pic demonstrates this fact:

    targetshooting3.gif
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    Premium Member MarineHawk's Avatar
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    Quote Originally Posted by OKElkHunter View Post
    Bullets really don't rise, they begin to drop as soon as they leave the barrel. It's the line of sight that effects the bullet impact. I believe the first response was fairly correct, your line of sitght is a straight line to the target, and that line of sight is higher than the end of the barrel. If the barrel and line of sight were parallel, even if the bullet did not drop, the bullet would alwasy hit below the point of aim. But as stated, the barrel actually cants slightly upward and crosses the line of sight at two points, the second is usually your long range point of aim, ie:100yds. If you can determine the first point you can actually sight your rifle in at that closer range. The attached pic demonstrates this fact:

    targetshooting3.gif
    I may be being too nitpicky, but when you point your rifle up into the air, and pull the trigger, the bullet rises, right? In fact, if you point your rifle straight up vertically and pull the trigger, it rises really high, really fast and does not begin to drop until it's a couple of miles or so above your head. And that's essentially what happens (though only by a few minutes of angle) when you sight a rifle in to shoot three inches high at 100yds and the barrel is 1-1/2" below the scope's sight line, right? If you're shooting horizontally level, the bullet rises 4-1/2 inches during the first 100 yds above (relative to the ground) where the bullet exited the muzzle. Maybe there's a different definition of "rises," but by rising, I mean going up, away from the ground. Bullets can do that, right?

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    Quote Originally Posted by MarineHawk View Post
    If you're shooting horizontally level, the bullet rises 4-1/2 inches during the first 100 yds above (relative to the ground) where the bullet exited the muzzle. Maybe there's a different definition of "rises," but by rising, I mean going up, away from the ground. Bullets can do that, right?
    Wal, Shore it can. If the barrel is pointed up from the ground.

    Smitty of the North

    Quote Originally Posted by bandhmo View Post
    Since a curve is any non straight line, calling the bullet path a curve is correct. A parabola is just a specific type of curve.
    Thanks bandhmo:

    I hope that squares me with Murphy.

    Smitty of the North
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    Has it ever occurred to you, that Nothing ever occurs to God? Adrien Rodgers.
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