POI shifts when shooting springers at elevated targets?

[bodiej]

Read in the NRA firearms handbook that when shooting at an elevated target, gravity will only act on the horizontal distance. Your POI will always be higher.  Drop a plumb line from your target and that will be the range.  Use that range for the elevation adjustment of your scope.  Or, just remember that and hold your aim a bit low.  Another poster above was correct however in reguards to wind effects (the actual flight time is greater).

[RedFeather]

I don't think that's exactly so.  Say you are ten feet from a tree and shooting forty feet up.  According to the manual, gravity is only working over the ten foot distance.  However, the pellet is transiting forty feet, or approximately taking four times the flight time VS a horizontal shot.  I would guess that the pellet is subjected to gravity for a longer period, producing a bit greater drop.

Anyway, the easiest way (and funnest, unless you are of the slide rule set) to determine the new POI is to practice on leaves, branches, acorns, etc.  Plinking, on other words.

[bodiej]

that's what I would have imagiined and i still can't quite wrap my nogin around it, but quoted:

"Shooting uphill or downhill can be a problem because the eye and brain percieve the range to be the longer slant distance to the target while the bullet drop more closely approximates that of the shorter horizontal distance.  Whether the angle is uphill or downhill makes no difference.  The strike of the bullet will be above point of aim.  For shots within 10 degrees of horizontal, the correction is so small that it can be ignored.  For example, a hunter using a 7mm 150-gr. bullet at a muzzle velocity of 3110 f.p.s. takes aim on a bighorn sheep on a 35 degree hillside at a slant distance of 400yds.  However, if a perpendicular line could be extended downhill from the ram it would intersect the level on which the hunter is standing at a horizontal distance of 328 yds.   With his rifle zeroed  at 200 yds, the bullet will strike 19.9" below point of aim at 400 yds. on level ground.  But in firing at the steep angle, the bullet drop equates to the 328 yds of horizontal distance.  At that shorter yardage, the bullet's strike below point of aim is 9.5".  If the hunter adjusts his his point of aim to allow for a 400 yd trajectory, the bullet will arrive 10.4 inches above his hold and quite possibly miss his quarry altogether."  (NRA Firearms Sourcebook 2006)"

The writer doesn't go into the associated physics of WHY this is though, and it's giving me a headache trying to solve it.  Bullet drop is supposed to be a constant acceleration as soon as the bullet leaves the muzzle   The distance is longer, therefore the flight time is longer, therefore gravity has more time to act on said bullet, right? (or does the uphill flight have a lower acceleration of gravity till it levels out? - that wouldn't make the "no difference for uphill / downhill" statement work).  I'm stumped

[neric]

BNations,  thanks for bringing up this question.  I've loved the discussion it brought and learning I gleaned from it all.   I reckon there'll be quite a few crows bitin' the dust around my house.   I've always missed, except for one lucky wing shot, now I know why.

Thanks a bunch everyone.

[bodiej]

found this in the library..
http://www.airgunexpo.com/calc/calc_hold.cfm?

[speedturtle]

Couldn't agree more! Practice is all it takes to be efficient in these kind of situations. I just did that and later on confirm my findings with the ChairgunPro and it works!

Example:

My target are those sycamore seeds ( 1" - 1.2" in diameter) situated at around 15 yards inclined distance (based on the AO of my scope) but is actually about 8 yards if I measure its horizontal distance.

My Leapers scope is set at its max magnification of 9X. At this magnification my mildot guide for an 8 yards target is about 2 mildots down, and my mildot guide at 15 yards is exactly at the crosshairs. What I did is to simply use the 15 yards P.O.A. and hit those seeds. Did I get it? Yup!

Now, looking at the inclined curve on my ChairgunPro simulation below, the new Ballistic curve positioned the 30 degree inclination P.O.A. only about 0.1" higher than the 15 yards P.O.A. that's why I can hit those targets just the same. The inclined compensation will have more pronounced effect at ranges farther than 25 yards and at inclination angles sharper than 60 degrees.



Hope this helps.

[bodiej]

but WHY!!!!???  I'm not gonna be able to sleep!!

[speedturtle]

My guess is that the more inclined is your barrel from the horizontal plane whether it's downhill or uphill the less is the immediate pull of gravity on your pellet in flight. Just look at the new ballistic curve based on the inclination angle (the broken blue/violet curve) and you'll notice that the steeper the angle the straighter the curve becomes. Just my observation bodiej, i'm not an engineer so I can't give you an exact explanation.

[bodiej]

ahh..  gonna have to ponder that, but it sounds like that would make sense, especially considering the curve - thanks

[speedturtle]

This link has a better explanation to what I am trying to describe: http://www.chuckhawks.com/shooting_uphill.htm

and this link can explain it further technically: http://www.loaD********o.com/Topics/April04.htm

NOTE: The moderated letters are actually "Alpha Mike Mike Oscar". Sorry, I had to disect it so you guys can view the link. My apologies to the moderators.

 

[bodiej]

"Remember that it is gravity working on the bullet during its flight time that causes it to drop. If you were to shoot straight down, say from a tethered balloon, the bullet would have no curved trajectory, it would travel toward the earth in a straight line, just as if you simply dropped it. Likewise, if you shoot straight up, the bullet travels up in a straight line until its momentum is expended. Again, there is no curved trajectory.

You can infer from this that the farther from the level position a rifle is held when a bullet is fired, the less the bullet's drop will be over any given line of sight distance, whether it is fired up or down. Since your sights are set to compensate for bullet drop, and there is less bullet drop when shooting at an up or down angle, you must hold lower than normal to maintain the desired point of impact. For example, if you are shooting up or down at a 40 degree angle and the line of sight range is 400 yards to the target, the horizontal range is only 335 yards. 335 yards is the distance for which you must hold."

Now that makes sense!
Thank you Mr Garcia, I think you just solved the issue