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General Discussion To Gateway To Airguns => Hunting Gate => : gamo2hammerli September 10, 2008, 01:13:46 PM

: Hold Over or Hold Under?
: gamo2hammerli September 10, 2008, 01:13:46 PM

Hi all, I usually shoot on level ground. I shot a few grackles from the second floor (Downwards) but only at close distances. Yesterday I took a shot at a crow (And missed) aiming upwards at 40 meters....the tree was about 3 stories high....from ground level.

If I remember correctly from my bow shooting days......if shooting from a treestand, you would shoot abit below the target to get on the bullseye. If you shoot at the bullseye, the shot would go over the foam deer in 3D. I believe shooting uphill is the same....aim lower.

Is this the same for a real rifle/gun/airgun? Shooting upwards or downwards....aim lower??? Thanks.

: RE: Hold Over or Hold Under?
: TCups September 10, 2008, 02:16:03 PM

Remember that if you shoot straight down, or straight up, the trajectory is flat. as viewed from your scope.  Call that 0 degrees and 180 degrees.  At 90 degrees, the trajectory is at right angles to the vector force of gravity and therefor, the drop of the pellet due to gravity would at first seem to be at a maximum.  But shooting straight up, the velocity is attenuated by gravity and shooting straight down, the velocity, while it would be accelerated in a vacuum, tends to be decelerated slightly less than it might otherwise be by air resistance.  And remember that gravity is a force that causes acceleration, or a change in downward velocity per unit time.  The longer the time the pellet takes to hit the target, the more time gravity has to accelerate the pellet earthward.  So, shooting downward, both the relatively greater velocity and the reduced vector force of gravity relative to the trajectory of the pellet cause less apparent drop in the pellet.  Aim high if you are shooting downward.  But shooting upward, while the vector force of gravity is reduced relative to the trajectory of the pellet, the pellet is being decelerated by gravity more than on a horizontal trajectory, the pellet takes a longer time to reach its target, and gravity has a longer time to act.  Don't aim high when you are shooting up, unless you are shooting up at more than a 45 degree angle.   Bottom line: forget the physics and take your rifle and your pellets to some place you can shoot upward or downward and see what happens.

: RE: Hold Over or Hold Under?
: tinbender September 10, 2008, 03:31:39 PM

Hey Tcups,

I should have known you had Physics education:) I've been an armchair sort of physics guy for a long time. My son is off to Denver for post grad work. Some of the best fun has been listening to him explain to me why something would not work, but after it does we usually don't have time to define why it did:)

Sometimes it can be a hinderance to think it too far, and your advice to get the elevation +/- shooting to real world results is a good one.

Good shooting Gamo

Dave

: Horizontal distance aimpoint.
: 3n00n September 10, 2008, 03:45:39 PM

Gravity effects the pellet over the hoizontal distance, not the linear measurement.

http://www.arld1.com/holdoverunderdonesteve.html (http://www.arld1.com/holdoverunderdonesteve.html)

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http://www.arld1.com/targetplottrajectory3.html (http://"http://www.arld1.com/targetplottrajectory3.html)

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http://www.arld1.com/impactpointvscantangle.html (http://"http://www.arld1.com/impactpointvscantangle.html)

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From: http://www.arld1.com/ (http://"http://www.arld1.com/)

: RE: Hold Over or Hold Under?
: TCups September 10, 2008, 03:45:47 PM

A skilled machinist can tell you quicker than several pages of calculations whether something that should work can actually be built, most times.

: RE: Horizontal distance aimpoint.
: TCups September 10, 2008, 03:51:08 PM

Of course, J is not only a skilled machinist, but also an engineer, an expert on air rifles, and a crack shot, were as I am a rookie.

: Due to distances involved, bbguns are relatively simple.
: 3n00n September 10, 2008, 04:24:08 PM

Not like long distance artillary that must even contend with coriolis effect calculations to arrive 'on target' afterseveral seconds of flight.

The advice to setupareas to "shoot at angles" is certainly best. Every boreis different, and thus has a different effect on the ballistic coefficientof every type of pellet, unique toits individual rifling.

Additionally, it's usually more fun to go plink, than it is to run a bunch of complex calculations . . . (http://../images/emoticons/wink.gif)

: Re: Hold Over or Hold Under?
: gamo2hammerli September 10, 2008, 05:09:38 PM

Thanks for the replies people.  TCups, looks like you have a Physics degree.  I like your explanation.  And you guys are right....real life shooting beats calculations on paper....every gun and pellet is abit different.  Just wanted a starting point and personal experiences....  Thanks again.

: Re: Hold Over or Hold Under?
: TCups September 11, 2008, 12:25:05 AM

I don't think I explained it very well, though.  Look at it this way.  Think about trying to throw a rock off the roof of the house at the ground to hit a tin can, then think about throwing the same rock from the yard at a tin can up on the roof.  Your throwing arm knows the physics even if your brain doesn't. It takes a lot more oomph to throw upwards to counteract gravity.  And the upward velocity will be slower. But you air rifle can only throw at one speed.

The earthward acceleration due to gravity is 32 feet per second squared.  The downward velocity increases exponentially with time, so the arc of the bullet in flight is parabolic.  The faster muzzle velocity of a lighter pellet means the pellet gets to the target in less time, in the flatter part of the trajectory curve, up to a point, but not necessarily with greater energy.  Conversely, heavier pellets, longer time to target, and a faster drop, but their forward momentum means a slower drop in velocity.  The same inertia that makes the heavier pellet harder for the compressed air to it accelerate down the barrel, resulting in lower muzzle velocity acts to make the pellet harder to decelerate due to air resistance once it leaves the barrel.  

This means that heavier (more massive) pellets carry more forward, down-range energy per unit of time than lighter pellets, independent of the drop due to gravity.  In a vacuum the parabolic arc of "Gravity's Rainbow" would be the same for .177 and .22.  In air, with forward resistance, the .177 is going to decelerate at a faster rate and though in a horizontal trajectory it has a flatter initial trajectory, its velocity is going to drop more rapidly and the effect of gravity over time will be more pronounced.  So, at the end of the trajectory curve, it will be falling at a relatively higher higher velocity than a heavier pellet. The ballistic coefficient (BC) tells you about how a pellet will decelerate in air relative to a known reference projectile, but BC's for air gun pellets are petty meaningless, it turns out.

But once you start shooting upward, not only air resistance, but also gravity work to uniformly decelerate a pellet. Lower velocity = longer time of flight to the target = more downward velocity or drop of the pellet before it reaches the target.  And when you compensate by aiming higher, the actual distance traveled by the pellet to a target, for example 50 yards down range, is no longer 50 yards, because the pellet isn't traveling in a straight line, it's traveling in a longer arc, and will take more time to reach the target, and will drop more due to gravity in that time, and will need to be compensated for by aiming higher . . . So, mathematically, you can't hit there from here (LOL).

Bottom line - forget all the calculations and BC's -- just understand in general what forces are working on the pellet once it leaves the barrel, and then go to the range or to the hills and figure out what your gun does with the pellets you are shooting.  Mr. Barnes begins to look even smarter.

BTW, regarding air temperature, the Cardew brothers (my new techno-geek heroes) took a test rifle and wrapped the compression tube in heating and refrigerating jackets, let the air in the compression tube come to a uniform temperature, before cocking the springer, and then looked at muzzle velocity.  Guess what?  Little or no difference over a wide range of temperatures.  Only when they heated the compression tube to the point that it began to vaporize the oil in the compression tube (at temps where the tube was too hot to touch) was there substantial difference in measured muzzle velocity, and that was due to excessive dieseling or detonation.  So forget ambient air temp affecting initial muzzle velocity -- it doesn't, no matter what the calculations say.  

So show me an air gunner with a calculator or a laptop with a ballistics program trying to figure out where the pellet is going to land, and I'll show you a man in trouble.  Time to go to the range and practice some more.

: RE: Due to distances involved, bbguns are relatively simple.
: KShep September 11, 2008, 01:17:45 AM

3n00n - 9/11/2008  12:24 AM

Additionally, it's usually more fun to go plink, than it is to run a bunch of complex calculations . . . (http://../images/emoticons/wink.gif)

Amen to that.  

Mild dyslexia means math is my mortal enemy.  Thanks for posting the holdover links.

: RE: Hold Over or Hold Under?
: RedFeather September 11, 2008, 09:13:53 AM

The short answer?  Yes.  Whether up or down, the gun will shoot higher than the point of aim established on level ground.  Best to do some practical shooting at different up angles to see by how much your shots are affected.  Doesn't matter if it's up or down, you get the same shift.  And plinking beats hammering on a calculator or computer any day.  Remember, Dan'l Boone didn't have the benefit of one.

: RE: Hold Over or Hold Under?
: shadow September 11, 2008, 09:22:39 AM

It comes to a point to where one doesn't think about it, it's automatic. Know your shooter and know your capabilities oh and practice, practice.:) Ed

: Re: Hold Over or Hold Under?
: gamo2hammerli September 12, 2008, 12:37:48 AM

Thanks again guys...I knew those bow shooting days were good for something.  :o)   TCups...I like that rock throwing comparision.  RedFeather, I like your response also...simple....no if's or but's.  heh heh   Shadow, I really would like to be VERY familiar with each of my airguns....but  I've got a few...not as much as some other members here, and I don't have a chance to shoot them as often as I like (Darn)....and not at extreme angles or distances.  As you said....being one with the gun....would be perfect.