I was reading in a reloading manual this guy at work had about bullet trajectories.

This guy stated that if you sighted in a rifle or what ever say at 100 yards on level ground. Then if you were hunting and had uphill or downhill shots the point of impact would always be high compared to a level shot. He showed graphs illustrating it but to me did not exactly explain why.

Ok, I'm all ears.

I may be talking B.S. here, and someone correct me if I'm wrong, but I believe that because of the trajectory, either up or down, the bullet is not effected as quickly as on a level shot.  Now don't get me wrong. Gravity is still the same, but since the bullet usually rises from the muzzle to a target say 100yds away on a level shot,  if you shoot at something that is lower or higher than you, you are increasing the trajectory even more, lessoning the effects of gravity, causing the bullet to go high.  That probably doesn't make any sense to anyone but me, sorry.  I have always been taught when shooting at an animal uphill or down hill hold slightly lower than you would normally do.
guns762  

What Troy said!!!!!!!!!!!!!!  edited because Troy is much more easily understood.
guns

If you look back to highschool geometry you have to think of it as a right triangle. The bullet path is along the hypotenuse (longest side), but gravity only effects it for the distance that it travels horizontaly. The greater the angle the higher the impact point. Just basic physics, right.

Yep, uphill or downhill shots will go higher than flat fire shots.

In flat fire, the force of gravity is almost exactly perpendicular to the travel of the bullet, pulling it downward with a "full value" force away from the bore axis (the quotes are a hint for folks used to thinking about wind drift in terms of wind direction or "value").

In uphill or downhill fire gravity acts along the horizontal projection of the bullet's trajectory; for a 45 degree uphill or downhill shot the "value" is about 0.707 [=sin(45 deg)=sqrt(2)/2].

The bullet is pulled less from the bore axis, so for a rifle sighted in for flat fire the bullet impacts higher than expected.

Hope this helps.

Yeah, it's all about the fact that gravity only pulls staight downward. If you shoot a bulletwith the barrel parrallel to the ground and drop a bullet at the same time they will hit the ground at the same time. Now if you shoot staight up and drop a bullet at the same time then they will not land at the same time.

For the detailed explination look at my post on the shooting of a monkey.

Imagine if you can that you could freeze the bullet in mid flight.  Sort of like a "Matrix" movie special effect.  So you walk out to where the bullet is frozen in mid-air.  You tie a weight on a string to the bullet.  This plumb bob represents the force of gravity, and is hanging from the bullet at a 90 degree angle.  Gravity operates pretty much perpendicular to the path, and center line of the bullet, assuming a level shot.  If you could adjust the force of gravity from shot to shot (assuming same point of aim and other factors being unchanged), for more gravity the bullet would hit the target lower, and for less gravity the bullet would hit the target higher.

Why?  Because gravity accelerates unsupported masses towards the center of the earth (center of gravity center not geometric center).

Now, back to our bullet hanging in mid-air.  We reach out and tilt the bullet up, at let's say a 30 degree angle.  What has happened to our plumb bob?  It is no longer perpendicular to the center line and new path of the bullet.  It is pulling less in the direction that would "lower" the point of impact and some towards the rear of the bullet.  So the effect of a changed gravity force "vector" in this case would be to move the point of impact up (decreased effect of gravity in that direction) and since the bullet is now headed uphill gravity wants to decelerate it.

Back to the bullet.  This time we tilt it down 30 degrees.  What happens?  The gravity vector is changed (decreased effect in elevation of bullet strike) exactly the same amount as in the previous example.  Except this time it wants to accelerate the bullet.

So, makes no difference if you are shooting some number of degrees up hill or an equal number of degrees down hill the bullet will hit high by an equal amount because the effect of gravity on the travel of the bullet is greatest when working perpendicular to its (level) path.  Aiming up or down can only decrease the force vector acting perpendicular to the bullet's centerline, and move the point of impact up.

The effect of gravity to speed up or slow down the bullet is negligible and can be disregarded.

Hope this helps.

Regards,

Vic.

Surprisingly, most unexperienced folks will still shoot low at an animal that is positioned up or downhill from them.  Despite the fact that point of impact is higher, the tendency is to range the target as closer that it actually is.  
Consider this:  when looking at an object that is 500 yards away on level ground as opposed to 500 yards horizontally and then perhaps another 300-400 yards vertically.  This works out to a range much greater than 500 yards, and hence the low bullet impact.  I worked with a guide in Montana one summer, and saw it happen even after we told the guy to aim higher!