I admit I did not read every post in this thread, and I am no expert, but I do remember this from Physics. I am sure many of you already know that gravity is a generally constant force of acceleration. I say generally constant because it is weaker as move away from the center of Earth, but in the few thousand feet range we live in, this change in gravity is insignificant.
Say you take 2 bullets that are at the same elevation with each other - If at the same instant you drop one bullet from your hand, and have the other one fired from a perfectly level rifle; both bullets will hit the ground at the same time. (Presuming the bullet doesn't hit anything in between and that the ground under your feet is flat, out to enough distance).
Gravity is a negative (that means towards earth) acceleration (that means velocity change over time) force of about -9.8 meters per second squared (Think it speeds up by the velocity of 9.8 meters per second, each second.) And in a vacuum a falling body would do that, consistently increase rate. However, air creates friction and that drag can curb the acceleration. As the velocity climbs so does the drag and eventually you hit the "terminal velocity" where the drag prevents further acceleration.
So what matters is how fast your bullet is moving forward, the push down from gravity is constant no matter where you are on the surface of Earth. The faster it goes forward the less it will drop for any given distance. This is only because the bullet has been on it's own in the air under the influence of gravity for less time at the given distance, if you increase its forward velocity.
So you are concerned with altitude which is all about the air pressure - Lower elevation has denser air and therefore higher drag to slow your forward velocity ("air speed" if you are familiar with aeronautical terms). Since your bullet has more drag in the denser air slowing its airspeed, by the time it gets to your target it has been in flight longer, and that additional time has allowed gravity to affect it longer. Since gravity is acceleration, this small additional time can have a very pronounced effect on the drop.
I would think shooting up or down wind would have a greater effect then elevation change, but have not tested that... at least you could calculate well for air pressure change. A barometer doesn't lie but the old windsock can really shift around on you.
Edit: I just read some posts... Unless you are shooting an artillery piece at over 100KM you do not have to worry about the Coriolis effect. - That opposite rotation for southern hemisphere thing.