As far as I am aware the only weapons that are effected by gravity are the bow-and-arrow (plus special arrows) and the projectiles (Molotovs and Grenades) - at least in Xbox version.
In fact, it has been discussed on FC3 forums that the bullets actually disappear after 250 meters anyway, long before bullet drop would occur.
I have been curious about gravity in FC3 myself, but testing with the sniper rifle from quite a distance I can still get headshots by aiming directly at the head, and miss if I aim higher (although it could be that I'm a bad shot too).
Also, I have never noticed any difference in accuracy in different weather conditions, or between day and night (not that I would expect it their either).
It's pretty much just a case of pointing at who you want to kill and pressing the trigger, and you hit them.