Nitestalker- bullets begin to drop the instant they leave the barrel- IF the bore is level.
However, as that article explains- almost every rifle will be tipped up to some degree. The bullets travel in an arc (a parabola) that passes upward thru the line of sight, and drops BACK to the line of sight at the target.
Except for my phased plasma rifle in the 40 watt range. It shoots flat.