The air pressure against the bullet nose slows it down. Gravity pulls it down
In a perfect planet smooth no hills if you drop a bullet the exact same time you fire a bullet perpendicular to the force of gravity they will both hit the ground at the same time.
Technically in a scenario where one drops a bullet and coincindentally fires a rifle that is aimed horizontally, the fired bullet will impact the ground a very tiny amount of time later than the time it takes for the dropped bullet to hit the ground. This is due to the curvature of the planet.