your physics equation abovehold true when you neglect fluid dynamics,but i think he meant to say if you assume the ground is 100% flat, neglecting the earths curvature and any altitude change. just from a pure initial velocity, the initial Vo(x) (initial muzzle velocity, has no impacton the gravitational constant. anyways, to answer your Q, NO they will not hit the ground at the same time due to fluid dynamics, look at any bullets trajectory, even one fired in parallel to an assumed "flat" ground. the projectile, will ALWAYS shoot in an arch where its maximum height (y) exceeds that of the muzzle beforeit reaches its "zero". with that in mind, the fired projectile has a further distance to travel to the ground, taking longer than a "dropped bullet"