Drop of a Bullet

If air friction is neglected, then the drop of a bullet fired horizontally can be treated as an ordinary horizontal trajectory. The air friction is significant, so this is an idealization.

If the muzzle velocity is
v = m/s = ft/s = mi/hr = km/hr

and the distance downrange is
R = m = ft = yards,

Then the amount of drop of the bullet below the horizontal would be
d = m = cm = inches

If the gun is fired on level ground at a height of m = ft, then the bullet will hit the ground in seconds, having traveled a distance of meters = feet.

To hold the drop to cm = inches at the downrange distance R above would require a muzzle velocity of m/s = ft/s.