It’s gravity, of course, that makes things fall to the ground. But if there’s no air pressure, all things, no matter how heavy they are, fall to the ground at the same speed.
Usually, air pressure pushes up more strongly against lighter objects or objects with a certain shape, and that’s why a feather or a piece of bread falls to the ground more slowly than a metal ball.
Since the 16th century, scientists have known that if it weren’t for air pressure, a feather and a metal ball would hit the ground at the same time if they were dropped from a tower.
So, two bullets dropped from a tower at the same time would hit the ground at the same time. This would be true even if one of the bullets was fired from the tower and the other was dropped.
That means that if you fired a bullet straight from a standing position, and dropped a bullet from the same height as the gun, the tw6 bullets would hit the ground at exactly the same time, less than a second later!
In response to the above statement:
A bullet dropped will come down at its termnial velocity in air. This will be enormously below its muzzle velocity, because the air resistance only has to overcome gravity to bring the bullet to a steady, low speed rather than the explosion of firing.
For a small object such as a bullet, terminal velocity will be under 100 mph. Muzzle velocity will likely exceed 600 mph. Hence the energy carried by the falling bullet will be less than 3% of that when fired, and it will be harmless.
Therefore a bullet fired downwards from the top of a building will indeed be much more dangerous and travelling at a much greater speed than one that’s dropped.
However, should the bullet be fired horizontally from the top of the building, it will in fact land at the same time as a bullet dropped. this is due to the gravitational force on the bullet horizontally.
So basically in answer to your question, a bullet fired horizontally will stay in the air for as long as it takes for you to drop a bullet from the same height it was shot at (in most cases, shoulder height)
Mythbusters actually did an experiment on whether or not a bullet can be fatal when coming down from being shot directly up, and within that experiment they discovered that a bullet takes 39 seconds to return to the ground.
Reference: http://kwc.org/mythbusters/2006/04/episode_50_bullets_fired_up_vo.html