Why do alternate bullets drop when fired as shown in movies?

Answer by Ankit Malasi:

I guess you are referring to something similar to this scene:

Or, as seen in a movie (Matrix, in this case):

Since I'm a repository of useless, but interesting, information, I think I can help you with your dilemma.
Those are not bullets, they are bullet casing.
In case you don't really know what that is, let me explain the structure of a bullet.
Basically, a bullet consists of 4 important parts:

  • the Bullet Casing with Gunpowder
  • the Rim
  • the Primer, and
  • the actual Bullet

You may have noticed that in movies about colonial times, the soldiers carry a pouch of gunpowder with them. They then filled the barrel of their guns with the gunpowder – usually through the muzzle, unlike the pic below – before dropping in the bullet, then firing.

The energy created by the ignition of this gunpowder is what gives the bullet its speed.
Modern bullets eliminate the need to carry gunpowder separately, by putting the required amount of gunpowder in the Bullet Casing.
When the trigger is pressed, the hammer slams into the Primer, which ignites the gunpowder, pushing the actual Bullet out through the muzzle, and leaving behind the now empty bullet casing.

This casing is no longer needed, and just gets in the way of the next bullet. So, it's removed from the barrel of the gun, either automatically (as you see in the initial 3 pics), or manually (i.e. when you cock a gun), by a small contraption called the Extractor. The  Extractor uses the Rim to grab the bullet casing for removal.

Every type of bullet has a different design, but they basically follow a similar design principle – as you can see in the bullet cross sections below:

Why do alternate bullets drop when fired as shown in movies?