Internal

What happens inside the barrel from the moment the primer is ignited to the moment the bullet unplugs from the muzzle.

Intermediate

From the moment the bullet unplugs from the barrel to the moment the bullet is clear of the cloud of gas that forms in front of the muzzle.

External

The time that the bullet takes, from the moment it is clear of the muzzle to the time that the front of the bullet touches the target.

Transitional

The short time the bullet takes from touching the target with it's front, till the bullet is completely submerged into the target, if it is applicable.

Terminal 

The bullet's reaction within the target and the bullet's reaction, to the time that the motion of the bullet has stopped.