A gravity-fed hopper is simply a container that holds paintballs and sits on your gun; it is the simplest, cheapest hopper available. The rate that paintballs can load into your gun is not based solely upon the speed of gravity, but on the rate of the balls inside the hopper bouncing off of each other. When a player is firing rapidly, the balls still in the hopper will form a sort of an arch over the feed hole, and no more paintballs will drop through. This is easily remedied by shaking the hopper, disrupting the arch and freeing the balls, but it is a hassle and slows down the rate of fire. To prevent this slow down in firing, the agitated hopper was developed.
An agitated hopper (often referred to as a 'loader') has a small motor that is attached to a paddle that spins and disrupts any arches that paintballs form over the feed hole, thus keeping a steady feed of paintballs into the gun. While the speed of an agitated hopper is no greater than that of a gravity-fed hopper, it is much more consistent and does not require any manual shaking of the hopper. Agitated hoppers are more expensive than gravity-fed hoppers, starting around $25.
Differences in Agitated Hoppers
Agitated hoppers come in many different forms. Some have simple straight paddles that feed the paintballs while others have propeller shaped paddles that are able to actually move balls more quickly into position to be fed. Most agitated hoppers do not spin constantly (though some lower-end models do) - they only spin when balls are not being fed into the gun. These hoppers are either actuated by a sensor that detects the lack of a paintball in the feed neck or else by an audio sensor that responds to the firing of the gun and rotates the paddles after every shot. Agitated hoppers typically run on one 9-volt battery while some run on two 9-volt batteries and rotate faster and with more power.
Force Fed Hoppers
Force fed hoppers (or loaders) do more than simply keep paintballs falling smoothly, they physically take the balls and force them into the chamber. A force fed hopper typically lets paintballs fall to different sides of a cone which then rotates and forces the balls into the paintball gun. A force fed hopper continues to force paintballs into the gun and keep a certain amount of tension on the paintballs. It is able to deliver paintballs much faster than an agitated hopper but costs more than $100. Force fed hoppers usually run on two 9-volt batteries or six to eight AA batteries.