*please replace the word "clips" with "mags" or "magazines"
Real Capacity (Real-Cap) Airsoft Clips
Real-cap clips are designed to operate in the exact same way as the real weapon. The will hold exactly the same amount of BBs as the real life cousins will hold. Only the most serious airsoft players looking for military simulation and authenticity need this kind of clip.
Low Capacity (Low-Cap) Airsoft Clips
As the name states, these clips are designed to carry less ammunition, and are intended for the relatively serious airsoft player who wishes to be in life like situation, in which the airsoft gun will not have many rounds. For automatic electric guns (AEGs), these clips will generally not hold more than about 100 rounds.
Medium Capacity (Mid-Cap) Airsoft Clips
Without the need for extra moving parts, as in the case of high capacity clips, the mid-cap clip still enjoys the advantage of quietness, like the low-cap magazine. In the case of the mid-cap airsoft clip, you will generally be looking at somewhere between 100 - 200 rounds.
High Capacity (High-Cap) Airsoft Clips
To get the extra advantage of being able to pummel your opponents with a rain of fire, or to be able to cover a teammate well, the high capacity magazine will be your choice. A rotating toothed wheel may be used to transfer the BBs from a reservoir to the chamber, else a battery operated, pressure sensitive pad may be used to feed a BB into the chamber of the gun.
Although this type of magazine is great for automatic firing and suppression of the enemy, the high capacity magazine may have a tendency to rattle, as loose pellets roll around in the clip. This could be the disadvantage that allows the enemy to locate you, and thus you may be taken by surprise.
Like most everything, the different airsoft clips out there have advantages and disadvantages. So make sure that you know how you want to use the clip, before you go out there and get one of your own.