As already stated there are two styles of airsoft magazines low and high capacity. Within the Low capacity style are Real capacity and Standard magazines.
Standard mags are the ones that would normally come with the rifle and hold approx 70 rounds (for you anal types 68 ball capacity)
Today we are going to talk about Real Capacity or “Real-cap”. Real-cap if you haven’t already figured out are designed to mimic the real magazine in both size and ball capacity. A standard m4 magazine can hold 30 rounds. Now let me address the old wives tale that you would only put 28 rounds in a real steel magazine to prevent jams. This may have been true in the early days, however modern real steel mags work fine with 30 rounds. Airsoft magazines don’t have this problem.
One note though. You may only get 28 balls out of your 30 round magazine with the first mag. The reason is when the magazine in inserted in the rifle a gap or approx 2 balls is inside the rifle. From the top on the mag to the breech. If the magazine push follower gets stuck near the top of the magazine it will not push the balls up fully and not all the balls will fire. This is why you always treat a rifle as loaded in the safety area. If the rifle is put sideways or upside down the the last remaining balls that haven’t fired off will fall into the breach, and if the trigger is pulled will fire off a ball.
Real-cap magazines come in all varieties M16, AK47, MP5 etc. In addition are numerous variants. M4 mags come both in metal and plastic, to mimic Bakelite.
In addition is the M16A1 twenty rounders. The manufacturer STAR makes a large assortment of mags of this style.
Of course the question is why would you want low capacity mags at the playing field. The most obvious reason is for airsoft reenactments looking for the authentic magazine. In addition the military uses airsoft for training and don’t want soldiers running around with hi caps. The other reason already stated in the experienced airsoft player post. If you are an experienced player going against new or just bad players switch from a hi cap to a low cap magazine, the restriction you are under will help level the playing field and make it more fun for all. You will still come out on the winning side. The game will be more fun and challenging for you and your skills will increase dramatically. This is my favorite practice tool.