Mid-Capacity magazines offer stealth operation by eliminating BB rattle often experienced with Hi-Caps. They feed smoother than Hi-Caps due to their high tension internal spring, similar to a real steel magazine. Since 470 round Hi-Capacity magazines don't exist in real life, a good amount of Mil-Sim players prefer Mid-Cap magazines for an even more realistic type of gaming experience.
Due to the design of all Mid-Cap magazines, they actually function better once you break them in. To do so spray some light silicone oil into the magazine, load and unload it a few times until BB's feed smoothly. This process breaks in the internal spring allowing it to function without problems each and every time.