While 5.56mm rifles (like the M-16, M-4, SCAR and so on) have long used 30 round magazines, larger capacity (especially 60 and 100 round) ones are becoming increasingly popular. These larger capacity magazines are not all that larger than the standard 30 round one. The 60 round magazine is 22.1cm (8.7 inches) long and weighs 918 gr (2.02 pounds) loaded. They cost $130 each. The 100 round magazine is 31cm (12.2 inches) long and weighs 1.5 kg (3.3 pounds) loaded. They cost $180 each. By way of comparison, the 30 round magazine is 18.7cm (7.5 inches) long and 509 gr (1.1 pounds) loaded. These larger capacity magazines work by having more than one column of ammo, and simple, sturdy and effective mechanical components to keep the ammo coming when the trigger is pulled.
The new high capacity magazines, while expensive, take advantage of new materials and are well designed. They are durable and reliable. While most NATO troops fire their 5.56mm assault rifles one shot at a time, one in every 5-6 troops is usually designated a machine-gunner, and often carries a 5.56mm machine-gun (a heavy duty assault rifle with a heavier barrel).
The high capacity magazines were initially meant for these 5.56mm light machine-guns. But non-machine-gunner troops found the higher capacity magazines useful in a firefight, where the 4-5 seconds it takes to load a fresh magazine and get back on target again, was often too dangerous. For many soldiers and marines, spending a few hundred bucks on some high capacity magazines seemed like a good investment.
High capacity magazines have been around for decades, and as more reliable designs and materials became available, capacities increased, along with popularity. The M-16, when it first appeared in 1963, was equipped with a 20 round magazine. But by the late 1960s, 30 round magazines showed up, and were very popular. Even higher capacity magazines were slow to appear because of design and materials problems.