Why does NATO keep reducing the size of rifle cartridges?

Answer 1

NATO isn't the only one who did it - the Bloc did, as well. The decision for NATO to adopt the 5.56 cartridge was based on the US announcement that all American units in Germany would be equipped with M16 rifles, rather than the NATO standard 7.62 rifles. An evaluation of various cartridge designs ensued, including the British 4.85mm cartridge, and the Belgian SS109 cartridge (which was 62 grain, as opposed to the American 55 grain M193). The SS109 was chosen.

The concept of intermediate cartridges was the result of German studies of WWI battles which showed that most firefights took place at ranges of less than 200 metres. From this, the 7.92x33 'Kurz' cartridge was developed, and was most notably for the famed StG43 rifle. The Soviet Union adopted a similar principle with their 7.62x39 M1943 cartridge, used first in the SKS, then the AK47. The idea being that it would be a compromise between a submachinegun - which had full auto capacity, but terrible range - and contemporary battle rifles, which had poor rates of fire, the capability to engage targets at longer ranges than necessary, and would be near impossible to control in full auto fire (as the NATO FAL and G3 rifles later proved).

The US was interested in a similar concept, using a smaller, but much higher velocity cartridge, and, from this, the 5.56x45 cartridge was borne. After evaluating US trends, the Soviets decided to follow suit, and began fielding the 5.45x39.5 AK74, with most (but not all of) the Bloc following suit shortly afterwards.

Today, most countries use intermediate cartridge rifles as standard - 5.56, 5.45, or 7.62x39, with China being the only country to introduce a new and proprietary cartridge for general issue - the 5.8x42mm, which replaced the 7.62x39 cartridge as the mainstay of Chinese forces.

These smaller cartridges are lighter in weight, allowing for a larger basic load to be carried by soldiers - typical basic load for a 7.62 NATO rifle was only 100 rounds vs. 150 rounds for most Bloc militaries carrying 7.62x39 rifles, whereas the lighter weight of the 5.56 allows for a larger basic load - in the US, it's 210 rounds. This is particularly important, as most ammunition is expended for the sake of suppression fire.