quality:correction, structural:summarization
5.56mm NATO

The adoption of the M16, the H&K33, and the 5.56×45mm cartridge inspired an international trend towards relatively small-sized, lightweight, high-velocity military service cartridges that allow a soldier to carry more ammunition for the same weight compared to the larger and heavier 7.62×51mm NATO cartridge. The 5.56mm cartridge is also much easier to shoot. In 1961 marksmanship testing, the U.S. Army found that 43% of AR-15 shooters achieved Expert, while only 22% of M-14 rifle shooters did so. Also, a lower recoil impulse, allows for more controllable automatic weapons fire.

In March 1970, the U.S. recommended that all NATO forces adopt the 5.56×45mm cartridge. This shift represented a change in the philosophy of the military's long-held position about caliber size. By the middle of the 1970s, other armies were looking at assault rifle-type weapons. A NATO standardization effort soon started and tests of various rounds were carried out starting in 1977. The U.S. offered the 5.56×45mm M193 round, but there were concerns about its penetration in the face of the wider introduction of body armor. In the end the Belgian 5.56×45mm SS109 round was chosen (STANAG 4172) in October 1980. The SS109 round was based on the U.S. cartridge but included a new stronger, heavier, 62-grain bullet design, with better long-range performance and improved penetration (specifically, to consistently penetrate the side of a steel helmet at 600 metres or 2,000 feet).

Also during the 1970s, Finland, Israel, and South Africa introduced AK type assault rifles in 5.56×45mm. Sweden began the transition with trails in 1981 and full adaptation in 1986. During the 1990s, Russia developed the AK-101 in 5.56×45mm NATO for the world export market. In addition, Bulgaria, Czechoslovakia, Hungary, Poland and Yugoslavia (i.e., Serbia) have also rechambered their locally produced assault rifles to 5.56mm NATO.