<P> Despite its early failures the M16 proved to be a revolutionary design and stands as the longest continuously serving rifle in American military history . It has been adopted by many U.S. allies and the 5.56 × 45mm NATO cartridge has become not only the NATO standard, but "the standard assault - rifle cartridge in much of the world ." It also led to the development of small - caliber high - velocity service rifles by every major army in the world, including the USSR and People's Republic of China . Today, many small arms experts consider the M16 the standard by which all other assault rifles are judged . </P> <P> During the 1960s other countries would follow the Americans' lead and begin to develop 5.56 × 45mm assault rifles, most notably Germany with the Heckler & Koch HK33 . The HK33 was essentially a smaller 5.56 mm version of the 7.62 × 51mm Heckler & Koch G3 rifle . As one of the first 5.56 mm assault rifles on the market, it would go on to become one of the most widely distributed assault rifles . The HK33 featured a modular design with a wide range of accessories (telescoping butt - stocks, optics, bi-pods, etc .) that could be easily removed and arranged in a variety configurations . </P> <P> 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 5.56 mm cartridge is also much easier to shoot . In 1961 marksmanship testing, the U.S. Army found that 43% of ArmaLite 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 . </P> <P> 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 meters). </P>

Where did the word assault weapon come from