The first gun that may be considered an assault rifle by modern definitions was the Fedorov Avtomat, designed in Imperial Russia and seeing limited service in World War I. The biggest breakthroughs in assault rifle design came in World War II, with the German-developed MP-44, dubbed the Sturmgewehr by Hitler. Literally translating as "storm rifle", this was the origin of the modern term "assault rifle". It was significant in that it introduced the intermediate cartridge, which the Fedorov Avtomat lacked. It also had a greatly increased magazine capacity. These features combined to make the Sturmgewehr (StG-44) the first true modern assault rifle. The Soviet Union had already begun experimenting with rapid fire urban warfare weapons, with guns such as the PPSh. The PPSh, however, used a pistol round and was therefore a submachine gun. When the Soviets defeated Germany in 1945, many StG-44s were taken and their features were built into the next major step of assault rifle development, the AK-47.
The Avtomat Kalashnikova 47-go goda, known as the AK-47, incorporated elements of the PPSh and the StG-44, as well as several innovations. Mikhail Kalashnikov led the development of the gun, and it was eventually introduced as the Soviet military's standard rifle. The AK-47 was tremendously popular and successful, being simple to manufacture, requiring little maintenance and operating successfully under extreme stress. For a time, the AK-47 was unmatched in terms of firepower and versatility. By the 1950s, though, the West had begun developing their own assault rifles, such as the American M14 rifle.
The classic Western assault rifle was the AR-15, introduced into the US military as the M16 rifle. Designed by Eugene Stoner at Armalite in the early 1960s, it used revolutionary materials such as polymers and aluminum to significantly lighten the gun. It was also chambered for the 5.56x45mm round, which was significantly smaller than the AK-47's 7.62x39mm. This allowed for a greater capacity without compromising on stopping power. However, the gun proved difficult to maintain and required frequent cleaning, unlike the AK-47. This was demonstrated during the Vietnam War, when the two guns faced off for the first time.
Following the Vietnam War, both Cold War superpowers looked over their main assault rifles. The 1970s saw a trend in favor of smaller cartridges. In the Soviet Union, the AK-47 and its various versions were replaced with the AK-74, which used a 5.45x39mm cartridge, instead of the earlier 7.62x39mm. The United States kept its 5.56x45mm round, which had since become the NATO standard, but evolved the M16 into the M4 carbine. The M4, developed in the 1970s and '80s, was lighter and shorter than the M16, and had various modifications to allow for attachments.
While the United States and Soviet Union made their developments, the arms industry of Western Europe was beginning to mature. German manufacturing had recovered since the 1960s and Heckler und Koch came out as a leader in German weapons development. The G3 was widely exported in the 1960s and is still used by German special forces as a marksmanship rifle. In the 1990s, however, it was fully replaced by the G36, a polymer-based system that used STANAG 5.56x45mm NATO magazines. In France, the FAMAS was developed, also in the 1970s, and was unusual in that it employed a bullpup system where the magazine was behind the trigger.
Today's assault rifles are largely the products of development near the end of the Cold War. The United States is testing alternatives for the M4, but no concrete plans have been reached. In Russia, the AK-74M is still the standard rifle, but is slowly being replaced by the innovative AN-94. The AN-94 is difficult to produce, however, and the new AK-12 is favored by the government as a successor, entering preliminary stages of deployment. China and India have developed their own assault rifle systems, but they are not as technologically capable as the newest Western and Russian weapons.