Few firearms have shaped the planet to the same extent as the AK series. With its basic action originally designed and implemented in the experimental AK-2 prototype in 1947, the design later went on to be adopted as the AK (Automatic Kalashnikov; or more correctly, "Kalashnikov's Automatic") by the Soviet military in 1949 in limited roles, and was later adopted as the main infantry weapon of the Soviet military in 1954. Production of the new AK (1949) proved difficult, however, and its design was changed quickly within a year. This new model did away with the very crude and basic stamped receiver of the original AK and replaced it with a milled receiver (and other minor changes were made). This second model was still somewhat flawed, and in 1954, it was again updated while retaining the AK designation. Due to the three different variants all retaining the same "AK" designation in the Soviet military, collectors and historians have started referring to the rifles by their receiver types for clarification purposes: Type-I (1949), Type-II (1949-1954), and Type-III (1954-1959). The term "AK-47" therefore does not refer to any specific rifle whatsoever, despite how often the term is misused. However, the Soviets did use the term "AK-47" as a catch-all reference to the first generation of AKs later after the AKM was adopted. The first generation AKs being, of course, the AK-2 prototype and the three different iterations of the "AK." As such, the term "AK-47" should only be used when referencing the first generation of AKs in a non-specific manner. For collecting or research purposes, the AKs should be identified by receiver type to reduce confusion when referencing a particular model.
In 1959, the Soviets decided that the AK series needed yet another update from the milled Type-III. Since stamping technology for steel had more or less been perfected for the AK action at this point, it was decided that this new receiver would be used for the updating process of the AK. This new rifle featured the new, stamped receiver, a ribbed/reinforced dust cover, more in-line stock comb to reduce recoil, and a slanted muzzle brake to reduce muzzle climb, along with many other new innovations. This newly adopted rifle was designated as the AKM (Avtomat Kalashnikova Modernizirovanniy), which translated to "Automatic Kalashnikov Modernized." The AKM was adopted by dozens of other countries, and it and its countless copies are the most widespread of all AK variants across the globe.
In 1974, the Soviets once again decided that the platform needed an upgrade. After having observed the results of the United States switching to a lighter intermediate cartridge (5.56x45mm NATO) during the Vietnam War, the Soviets decided that they, too, needed a lightweight, higher-velocity intermediate cartridge to replace the 7.62x39mm cartridge used in the AK, AKM, and SKS (among other weapons). They had settled upon the 5.45x39mm cartridge as their new round, and a highly modified AKM variant chambered in this caliber won the rifle trials. This design was then focused on and perfected. In 1974, that was accomplished with the AK-74. The AK-74 was essentially a modernized AKM that included such changes as a threaded front sight block (opposed to the barrel like on the AK/AKM) that housed the new muzzle brake that featured an enlarged expansion chamber, a thinned bolt stem to improve bolt unlock reliability, eventually a new gas port angle to reduce bullet shear in the bore, and slightly improved and modified furniture, among many, many other features. The rifle proved to be very reliable, much more accurate, and with the lowest felt recoil of possibly any infantry firearm ever fielded by any military to date.
In 1991, exactly 100 years after having adopted the M91 of Mosin's designing, the Russian Federation adopted the AK-74M, a modernized variant of the AK-74. This new model differed primarily from the previous AK-74s in that production techniques and materials were superior, but a few other things were changed, as well. The fixed buttstock of the AK-74 had been replaced with a side-folding, full-profile polymer buttstock. This stock let the soldier fold the stock for storage, but when extended for firing of the weapon, it offered a cheek weld and length-of-pull comparable to the standard fixed stock while being just as stable. The rifle also took the receiver-mounted optics rail of specialized variants from the AKM and AK-74 and made it a standard feature, enabling all soldiers to attach an optic to their weapon if needed or desired.
Shortly after the introduction of the AK-74M, the Russian Federation began production of the AK-100-series of rifles. To put it simply, the AK-100 series rifles were simply AK-74Ms that had been rechambered to different calibers and also introduced some 12.5"-barreled carbine variants, as well. The AK-100-series includes 5.56x45mm variants (Rifle: AK-101; Carbine: AK-102), 7.62x39mm variants (Rifle: AK-103; Carbine: AK-104), and the carbine variant of the AK-74M itself has been included in the AK-100-series as the AK-105. It is worth noting that the AK-105 is an "in-between" of the AK-74 rifle and the older AKS-74U carbine from the late 1970s, but has been on the path to fully replacing the AKS-74U in service use.