It is a carbine version of the AKM-74/2 assault rifle intended to be used by special forces, paratroopers, vehicle crewmen and other branches of militaries as a compact rifle. In the games it is widely used by the Ukrainian military and rookie stalkers of numerous factions. This firearm is also the signature weapon of Strelok.
In Shadow of Chernobyl, the AKM-74/2U is overall underpowered. It's has the lowest accuracy and damage rating for any assault rifle in the game, but it is light due to it's compact nature, and boasts very good handling, coupled with an above average rate of fire. These two advantages make this weapon the most easy-to-handle rifle in the game, as even when firing at full auto, the weapon remains largely easy to control at close to mid range. It is also rugged like its big brother and starts to inconveniently jam only when 1/3 damaged. Regardless of its shortcomings, it is a very useful as a player's first fully-automatic weapon. It's caliber also allows it to punch through light and medium armored enemies.
Also the earliest assault rifle to acquire, and possibly the easiest to get. Held by Major Kuznetsov in the Cordon, along with all of the military stalkers in the area, along with a unique pistol. Its an invaluable weapon at this early point in the game.
It is slightly more useful and powerful than in Shadow of Chernobyl and it's easy to maintain and upgrade offset by it's somewhat lacking reliability and overall stopping power. The AKM-74/2U does not have many upgrade options as it can only be upgraded for either better firing rate/less recoil or better damage/accuracy, though not to the extent of regular assault rifles. All in all though it is a good starter weapon with its light weight, reasonably strong stopping power and deadly armor piercing ammunition capable of tearing through most of the enemies the player will encounter in the early parts of the game.
In CoP, it has massive recoil and overheating problem is represented by atrocious durability. Fully upgraded it can be a fairly effective CQB weapon, but just as common AK-74 is better in nearly every way, and both are outclassed by the unique "Tunder" 5.45.
In Shadow of Chernobyl, the handguard near the muzzle of the weapon was synthetic grey while in Call of Pripyat and Clear Sky, it was made of wood.
Its stamped steel magazine is unusually large for a 5.45x39mm rifle, infact it's size is more akin to that of the AK 47, which uses larger, 7.62x39mm rounds. This would not make sense as AK-47 series rifles uses larger rounds while the AK-74 series rifles uses smaller rounds in comparison, so the magazine of an AK47 would not even fit into the feed of an AK74 nor would the former's bullet fit into latter's chamber. In reality the AKS-74U uses the exact same magazines as the AK-74, which have much less of a curve as well as a composite construction.
This is the only carbine in the series that is present along with its assault rifle counterpart.
The ubiquity of the AKS-74U's usage by numerous rookie Loners, Bandits, and Military NPCs through the games is highly implausible, since the AKS-74U was (as already mentioned) nowhere near standard-issue. Its widespread usage is even less plausible when those NPCs try to use such a short-ranged weapon against distant human targets in the Zone's largely open environs, a role which the AK-74 would be much better suited for.
However, it is also possible that the military's standard weapon in the zone are AKS-74Us as the carbine is much cheaper than the full-sized AKS-74. This is possible due to the fact that the Ukrainian government seems to have a very small budget for Zone's military boarder guards, which is referenced by Major Khaletskiy, who himself is barely paid anything for his service. And so, because of the military's illicit trading with stalkers, the AKS-74U became widespread due to military trade.
That said, there are a few areas where NPCs using the AKS-74U would not be out of place, such as the soldiers patrolling the Agroprom Underground or the Spetsnaz squad sent to clear out Lab X18. Lt. Sokolov is also another logical user of the AKS-74U, as he is a helicopter pilot (rather than an infantryman) and needed a weapon that could fit into the confines of his vehicle(s) while still possessing the armor-piercing abilities of the 5.45x39mm round.
The player's hand goes through the weapon's handguard. This can be seen if one looks at the heat vents.