The AK/50, in real life, is a Kalashnikov-based rifle, designed to fire the powerful .50 BMG cartridge. It is a fairly recent weapon, being first seen in 2016. It uses unmodified Barret M82 magazines and has the same long-stroke piston operating system seen in the Kalashnikov design, as well as tactical rails like more modern-day rifles.
The AK/50 seen in-game is completely different. It appears to be a custom-made Kalashnikov rifle, modified to fire the .50 Beowulf cartridge, like the AR-15\50.
The AK/50 was a Battle Rifle variant of the AK-47, possessing much more damage and recoil than its predecessor. Considering how the AK-47 already possessed very high damage, the AK/50's damage was incredibly high, being able to 2SK enemies at its maximum damage, and 3SK enemies at its minimum. However, as with all Battle Rifles, the AK/50 suffered tremendously from high recoil, but even more from its low magazine capacity, only being able to hold 10(+1) bullets, which decreases its effectiveness in prolonged fights.
The AK/50 was available to use in the CTE only and was unlocked by default, as was any other weapon there. Since the CTE was shut down, the AK/50 has not made a reappearance.
Pros & Cons
Extremely high damage.
Extremely small magazine size.
Restricted to fully-automatic only.
Aside from the usage of .50 cartridges, the AK/50 works exactly the same as any other Kalashnikov weapon, even though it is custom made.