Files of Metro 2033 suggest that an

Files of Metro 2033 suggest that an unsuppressed scoped and an un-scoped silenced Kalash 2012 variants were originally to appear in the game, but were cut at some point in development.
Despite not having any openings for spent casing extraction on both side of the weapon (or a charging handle, for that matter), the Kalash 2012 works just fine, with casings "ejecting" from the right part of the weapon, supposedly appearing from thin air. The Kalash-2012 may use the similar case ejection mechanism as FN P90, which the fired case is dropped from the bottom, but if it does design this way, the spent case should not be seen in the first person view.
The weapon appears to be based heavily on the OTS-14 Groza, but with a magazine similar to that of the FN P90, and firing the 5.45x39mm ammo used by weapons like the AN-94, AK-74M and RPK-74, rather than the 7.62x39mm or 9x39mm rounds that the OTS-14 actually uses.
Though a fictional weapon, from first-person it looks much like the FN P90, which may confuse players when they first see it in the tutorial. This confusion is usually terminated when they see the inventory model for it, when it appears later in-game.
This fictional weapon is an interesting concept for an assault rifle but is a fairly implausible gun from an engineering standpoint. The weapon has at least five substantial design issues, as follows:
The weapon seems to have no clear ejection port for case extraction, unless it uses caseless ammunition, which it clearly does not. Closer inspection of the weapon while firing shows the casings eject from an area to the right of the barrel, suggesting a forward-ejecting system like the FN F2000, but no tube to divert the spent casings exists on the weapon. Casings ejecting from the front of the firearm is clearly a developer oversight, or an error.
As of Metro: Last Light, it now has a new model with an ambidextrous ejection port.
The weapon lacks a charging handle necessary to operate the gun, chamber rounds, and clear gun jams.
The weapon likely has ways of chambering a new round after a fresh magazine. It must have an automatic bolt-release once said mag is inserted into the firearm. But there is still one missing factor, the charging handle.
The weapon has no room for the bolt and buffer spring (located in the stock/rear-receiver) needed to function.
It could be using a vertically moving bolt like the TKB-022PM, another Russian bullpup assault rifle. Interestingly, there were concerns about the weapon's displaced center of gravity as a result of the magazine location, this is a problem the Kalash 2012 would solve with it's top-mounted magazine.
The weapon could not support an AK style gas operating system as it appears to in the current design.
The weapon's handle and trigger are too far back for comfortable use in tandem with the stock. The user would have to crook their arm to shoulder the weapon, inducing fatigue.
In Metro Redux, it isn't possible to chamber an extra round, indicating the gun must be an open bolt design. It could also be a mixed-mode weapon, which means it works in open bolt in full-auto and closed bolt in semi-auto.
Seeing as the weapon is referred to as "Kalash", it is possible that it was engineered by Mikhail Kalashikov, creator of the famous AK-47.
The AK-12 exists in real life and it comes as a substantially different weapon from Kalash 2012 seen in Metro.
During the Enlightened ending, the scoped and suppressed variant is seen on Artyom's back, as opposed to the VSV seen in the Ranger one.
In Metro 2033, Artyom never used Kalash 2012 against human enemies, but in Metro: Last Light, he can use it against them.
In Metro 2033, if Artyom stands idle long enough with it equipped, he will take his right hand off the gun and crack his knuckles.
The Kalash 2012 is the favored weapon of the Rangers in Metro 2033 and it is only seen in use by them. Only Miller and Artyom, if the player chooses to do so, are seen using a customized Kalash 2012 though.
In Last Light, the Kalash is only seen in the hands of Red Line elite forces, a Hanza soldier in Quarantine and Reich officers on Anna (Level). The Kalash 2012 is available in the Ranger arsenal, but few of them use it.
In real life, bullpup style AKs (and other AK-style weapons) are used by Russian Special Forces. Given how the Kalash 2012 is seen being used only by Rangers, and many Rangers are implied to be ex-military, the Kalash 2012 was likely designed for the special forces.
In Metro: Last Light, if Artyom is left idle, he will play with the Kalash 2012 as if he were using a guitar.
It is unknown if the Kalash 2012 exists in the books, as it is never described. However, it is possible that it may make an appearance in the upcoming novel, Metro 2035.