Which files does the skin replace?

WEP_1P_AK12_TEX.upk in KFGame\BrewedPC\Packages\Weapons\AK12, and WEP_1P_Optics_TEX.upk in KFGame\BrewedPC\Packages\Weapons\Optics for the sight.

How do I go back to the stock skin?

Only if you saved the original files first. Copy the stock .upk files to a safe folder before replacing them, otherwise verify game files in Steam to restore them.

Mods & Workshop

KF2 AK-12 Vulcan Skin: How the 2015 Texture Swap Worked

In June 2015 a modder called V3n0m published the first version of an AK-12 "Vulcan" weapon skin for Killing Floor 2, inspired by the Vulcan finish on the AK-47 in Counter-Strike: Global Offensive. At that time the game had no official custom-skin support, so the skin was installed by overwriting the game's own texture packages. This page records how that method worked. A download hosted on a file-sharing site was linked in the original guide and is not reproduced here.

Method

The skin came as two package files, both replacing originals inside the game folder (default Steam path C:\Program Files (x86)\Steam\steamapps\common\killingfloor2):

WEP_1P_AK12_TEX.upk goes to KFGame\BrewedPC\Packages\Weapons\AK12.
WEP_1P_Optics_TEX.upk goes to KFGame\BrewedPC\Packages\Weapons\Optics, which changes the sight's look as well.

Warning from the author

Before replacing anything, copy the standard files somewhere else. Once overwritten you no longer have the stock skin. If you forgot, use Steam's "Verify integrity of game files" to restore them.

Notes

This was Version 1; the author said the skin would change in future versions, and a showcase video was posted on YouTube.
Replacing core packages only affects your own client view and can be overwritten by game updates. Newer approaches such as custom weapon paint jobs with the SDK and weapon skin ini tweaks are safer.
The same author also made a 9mm Urban Storm skin.

FAQ

Which files does the skin replace?: WEP_1P_AK12_TEX.upk in KFGame\BrewedPC\Packages\Weapons\AK12, and WEP_1P_Optics_TEX.upk in KFGame\BrewedPC\Packages\Weapons\Optics for the sight.
How do I go back to the stock skin?: Only if you saved the original files first. Copy the stock .upk files to a safe folder before replacing them, otherwise verify game files in Steam to restore them.