Well, It have been a while since skins thread happened and nothing news about it so I decided to look more into it myself and here is the result.
How to install :
1. go to directory that you install game then follow with this A.V.A/avaGame/Content/Weapons
2. back up file named "Wp_Rif_M4.upk" (copy for reason file doesn't work) to somewhere else outside game folder.
3. copy this file to above directory.
P.S. Thank you, the skin maker, who don't want to be name for give me a little clue. I still have much to learn but thanks anyway.
PW is in this rep.
PW = my name in reverse and in case sensitive <======HIGHLIGHT HERE