Hi!
To Install this skin you need to do this:

1. Unrar ''materials'' folder from your choice (Red Camo,Savana,Simple Black ...)
to Steam/steamapps/common/Counter-Strike Global Offensive/csgo.

2.Next you need to go to csgo and open  pak01_dir.vkp (first make backup folder of that file)
with Notepad ++.

3.And you need to find 

$baseTexture "models\weapons\v_models\rif_aug\rif_aug 
and change it in 
$baseTexture "models\weapons\v_models\rif_aug\rif_aug1

and again you need to find 

$baseTexture "models\weapons\w_models\w_rif_aug\rif_aug 
and change it in 
$baseTexture "models\weapons\w_models\w_rif_aug\rif_aug

4. Go and play the CS:GO! ;)