Installation (by Spark for example):
Please make backup of entire GTA4\Grand Theft Auto IV\pc\models\cdimages\weapons.img
Start Spark, open GTA4\Grand Theft Auto IV\pc\models\cdimages\weapons.img
Select w_ak47.wtd and click Edit.
Replace all textures by clicking on Import Texture, choose those from directory "skin".
Click Save and close in up right conner.
Press Save, close Spark.
Enjoy!