Default mag is the camo mag.
To replace by the black mag :

In the materials/models/weapons/v_models/AUG.BHD.Gordon.Car15 open 556.vtf and replace :

"$baseTexture" "models\weapons\v_models\AUG.BHD.Gordon.Car15\556_camo"

by :

"$baseTexture" "models\weapons\v_models\AUG.BHD.Gordon.Car15\556_black"
