Hammer Tutorial V2 Series #14 “Env_Cubemaps, Static Reflections, Reflective Textures”

Env Map Information:

https://developer.valvesoftware.com/wiki/$envmap

Env_Cubemap Information:
https://developer.valvesoftware.com/wiki/Cubemap

Cubemap Weapon Information:
https://developer.valvesoftware.com/wiki/Weapon_cubemap