The cubemap issue requires external map data to reflect properly.
https://developer.valvesoftware.com/wiki/Cubemaps
"Many reflective materials must be combined with external data so that the Source engine can correctly generate their appearance. In other words, a cubemap creates the textures that a reflective surface will be reflecting."
So without a cubemap in the .bsp file, it will just reflect the shotgun. Not a problem, and making cubemaps are a bit time consuming.