I have added a custom model to a weapon and gave it a refract shader.
For some reason there is no refraction.
It does not refract the world behind it in "realtime", but instead it seems to have the world in the object itself.

And the strange thing is that when I use refractshader for the hands, the hands to refract like it should so the world is refracting in "realtime".

But why doesn't the added model to the weapon.

I think it has got something to do with "preloading" and calculating the shaders.
For the hands.vmt is "default" (although it uses a custom refract shader) it seems to load it correctly but the shader for custom model does not.