That's a PTW, I've seen it many times before on other airsoft forums, googling PTW, and also it's image link is http://i64.photobucket.com/albums/h167/turok_t/PTW
Doing a rough search of the image, it's owned by Isaac Marchionna of www.lawndartdesign.com
, the original link is http://www.lawndartdesign.com/guns/M4A1.html
, there is a contact link at the bottom of the page. His description does not state that it is an aftermarket barrel, which leads me to believe it is stock, he is a photographer, it is entirely possible that the colour was enhanced or altered.