after having seen the video, I have no idea how you spotted that, nice job. Looks like a new shotgun type weapon maybe? doesnt look like a replacement for the main gun to me im confuzzled as to its purpose.
My guess, is a cannon. It eplodes and sets anyone in the range on fire, that's my guess. I hope Valve dosen't screw up the unlock thing in console like they did the Medic. Lol. Nice find btw.