It's nicely disguised. The inner workings, grip, trigger, and barrel I'm 90% sure, are the front 2 thirds, of the back half of a Longshot. They chopped and channeled it, and then made a big ol custom slide housing to use the slide bolt action. While it undoubtedly looks great, from a technical standpoint, I hope there's still a way to clear jams, as the slide housing looks like it covers over the breech.
(OOG: Dering Sprague)