I've seen stuff like those rail-rifles before. Rich people in Florida love 'em. Windage and elevation adjustment on the bench, no sights, use a spotter setup to zero. Weird things.
Eyeballing the dimensions here, this is a weird loading. That's something like 20x90mm, best guess is small arms -- you'd just use the Vulcan if you wanted something not man-portable. Something like a payload rifle (but I'm pretty sure that program's dead) or an anti-materiel role is my bet, or maybe (if it's straight-walled) something like a cousin to that weirdo Russian 12.7mm round for individual weapons (they have a revolver and a combat rifle in it, the damn freaks). Either way, small enough the recoil isn't unmanageable for a human being. Not a fire-from-the-shoulder thing, unless it's straightwalled case, but yeah.
Pretty sure you can't share and I'm not trying to guess, more just... this is my nerd spot right here. I love this shit.