"Technically they are handguns, but a 15 inch barreled 45/70 or 308 is not a handgun to me; it's a rifle without the buttstock."
The PC term is "Specialty Pistol" they can't be a rifle without a buttstock, the BATF says so.
I'm on another board with some fellas who shoot things like 416's and 458's in Encores, and that's got the S&W 500 beat, at least on the recoil end. I wonder if you could chamber an Encore in 50bmg...