[MikeG]

I think a reasonable guess will be that the .480 should pick up velocity similar to what a .44 mag does when it goes from a handgun to a rifle.  A lot of loading manuals have data for both rifles and handguns so it's pretty easy to estimate the difference.  ~300fps?  That would be my guess, maybe a little more, depending on bullet weight.

Energy, trajectory, etc., should be pretty easy to calculate from there.