A six-round 500 would need to have a cylinder with an additional .6-.65 inch of circumference on the circle that runs through the centers of the charge holes. That pushes the diameter of the cylinder out nearly another two-tenths of an inch, which means the frame window for the cylinder has to get a quarter inch taller too. Suddely you are not talking about an X frame, but an XX frame.
You could get six .44 magnum chambers into an X-frame cylinder, but why bother? If you are going for max power per round, five rounds will be your limit. If you want at least six loaded chambers at one time, you are going to have to buy two X-frames and carry them both. The upside is that you will then have 10 rounds.