I'd Google "Ruger only 45c loads in the Rossi rifle", and see what is out there. Most loading manuals have Ruger specific loads. The XTP Mag bullet is designed for higher velocities then you would normal get from the 45lc in a pistol, though Casull like velocities should be possible in a rifle length barrel using H110 / 296 (same powder). I get 1800 / 1900 fps with the .429 44 mag 240 grain XTP, and cant see why you shouldn't get the same using that weight in a 45lc, provided the Rossi will handle Ruger loads, which I believe it will from what I have read, though check to be sure. Of course, you do not have to load that hot, as the standard XTP will work very well on deer at 1200 velocity range. By the way, 240 grain XTP's have been my go to bullet for years when hunting whitetail with an expanding bullet in my 44 mag handguns. It works well.
Larry