If I recall, Lowell is not a handloader.
The factory loads for a 255 bullet run up to about the mid 800's fps in a pistol. At the lower end of the 255 bullets is the Winchester "cowboy" loads and they run down in the 700's fps, again from a pistol barrel.
Shooting these loads from a rifle will likely increase the velocity, but I'd only be guessing as to how much. However, I wouldn't count on more than about 10-15% unless I actually chronographed them. Still, a 255 gr bullet going in excess of 900 fps is nothing to sneeze at for a short range deer use. I wouldn't take a "Texas heart shot" with it, but a nice broadside shot at 50 yds wouldn't bother me a bit.
