Good question.
I just got one in .50ae. It handles the 300 grain 1,450 ft lb. 1475 fps. cartridge with ease and grace.
It can be converted to both .44 and .357 magnum as well.
I wonder if the unique rifle style bolt system might need the rimmed style cartridge?
I don't know if my question/response to your query is in any way logical. Just a guess.
I have wanted one forever, for some reason that I can't logically explain.
But, it is a real hoot at the range, and just a beautiful piece of gunnery.