Here is a quick and dirty FPS conversion:
8mm .48g BB = 370fps = 3.04 joules
Converted to a .20g BB weight, the new readings will be 571.89fps, which far excedes current local (and national BofUSA event) limits.
Regardless of what FPS it's shooting at, I doubt it'll be much of an issue because I don't think we'll see you humping around a 20lb. rifle in the field all day.