From the high road archives
".30-30 brass is ~0.04" smaller at the head than .303 Savage and you'd likely get split cases that could damage the rifle's chamber or worse. According to Ken Howell's book Custom Cartridges you can form cases from .220 Swift cases by fireforming with an inert filler (corn meal) and a fast pistol powder. Then back off the .303 Savage sizing die so that the cases headspace on the formed cases shoulder instead of the rim. Trim them to length, deburr
Al