Daren is correct about pumps not being affected, by federal laws. State/local may have variances (witness the freedom-loving peoples of the PRK and elsewhere). Semi-autos are affected by mainly the furniture; pistol-grips, bayonet lugs and folding stocks for the most part. I'm not overly sure about the import parts count but I think it's the same criteria as the "assault weapon"'s.
Many shotguns can be legislated out of legality by reclassifying them as a destructive device (USAS 12, Street Sweeper, et.al).