Fully automatic weapons are indeed legal to own by civilians. They are called Class III weapons, you must pass a background check and pay a 200.00 transfer fee. There are other restrictions.
Your logic is flawed, older weapons are indeed more high powered, yet automatic weapons could be bought through a mail order catalog up until Al Capone and company started their bloodshed.
The crimes committed with automatic weapons is near zero, same with magazine fed rifles the media likes to call assault weapons.
Despite Hollywood movies criminals do not use magazine fed rifles in crimes on any routine basis.