Fully automatic rifles are only useful in military situations. There is no reasonable use for them in civilian life.
You ca get them legally though, and I know a couple of people that do have them - and legally. There is a lot of documentation and registration to getting them though.
Would you have military grade explosives, such as C-4, available to all as well? Should an individual be allowed to develop their own nuclear weapons, if they have the capability to? There has to be a line in the sand somewhere.