Just to be nitpicky: it is legal to own, you just have to file some paperwork and pay a $200 tax. Where do I get one?
Depends. Some DD's are listed under the NFA of '34 and as such had to be registered pre-84. Unless you are a government agency, newer Class III toys and some DD's are strictly off limits. The Master Key and the AA-12 are both NFA DD's.