yes, you can buy them
a) deactivated or as a parts kit without barrel or bolt: only show ID
b) semi-auto: a whole bunch of paperwork (minimum 3 months of writing, copies, investigation, permission from the governor, ...)
c) full-auto: collectors license, present a list of the weapons you gather to the governor and that's it.
(but before you get a collectors license, there's also a lot of paperwork, copies, ...)