The other ones that can be affordable are the Tippmans... They are a bit of an underground secret among miniatures collectors. Way more fun and interesting than an AK.
Last, if you watch the site of a guy named Wayne Driskill Miniatures, he occasionally gets .22 Furr Gatling guns. They run about $5k. But no papers, no transfer fee. And way 'cheaper' than most full autos. They are transferred just like any other .22. Lots of these show up on the market, built by machinists and then sold to fund a 'next' project. Wayne is probably the best dealer for them. They are little pieces of art!
Cheers,
Sirhr