It's an ordinary Title 1 firearm (and NFA arms are Title 2), but as stated they're expensive. One I saw in .45-70 was ~$15k without the carriage, $22,500 with.
If you have a small machine shop and are competent with it, you can make your own "from scratch" for .22LR. A number of vendors sell plans; at least one also offers them in every degree of completion from a box of metal pieces to ready to fire.
If you hook up an electric motor to replace the hand crank, one might argue you now have a machine gun... The Vulcan and Mini-Gun, for examples, are basically motorized Gatlin guns.