In some states, yes – regardless, they must have been manufactured before 1986. I think the cheapest that comes these days is around $6k for a shitty Mac-10. An M16, for example, starts around $20k.

A post-86 can only be a dealer sample or a Class III build.

Basically you can 1) buy an old machine gun for a 200$ tax stamp under much more stringent regulations, 2) attempt to get a "Love Letter" (a letter given to you by a Chief Law Enforcement Officer (CLEO) to request guns for "testing"), or 3) build one for a CLEO as a Class 3 Manufacture with a provable intent to sell.