Once a firearm, ALWAYS as firearm until it has been deactivated. Deactivation does not involve a barrel- it DOES involve cutting the receiver into 3 pieces with a cutting torch, or totally melting it.
There ARE blank firing full autos- a number of European companies specialize in lookalikes (because in many cases, folks there cannot own the real thing) HOWEVER- a blank firing gun may not be capable of firing ball ammo, and may not be "readily convertable" to doing so.
Yes, movie guns DO come up for sale. ATF rules apply. The Gatling gun used in Predator has been sold a couple of times. (HOW much? ) Even the studios get themslves in trouble now and again. They made up a cut down lever action rifle for Steve McQueen in the series The Bounty Hunter. Problem was, the "Mare's Laig" became a Short Barreled Rifle- and they did not touch second base with the ATF. Whoops!