Machine guns registered before May of 86 are grandfathered. They are still transferrable. MG's made after then can be sold to Class III dealers IF (and only if) they have the proper paperwork from an agency requesting a demonstration of the gun. These demo guns are only transferrable between Class III dealers and when the dealer is no longer a dealer, he must transfer them to another Class III dealer or surrender them to the ATF.
Of course they can shoot them while they own them.