Yes, you can purchase a machine gun with a Gun Trust but you cannot purchase a new machine gun. (As long as it is purchase in a state where they are permitted) You may be wondering what the difference between and old machine gun and a new machine gun. As of May 1986 individuals, Trusts, and business entities can only purchase machine guns that were made and registered as transferable prior to May 19th of 1986.
A new M-16 can be purchased for $1200-1500, but can only be purchased by police or military, while a legally transferable pre 1986 Machine gun will cost you 10 times that amount.
So for those of you who are looking at a Gun Trust as a way to purchase an inexpensive machine gun, forget it. Not only is a machine gun expensive to buy, but its more expensive to feed. Many shoot 600-900 rounds per minute and the typical magazine will last a second or two.