It is a common misconception that machine guns cannot be owned by law-abiding citizens. This comes from the creation of a variety of confusing laws that have made purchasing a full-auto gun more difficult than purchasing a "normal" gun. But, if you can comply with the law, you may qualify to own a machine gun.
First a brief history: In May of 1986, certain laws went into effect that made it illegal for 'civilians' to own fully automatic firearms that were manufactured AFTER THAT DATE. Many fully automatic weapons manufactured, registered and tax paid BEFORE MAY, 1986, MAY BE OWNED BY AND SOLD TO INDIVIDUALS. The full-auto guns that may be owned by individuals are called 'transferable'. Some states DO NOT allow machine gun ownership at all, no matter when the gun was made, but many states do.
To purchase a transferable machine gun, you must meet certain requirements (generally the same as when you purchase another gun, but with additional scrutiny), fill out special paperwork (called a 'form 4'), and pay a $200, one-time, transfer tax. Every time a machine gun is transferred, the $200 tax must be paid-- usually by the purchaser. The steps to take to purchase a transferable machine gun are: