First a little 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. Most fully automatic weapons manufactured and registered 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 most states do.
To purchase a transferable machine gun, you must meet certain requirements (generally the same as when you purchase another gun), 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:
Get your fingerprints (either by a police dept. or by a qualified fingerprinter, two imprints are needed) and two passport sized pictures taken. These will be used to perform a comprehensive criminal background check on you.
Have your local dealer help you fill out an "Application for Tax Paid Transfer And Registration Of Firearm" for, known as a "form 4".
You must have the signature of the Chief Law Enforcement (CLEO) officer that has jurisdiction over the municipality in which you live on the form 4. This could be the City Chief or the County Sheriff, for example. This is usually not a problem-- in machine gun friendly states.
The form 4, CLEO signature, 2 fingerprint cards, 2 pictures, and a $200 check (your one-time transfer tax) must all be mailed to the BATF and an approved tax stamp returned before you may take possession of the gun. This may take anywhere from 2 to 5 months.