How do you legally acquire a fully-automatic gun license?

First, in the US, there is no "Fully automatic gun license". Fully automatic firearms must be transferred through a dealer that holds a "Class III" addition to his 01 Federal Firearms License. The transfer must be registered with, and approved by the Federal Bureau of Alcohol, Tobacco, Firearms and Explosives, and a $200 Transfer tax paid. The transfer must also be approved by the Chief Law Enforcement Officer of the place you live, and must comply with any state laws. You will also need a VERY large checkbook. There are a very limited number of full auto weapons that can be transferred to a private citizen- only those already registered in the system as of May 1986- no new ones. An M16 rifle that has been registered is currently selling for about $16,000. A Thompson sub machine gun may be $25,000 or more. The last M60 machine-gun I saw sold was over $35,000.