How do you buy a fully automatic weapon?

In the US- you must be 21, reside in a state where State law permits private ownership of a machinegun, and be legally able to purchase a firearm. Next, find a Class III dealer in your home state- transfer of the gun to you will be through that dealer. The dealer should help guide you through the process of applying to the BATFE for a transfer of the gun to you. Send that paperwork, copy of your fingerprints, approval from your local Chief Law Enforcement Officer, and payment of the $200 tax to the BATFE. When approved, the dealer will notify you, and you can take possession of the gun- AFTER you pay for it. That is likely to be the hard part. A LEGAL, transferable M16 rifle is between $12,000 and $16,000.