Do you want to own NFA Class 3 firearms or become a FFL Dealer with a class 3 license to sell ?
To become a class 3 dealer, first apply for and be granted an FFL .
Then you pay $ 500 a year , every July 1st., for the class 3 SOT license.
NOT worth doing in most cases. Transferrable machineguns must have been registered prior to May 19, 1986. New MG's are restricted to sale to law enforcement, military or export only. Dealer sales samples of these require a LE letter requesting a demo to get one. You cannot keep POST-86 samples after you go out of business.
As an Individual wanting to own Class 3 toys : Find a class 3 dealer in your state, purchase the item you want, submit the NFA transfer paperwork, fingerprints, photos and the one time $ 200 transfer tax...and wait up to 3 months for approval. If you have a corporation or LLC to register the firearms to, no LE signature is required and the transfer time drops to 4 to 6 weeks. You become a SOT : special occupational tax payer, it is NOT a class 3 license.
Hope that helps !