Class 3 liscence with the SOT tax aditional and then you can purchase post 1986 weapons as a dealer. You will hafto actulally sell some firearms to the police or some other agency that can legaly own them, otherwise sitting on invetory and not selling anything is considered tax fraud (due to the $200 transfer tax waived for dealer to dealer transfers)
Pre 86 weapons are a matter of getting local law enforcement to sign off on a batf-4 (I think thats the form) and then it can be transfered to a private citizen.
If you start your own incorpirated buisiness, then you can list class III weapons as an investment, or as a business expense if you incorparate as a security firm, but the weapons will never belong to you personally, and if the business gets sued, they are part of the assets.
You can also become a class II SOT and maufacture your own, but its an ass ton of paperwork.
All tings aside, yes I would love to own a few 18s, and if it where posible to keep a full auto .45 from going straight to the sky, Id rock it.