You would first need to obtain a Manufacturers FFL...regular FFL application plus 30+ pages of EPA stuff, on how you will deal with generation and disposal of hazardous materials generated in the manufacturing process...you cant legally put in a trash can. You MUST be in compliance with all state and local business/zoning laws as well. You will be personally interviewed by an BATF&E agent prior to being issued a license.
Once granted to you, the file for Class 2 NFA manufacturers license,$ 500 per year, then $ 1000 per year if you do over $ 500K in business receipts.
Once you have both of these in place, you may manufacture Post-Ban sample machineguns when ever you want and report daily production to NFA Branch of BATF&E via fax. ALL machineguns made are post, DEAD machineguns...for sale, upon approved transfer, to law enforcement, military only and MUST be disposed of prior to closing your business.
You may also mfg. short barreled rifles and shotguns as well as supressors under this mfg. license.
An awful lot of work to make a "cheap" machinegun just for shooting...and end up in a close relationship with an annual BATF inspection !
Where an investment in a pre-86 machinegun is a one time $ 200 tax and the value just keeps going up ! The $ 500 per year for the mfg. licenses adds up pretty fast...unless you do make a business out of it. A Mfg license includes all dealer priviledges to sale and transfer of firearms would be an option to make some money selling both sporting and NFA firearms.
Check your state and local laws before starting out....you MUST comply with all those laws as well. Since it is totally regulated the BATF will know exactly what you have in inventory and how many transfers, if any, you do per year to justify a renewal of such a license.
Hope that helps !