You are wrong. All MGs were banned in 1986, with the only exception made for those that had already been legally registered prior to that date. Made after 86, no way a civillian can own one.
Now, manufacturers can still make them, but can only sell to the government or law enforcement, or a licensed dealer. Dealers can get a single sample copy for demo purposes only, and need a letter from an LE agency requesting that demo. Dealers and manufacturers must have the appropriate FFL and pay an additiional "Special Occupational Tax" every year to handle NFA firearms like MGs. Currently the tax is $500 or $1000 per year based on sales volume. Let the FFL or tax lapse, they have to dispose of any post 86 MGs in stock. Manufacturers can of course make as many MGs as they want to, but be aware ATF makes sure you have an actual manufacturing facility that meets all local regs regarding zoning, OSHA rules, hazardous waste disposal, etc. IOW, a big expensive process that is not worth it unless you really are going into business to sell NFA firearms.