You do need the Class III license to sell, manufacture or import any Class III device. ( Machine Guns, Short Barreled Rifles, Suppressors, etc )
Civilians are not allowed to own machine guns manufactured after 1986, but are allowed to own any manufactured prior. They are, however, prohibitively expensive for anything decent and, like all other Class III devices, have special rules about who has access to them, where you can take them, prior approval from the BATFE if you travel across State lines, etc.
If you own a Class III device ( or several ) then they have to be kept in a safe or locked unit that only YOU have access to. A smaller locked safe within a larger one would work and allow others in the home to access the main safe, but not the smaller one containing the special devices. Unless the others in the home are also listed as users, they are not allowed access to the device at all. A BATFE gotcha if you're not aware of it.
Considering how much ammo costs these days, unless you have a Government budget, machine guns are prohibitively expensive to operate. I'd rather have the semi-auto variants myself. Far easier to control. If you need a dozen shots to hit your target, you need to work on your marksmanship I think.
The $500 Tax is for Dealers, Importers or Manufacturers.
Individual ownership Tax Stamps are $200 for most Class III devices. Gets attached and sent to you via a Form-4 from the BATFE. Devices that are registered to individual users need to have a copy of the Form-4 on hand at all times if the device is in use or in transport. I don't know the rules when it comes to Corporations, Agencies and the like.
I've never been harassed by any LE demanding to see the form ( don't show it to just anyone who demands to see it as it contains your name, address ( where the device lives ) and other personal info on it ) but I have been approached by them out of curiosity about the device itself.
Other categories of things civilians can own with the right paperwork and stamps include AOW ( All other weapons, such as short barreled shotguns ) and Destructive Devices ( Explosives, mini-guns, and the like ). Lots of fun paperwork to deal with. Subject to both State and Federal laws. ( Means even though the devices are legal at the Federal level, your State may not allow them )