Oh, I get it. But a machinegun is registered with specific details; if those are changed, BATFE must be notified even if the nature of the firearm is not changed. If it is, the registration is void.
And yes, there are many companies making machineguns. And many more companies that have an NFA manufacturer's SOT in case they want to make machineguns. But I don't see FNH or Remington or the Class III manufacturer I know posting on here about some ingenious (and, of course, "legal") way to add to, or make, or modify something or other to fire full auto. I repeat, I admire the ingenuity, and I am sure BATFE would also, but they might not let their admiration stand in the way of reaching for the cuffs.