Just think that up until the NFA was enacted anyone like Maxim or Browning could design and sell full auto.
and since "normal" citizens have been disallowed to invent and manufacture, how many really good, new designs have come about?
Actually quite a few. Unfortunately, most are from overseas by "state" sponsored corporations. The home inventor used to be the backbone of US national security :>)
Well, I've got a patent for a point-detonating shotgun slug from 1991. The NFA didn't stop me. Although I did have some brief issues with the local constabulary until I rightly pointed out to them that there were no laws on the books to cover my invention (which is sold in very modest numbers only to the US military).
In its extreme application my little invention makes a shotgun into an effective light-anti-tank weapon. It won't take out a heavily armored MBT, but a BMP or similar vehicle is susceptible to the HEAT version.