In this country, it was made a criminal offence to convert a semi auto into a fully auto (part of the 1977 legislation that grandfathered fully automatics). Yet for years afterwards, firearms prohibitionists carried on about how easy and widespread converting from semi to fully auto was, but in reality it is a very difficult and expensive thing to do.
Familiar with Gatling guns? Know anything about motors? Ever program a $35 raspberry pi computer?
If a pressing need for civilian automatic weapons ever appeared, American ingenuity would rapidly fill the need.