The percentage of legal automatic weapons is minuscule in reality. They’re prohibitively expensive, even for a piece of shit. Last I looked into prices, over a decade ago on a shits and giggles whim, a roached out mp5 carbine with issues was the cheapest on the market at about $15,000. Then there’s the tax stamp, insane background checks, relinquishing your privacy due to residency checks that you can’t opt out of, and many other unconstitutional roadblocks to ownership. Autos are actually the least likely category to be used in any fashion, legal or not.
At any rate, the fathers would definitely be down with citizens having them, as they made sure citizens had access to the same weapons the military had back then.