I heard and read that "the Founders couldn't have even imagined the horrors of today's guns..." Well, not only were people aware of something faster than single-fire muskets, it turns out that George Washington tried to buy some for the army. The Belton Flintlock.
https://billypenn.com/2018/02/16/a-...nted-one-of-the-first-semi-automatic-weapons/
https://billypenn.com/2018/02/16/a-...nted-one-of-the-first-semi-automatic-weapons/