Your points are all valid, but at the end of the day...if nut-case ‘larry’ wants to go out in the trailer-park and shoot twelve people, with just a single-shot Derringer, he can probably do the job. I don’t think this crowd grasps that or intend to go and make Derringers into forbidden guns or associate them as ‘assault-Derringers’.
Charles Whitman did quite a job from the Texas University Tower back in 1966, using a Remington 700 bolt action 6MM rifle.
But if you really want to take out a lot of people use the method Julio Gonzalez did when he burned The Happy Land dance club. One match and a gallon of gasoline.
Rest assured if such massacres happened today there would be an outcry to ban the rifle but not the gasoline.
That’s not the point, public safety was and is never the point of disarming a population. Humans are very effective murdering machines. We have the innate ability to to craft ever increasingly effective ways of killing each other. If a person is bent on murder in a mass scale they will succeed. a firearm should be the last choice for a mass casualty event. There is so many more effective means for someone of intelligence. Joe shootem up is a suicidal loon usualy a coward who has a complex and needs validation before checking out. True sociopaths intend to live and keep killing for the sport of it.