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I believe part of the reason is that effective leftist (non-anarchist) gun activism mostly revolves around self-defence organisations (think BPP or AIM) rather than combat or terrorism. And history has shown that even the slightest signal of successful organisation from marginalised groups in the US having guns will trigger the most violent of reactions from the State and bourgeois media.

Another part is how hard it is to have access to the ruling class or their lackeys. You'll probably never run into a board member of Amazon or Lockheed Martin on the streets (and they're usually not very well known). My hypothetical train nerd would have a lot of trouble trying to locate and enact the Trolley Problem on US Secretary of Transportation (and Mayo) Pete Buttigieg, even after the East Palestine derailment earlier this year or strike block last year.

But it's always worth remembering that the USA has enacted the most all-encompassing surveillance state in history, even if they sometimes suffer from the typical imperialist incompetence form hubris. Most adventurist plots are probably silently caught before the plans are put into motion.