Note that just because Marx said something doesn't mean you should treat it as a gospel.

Marx lived from the early 19th to the late 19th century and didn't witness WW1. Since then, the military has become almost infinitely more powerful compared to Marx's days, where armed workers stood an actual chance against the military. Nowadays though? Not even remotely. Shut off the internet and electicity in a region, ban the press from reporting, send in the military and wipe out any protest, regardless of how many arms there are. Just take a look at Chechnya and how well they've been doing after declaring independence.

The US is an exploitative capitalist country despite having an awful amount of guns.

Northern European countries are significantly less exploitative, while still capitalist, countries without nearly as many guns.

The amount of arms in worker's hands and the severity of capitalism has no relation anymore. Hell, by having so many guns, the US can reasonably argue for ever greater police budgets without much backlash. You wouldn't want children to die in school shootings, would you, you monster?