I think it’s worth bearing in mind that when Marx wrote that, guns were still using powder and percussion caps.

More importantly than that, there were no machine guns, no tanks, no airplanes, no helicopters, no guided missiles, no rockets, no cluster bombs, no satellites, no drones, etc.

The quote is from Marx’s address to the communist league in 1850, so approximately the time of the US civil war. This was a time where the most powerful weapon of war was the cannon. Most cannons at that time were smoothbore breech loaded weapons. They were slow to load and inaccurate. In WWII up to 75% of all casualties were the result of artillery, but in the civil war it was only 12% of all casualties.

Aside from cannons, everything else was weapons carried by individual soldiers. Grenades sort-of existed at the time, but were very unreliable, and very difficult to use. So, it all came down to individual soldiers and their muskets.

Also, consider that in the 1850s a professional standing army was rare. At the peak of the US civil war there were 700,000 soldiers on the Union side, but it started with only 18,000 soldiers. That means that in wars during Marx’s time, most soldiers were conscripts or newly recruited and barely trained.

All that to say that in Marx’s time, it might have been possible for civilians armed with personal weapons to take on a government and win an armed conflict. The “proletariat” army would more or less be on an even footing with the army of the state / bourgeoisie. They’d have more or less the same weapons and the same level of training. The only thing they wouldn’t have would be the slow, inaccurate and unreliable cannons that were more scary than effective. But, presumably they could more than make up for that in sheer manpower.

Finally, even though it probably didn’t matter to Marx, consider what having a gun at home would mean in the 1850s. If an intruder comes and breaks into your house, are you going to defend your property with your musket? Probably not. It takes minutes to load and once fired, minutes more to reload. Are you going to use your musket in a “road rage” incident while riding your cart to market? Probably not. Were there mass shootings by musket? Of course not. Were there homicides and suicides? I don’t know, but I assume it happened occasionally, but it was a very different weapon back then.

Marx was concerned with the great forces of history, so he probably wasn’t the type of person who was going to consider the negative consequences of firearms lying around the house. But, even if he had considered it, back in the 1850s having a musket at home probably wasn’t a major danger to the household or to society at large.

So, let’s say what Marx said was gospel. Even in that case, it was gospel for the 1850s. What would a modern-day Marx say about things today? Maybe a modern-day Marx would say that modern standing armies are so overwhelmingly powerful that it’s pointless to pretend that they can be beaten by civilians with small arms and no training. Maybe he’d take lessons from Gandhi and MLK and suggest non-violent resistance. Or, maybe he’d be a prepper and suggest that civilians stash grenades, machine guns, rocket launchers, etc. But, IMO, his advice probably wouldn’t be that civilians just have muskets (or their modern day equivalents) because he’d have to know that in the modern world a bunch of untrained dudes with AR-15s isn’t going to win against the US military.