Marx lived during a period of popular democratic armed uprisings against reactionary autocracies, so his opposition to disarming the insurgent masses was quite understandable then.
However, the historical context has radically changed since 1848, 1871, and 1917, and the current impetus for gun ownership in the US now mainly comes from reactionary whites who are opposed to all of the values and institutions which we support. There is no serious opposition to gun control coming from the trade unions and representative organizations of blacks, hispanics, gays, women, etc. whose members are, metaphorically speaking, in the gunsights of those agitating most vociferously against any form of gun control. It is not the latter who are demanding arms.
I was once, and would again be, for the unrestricted right to gun ownership as a defensive measure were there to reappear a mass socialist movement vying for power and experiencing violent repression by the state and right-wing paramilitaries. I'm not a pacifist. I think left-wing calls to arm the German and Spanish masses against fascism in the 30's and, later, against the gathering right-wing coup in Chile, were entirely correct.
But there is no absolute principal here; like most issues, gun control is a tactical question which can't be viewed ahistorically. What was once a progressive demand has been appropriated by the most backward sectors of white American society threatened since the 60's by the growing social and political weight and assertiveness of non-white minorities, women, and gays, in particular. The right has been able to mobilize support for the issue beyond its ranks because large numbers of white Americans continue to share its anxieties about these groups to greater or lesser degree and/or because they have been persuaded that the individual right to bear arms is a fundamental democratic right dating back to the (mythologized) American Revolution. In the current context, the movement against gun control benefits only the right and has absolutely no resonance on the left of the political spectrum. Not even the most feverish revolutionary Marxist sect would today agitate around Marx's injunction to the Communist League in 1850 that: “… the workers must be armed and organized. The whole proletariat must be armed at once...Under no pretext should arms and ammunition be surrendered; any attempt to disarm the workers must be frustrated, by force if necessary.”
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CB I agree with Marv. A further specific American historical point is that US Communists were substantially defeated as a mass political party in the late 40's and early 50's in part by the feds criminally framing it as advocating the overthrow of the US government by illegal force and violence, armed struggle, guns in hand. This claim was false, as I say in the sense that the CPUSA was advocating only use of legal , electoral means for the working class to attain state power. The government used passages from _The State and Revolution_ by Lenin to claim that Marxists didn't believe state power could be attained unarmed or by just voting. Engels is quoted there as saying suffrage will always only be a measure of the maturity of the working class or something like that; and other parts of the book.
Thus, all US leftists must be very careful not to advocate armed insurrection (as the rightwingers are now privileged to do today). In fact, Communists have to be "extra" careful and explicit that they are _only_ advocating voting. Of course , the need for this care is even greater with the Patriot Act, new anti-terrorist laws.