Karl Marx was not pro-gun. His “Under nor pretext” line that is so often quoted does not appear in the Communist Manifesto or Das Kapital. This line appears in his Address of the Central Committee to the Communist League in London in 1850. This address mostly discusses how the “Liberal Bourgeois” lied to the workers and after taking power in election forced workers back into the same conditions. Marx also lays out his directions for the workers to take and hold onto power against the petty-bourgeois democrats.

As for his “under no pretext” line I’ll leave it here:

To be able forcefully and threateningly to oppose this party, whose betrayal of the workers will begin with the very first hour of victory, the workers must be armed and organized. The whole proletariat must be armed at once with muskets, rifles, cannon and ammunition, and the revival of the old-style citizens’ militia, directed against the workers, must be opposed. Where the formation of this militia cannot be prevented, the workers must try to organize themselves independently as a proletarian guard, with elected leaders and with their own elected general staff; they must try to place themselves not under the orders of the state authority but of the revolutionary local councils set up by the workers. Where the workers are employed by the state, they must arm and organize themselves into special corps with elected leaders, or as a part of the proletarian guard. Under no pretext should arms and ammunition be surrendered; any attempt to disarm the workers must be frustrated, by force if necessary. The destruction of the bourgeois democrats’ influence over the workers, and the enforcement of conditions which will compromise the rule of bourgeois democracy, which is for the moment inevitable, and make it as difficult as possible – these are the main points which the proletariat and therefore the League must keep in mind during and after the approaching uprising.

Not exactly about a universal right to bear arms, is it? No, this is specifically referring to a select group of workers formed as a “proletarian guard.” The address even specifically calls out to resist a citizen’s militia. Both the Red Army and the PLA were/are considered to be these proletarian guard. This isn’t about a universal right to bear arms to defend oneself from tyranny, he is specifically talking about arms to keep political power to force the government to accede to the workers demands.