Revolution doesn’t purely mean guns or the over throw of power necessarily. The industrial revolution for example. Marx himself realized that this was ongoing. Communists don’t really aim for revolution but are aware that revolution in the decisive sense (violent seizure of power) will be necessary. The very nature and existence of the working class, as the antithesis of the bourgeoisie means that it is the working class that will cause these revolutions in society. As the working class struggle, the bourgeoisie are forced to change things that increasingly phase them out and make them useless. Every massive change in capital was caused because of class struggle.