"The man has a weapon."
very good explanation of this on another thread which I've now completely forgotten :-) The upshot seems to be that for aesthetic reasons they use "las armas", but arma itself is masculine so it's el arma and un arma, but las armas.
Almost, but you have that backwards. Arma is feminine. The article is changed to el/un in the singular case for aesthetic reasons, but it's still a feminine noun and is treated as such in all other cases. Hence unas armas, las armas, El arma negra, Las armas negras.