The CC company association has created a special classification for firearms and firearm sellers. So, no it's not banned yet, but one has to wonder why they would do that unless there was intent to limit or eliminate transactions.
It's a step toward the same thing you're talking about with cannabis sales. A legal product that is being shadow-banned by the inability to conduct business with a common form of exchange.
In the case of cannabis though, I wonder if it doesn't have to do with federal law. AFAIK, the feds haven't legalized it yet, and maybe the card issuers are concerned about being involved in what is still a federally illegal activity.