Here is an article about a new ISO code for tracking purchases from firearms stores. It seems there is more to be figured out with how it would be implemented, but it is a merchant code, for a store, not necessarily the items. So any larger purchases would likely be scrutinized. It will also make it easier for card companies or banks to say they will not process from firearms type retailers.
Visa, Mastercard and American Express will now label sales at gun stores separately. Gun control advocates argue that the separate category will help curb gun violence.