This is over a year old but in the current climate, it may potentially be a bigger problem now. Consider the wording of 15 U.S. Code § 1691. Would one possible solution be to add a provision preventing a lending institution from singling out lawful business activities for a prohibition. I also wonder why Bank of America (as one of many examples) isn't vulnerable to accusations of restraint of trade because of their refusal to lend to manufacturers of legal firearms.
What would prevent NRA, GOA, etc. from setting up a credit card for members (or non-members) to use for firearm purchasers, as well as general use? It looks like they would gain a profitable business opportunity if established card companies balk at gun sales.