There is precedent for credit card issuers to ban the purchase of completely legal products. Just this month, JPMorgan Chase, Citigroup and Bank of America banned the use of their cards to buy Bitcoin and other cryptocurrencies.

To be clear: Those three banks won’t let you use your credit card to buy Bitcoin, but they will happily let you use it to buy an AR-15-style semiautomatic rifle — the same kind of gun used in mass shootings in Parkland; Newtown, Conn.; San Bernardino, Calif.; Las Vegas; and Sutherland Springs, Tex.

Visa, oddly enough, is the card of choice of the N.R.A.: There is actually an N.R.A.-branded Visa card issued by First Bankcard, a division of First National Bank of Omaha. And Mastercard proudly announced last year that it was the branded card for Cabela’s, an outdoor gear megastore with a seemingly limitless assault-weapon catalog.

Visa spokesmen did not reply to several emails seeking comment. A spokesman for Mastercard sent a boilerplate statement that expressed “disgust with recent events, including last week in Florida.”

“Our payments network is governed by standards that have been established over time,” the statement said. “Chief among these is that we do not and will not permit merchants to engage in unlawful activity on our network.” He said the company would continue to talk to customers and lawmakers about its policies.

If Visa and Mastercard are unwilling to act on this issue, the credit card processors and banks that issue credit cards could try. Jamie Dimon, chief executive of JPMorgan Chase, which issues credit cards and owns a payment processor, has talked about how he and his bank have “a moral obligation but also a deeply vested interest” in helping “solve pressing societal challenges.” This is your chance, Mr. Dimon.