Visa Inc. announced in September that it will separately categorize sales at gun shops—a move gun control advocates say could help flag suspicious sales ahead of a mass shooting.

"The argument in favor of it is that somehow potentially this data could be used to interdict a potential mass shooting, to flag the purchasing activity of someone that may be likely to engage in that. The example that gets used is the shooter at the nightclub in Florida several years back apparently purchased $26,000 worth of firearms and ammunition prior to that mass shooting. That’s a big number," says Grant Reeher, professor of political science and director of the Campbell Public Affairs Institute.

"But then it invites the question: what would be the threshold? And then what would you want the credit card companies to do with that information?"