Guns Down America said it had shared its methodology and individual grades with all 15 banks and was involved in continuing talks with some of them, but several banks contacted for comment said that they had not been consulted and that the group had relied on arbitrary metrics.

In the last year, gun safety groups have begun looking more closely at the firearms industry’s corporate ties, with the scrutiny leading some businesses to make changes.

After a mass shooting in Parkland, Fla., Delta Air Lines, Hertz and several other companies broke with the N.R.A., and retailers like Dick’s Sporting Goods made some guns harder to buy. Influential Wall Street figures like Laurence D. Fink, the chief executive of BlackRock, publicly embraced gun safety efforts.

In March, estimated firearm sales in the United States slipped 12.6 percent from a year earlier, to nearly 1.4 million units, according to the research firm Small Arms Analytics & Forecasting.

The financial industry’s connections to gun makers have drawn attention before. In December, The New York Times reported that many of the mass shootings in the last decade had been financed with credit cards.

[Read out how banks unwittingly provide financing for mass shootings.]

Some banks have already started to revise their policies and practices on firearms. Citigroup, which earned a score of 84 from Guns Down America, said in March 2018 that it would work only with clients that agreed to certain restrictions on firearm sales.

A short time later, Bank of America said it would stop lending to companies that made military-inspired firearms for civilian use. It was subsequently criticized for its commitment to contribute $43.2 million in bankruptcy financing for Remington.