Investment bank Citigroup has said it will introduce new restrictions on business customers who sell guns.
It joins a growing corporate backlash against gun violence in the US, following the deadly school shooting in Parkland, Florida last month.
Citigroup said it would not work with firms that sell guns to customers who have not passed a background check or who are younger than 21.
It has also barred the sale of bump stocks and high-capacity magazines.
The measures would apply to clients who offer credit cards supported by Citigroup, or borrow money or use banking services through the firm.
The Parkland massacre, in which 17 died, has sparked a consumer boycott of firms who have partnerships with America's powerful gun lobby, the National Rifle Association.
It has led many firms to cut ties, including United and Delta airlines and car rental giant Hertz, which have ended discounts for NRA members.
Others, such as tech firm Amazon, which distributes NRA television programmes, face pressure to act.
Citi, which is the first Wall Street bank to take a stand on gun control, called its measures "common sense".
Chief executive Michael L Corbat told The New York Times that he himself was "an avid outdoorsman and responsible gun owner".
But he added: "As we looked at the things we thought we could influence, we felt that, working with our clients, we could make a difference.
"Banks serve a societal purpose – we believe our investors want us to do this and be responsible corporate citizens."
The bank only has a few gun manufacturing companies as clients, but those it does work with will be asked to give details about their sales practices.
If customers refuse, the bank said it would work with them to "transition their business away" from Citigroup.
The company declined to outline the value of its work with gun-sellers, but said "real revenue is at risk" if customers protest and business relationships sour.
Seventeen students died when 19-year-old Nikolas Cruz opened fire at his former school in Parkland in February.
It was the deadliest school shooting since 26 people were killed at Connecticut school Sandy Hook in 2012.