Major US credit card companies will implement a new merchant category code for gun stores to use when processing credit and debit card transactions, a move that gun control advocates say will help flag suspicious purchase and gun traffickers.

Visa Inc, the world's largest payment network, as well as Mastercard Inc and American Express Co, said they plan to adopt the Geneva-based International Organization for Standardization's new merchant code for gun sales, which was announced on Sept 9.

Until now, gun store sales were often considered as specialty retailers or durable-goods sellers — categories that include a much broader collection of companies.

Gun rights advocates have argued that categorizing gun sales would unfairly flag an industry when most sales don't lead to mass shootings. They also say that tracking sales at gun stores would unfairly target legal gun purchases, because merchant codes only track the type of merchant using a credit or debit card, not the item purchased.

"The (industry's) decision to create a firearm specific code is nothing more than a capitulation to anti-gun politicians and activists bent on eroding the rights of law-abiding Americans one transaction at a time," Lars Dalseide, a spokesman for the National Rifle Association, said in a statement Sunday.

"This is not about tracking or prevention or any virtuous motivation — it's about creating a national registry of gun owners," Dalseide said.

But gun control advocates say that sales codes for firearm-related sales could flag suspicious purchases or more easily trace how guns and ammunition are sold.

In 2018, The New York Times found that electronic payments were used to purchase guns and ammunition used in some of the country's most lethal mass shootings, including in Aurora, Colorado; San Bernardino, California; Orlando, Florida; and Las Vegas.

Merchant category codes, or MCCs, are four-digit numbers that networks use to identify types of merchants by the goods and services they sell. Card networks assign specific codes to almost every kind of purchase, such as those made at fast-food restaurants, supermarkets, clothing stores and many other retailers.

Banks and credit unions made more than 1.4 million suspicious-activity reports in 2021 for other types of transactions that might suggest anything from identity theft to terrorist financing.

Earlier this month, Democratic lawmakers wrote to Visa, Mastercard and American Express to push for a new MCC for gun and ammunition sellers, citing recent examples of people using credit cards to buy guns that were later used in mass shootings.

Democratic US Senator Elizabeth Warren of Massachusetts and Democratic US Representative Madeleine Dean of Pennsylvania wrote in a letter signed by dozens of other lawmakers that the new code for gun retailers would be "an important step toward ending financial system support for gun trafficking, gun violence, and domestic terrorism".

The shooter who opened fire in a Colorado movie theater in 2012 charged more than $9,000 worth of guns, ammunition and tactical gear in the two months leading up to his attack, in which 12 people were killed and 70 injured.

The man who shot up the Pulse Nightclub in Orlando, where 49 people were killed in 2016, spent more than $26,000 on credit cards on firearms and ammunition.

The gunman who killed 59 people at an outdoor music festival in Las Vegas in 2017 charged almost $95,000 on dozens of guns.

"When you buy an airline ticket or pay for your groceries, your credit card company has a special code for those retailers. It's just common sense that we have the same policies in place for gun and ammunition stores," said New York Mayor Eric Adams.