The sellers of high-capacity gun magazines and other gun accessories have become an influential, if not always widely noticed, force in the National Rifle Association.

But their role, the Center for Public Integrity reports, could soon draw more scrutiny because of gun control legislation being proposed by Rep. Carolyn McCarthy, D-N.Y., and Sen. Frank Lautenberg, D-N.J. The lawmakers — moving quickly after the shooting spree that killed six and wounded Rep. Gabrille Giffords, D-Ariz., along with 13 others — are pushing to ban the possession of new high-capacity magazines such as the one used in the Tucson rampage. The legislation would apply to magazines holding more than 10 rounds of ammunition.

The NRA already has issued a statement signaling its opposition, calling high-capacity magazines “standard equipment for self-defense handguns and other firearms owned by tens of millions of Americans.”

One of the factors behind the NRA’s stance, the Center for Public Integrity suggests, is fund-raising by magazine sellers such as Missouri-based MidwayUSA. The company is the developer of a fundraising tactic known as the “Round-up” that since 1992 has pulled in $5.7 million for the NRA’s lobbying operations. The tactic asks customers to round up the cost of their orders to the nearest dollar or higher and, in turn, the company donates the extra money to the NRA’s Institute for Legislative Action, a lobbying arm.

Other companies that sell magazines and other gun gear also have adopted the fund-raising technique.

In addition, magazine sellers and makers have worked their way into positions of influence. Brenda Potterfield, the wife of Midway Chief Executive Larry Potterfield and a co-owner of the firm, has served as vice president of the NRA Foundation’s board. The Center for Public Integrity also cited the leaders of two other companies that have profited from magazine sales who joined the NRA’s Board of Directors over the last two years: Ronnie Barrett, the head of Tennessee-based Barrett Firearms Manufacturing, and Pete Brownell, who runs Iowa-based Brownells Inc.