Christmas was a few days away and Solomon Lehnerd was selling more grenade launchers than usual.

It was the first holiday season that his online gun store was selling the components to assemble the holy grail of enhancements for AR-15-style rifles: a 40-millimeter grenade launcher that mounts to the rifle.

Mr. Lehnerd’s shop, Rooftop Defense, thrives on such items as buyers try to recreate historically accurate military rifles, and he profits almost entirely from what has become the new frontier of American gun stores: accessories.

“The accessories are the meat and potatoes of revenue,” Mr. Lehnerd, who is 29 and goes by Sol, said as he surveyed his counter scattered with holiday orders. “It’s not really in guns; guns have almost no margins at all.”

Scopes, suppressors, handgrips and muzzle brakes were once inconspicuous options associated with firearms in the United States, footnotes to the hardware that often makes headlines. But with store workers reporting lower than expected gun sales in an election year — firearms purchases usually spike in fear of new restrictions from incoming administrations — expensive accessories have been “keeping the door open,” as one retailer from coastal Maine said in late December.

Even the best-selling AR-15, which in the last two decades has become known as “America’s rifle,” is owned by so many Americans that it is no longer in such demand. “Everyone’s got them,” one brick-and-mortar retailer in Minnesota said of his sales this summer. A 2021 firearms survey conducted by a Georgetown University professor found that nearly 25 million people had owned an AR-15-style rifle.