And Deseret Digital Media, which owns KSL.com, a Web site that has been criticized by Mayor Michael R. Bloomberg for allowing unregulated gun sales, said it was suspending classified advertisements for guns.

Elsewhere, though, consumers are hurrying to buy guns, leading to some models being out of stock, warnings of shipping and customer-service delays, and significant premiums on assault rifles.

“We are seeing a total madhouse of buying everything in sight,” said Bob Irwin, owner of the Gun Store, a Las Vegas shooting range and retailer. Thursday, he said, was the largest sales day in the history of the store, which has been open for 30 years. “We have not only a run on the guns, but a run on ammunition.”

Mr. Irwin has begun limiting how much of some types of ammunition customers can buy, and he has canceled employees’ days off to handle the demand.

Walmart, the largest retailer of guns and ammunition in the United States, indicated that several semiautomatic guns were out of stock at locations across the country. Kory Lundberg, a spokesman, said the company was not sold out of guns altogether, but had low inventory in some situations. Walmart carries guns in about half its stores, and about one-third carry so-called modern sporting rifles, the category including the Bushmaster and other AR-15 weapons.

Other retailers around the country were selling out of guns and accessories. On Friday on ImpactGuns.com, the Bushmaster .223 was out of stock. Davidson’s, a supplier to gun retailers, placed a notice on its Web site that said it was seeing “unprecedented demand,” and at MidwayUSA.com, more than 100 parts for AR-15 guns were out of stock and on back order.

On AR15.com, a gun-enthusiast Web site, a user posted that a barrel for a gun disappeared from an online shopping cart overnight, and is now on back order. Another user, named warplg8654, responded, “Dealers can’t keep anything in stock for what I think are obvious reasons given the current political climate.”