Sales of the firearm accessory that the Las Vegas gunman used to modify his rifles jumped this week, causing some companies to sell out of the so-called bump stocks even as Walmart and Cabela’s stripped them from their websites.

The products — conversion kits that allow semiautomatic weapons to fire hundreds of rounds per minute, similar to an automatic weapon — were once relatively obscure specialty items aimed at gun enthusiasts who own semiautomatic rifles.

But since the massacre on Sunday, when the gunman fired on a music festival crowd from the 32nd floor of the nearby Mandalay Bay Resort and Casino, killing dozens and injuring more than 500, interest in the accessory has surged. Officials said that 12 of the rifles found in the gunman’s suite were outfitted with bump stocks.