Brownells Inc., which claims to be the world's largest supplier of firearms accessories and gunsmithing tools, said it has sold 3½ years worth of ammunition magazines in three days.
Consumer demand has been "unprecedented" recently, according to a statement attributed to company President Pete Brownell on gun owner forum AR15.com.
Police say Adam Lanza used a Bushmaster AR-15 semi-automatic rifle on Dec. 14, when he went on a shooting rampage in Newtown, Conn. that left six adults and 20 children dead. He also killed his mother and himself.
In the AR15.com post, Brownell said he and his staff "absolutely apologize" for an order backlog on the company's website that is slowing down usually instantaneous transactions.
"The demand for magazines actually exceeded the ability for the system to keep up with the volume that was being ordered," he wrote. "We're working like crazy to get these orders to you as quickly as possible."
Brownells also noted on its Facebook page that winter storms, in addition to the "extreme order volumes," are delaying some shipments.
In the wake of the tragedy, several major firearms retailers have turned to a more subdued marketing strategy.
Cabela's, an outdoor products retailer, said it would not sell the AR-15 at its Connecticut store. Dick's Sporting Goods said it removed all guns from its store in Newtown and suspended sales of modern sporting rifles at all of its locations nationwide. Wal-Mart took down the online information page for Bushmaster's Patrolman's Carbine M4A3 rifle.
Gunmaker Smith & Wesson's stock is down 15 percent since the rampage. Competitor Sturm Ruger has slipped nearly 9 percent.
On Friday, the National Rifle Association said it would train armed volunteers to protect the nation's schools. In a speech, NRA Chief Executive Wayne LaPierre cited gun-free school zones, the media and violent movies and video games as causes for concern.