The Iowa firm says it has sold 3 1/2 years worth of ammunition magazines in three days, the Chicago Tribune reported Monday.
Brownells President Pete Brownell, in a statement attributed to him on gun owner forum AR15.com, said that level of demand is "unprecedented" and apologized for a backlog in completing orders.
"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."
The Tribune said several major firearms retailers are being more muted in their response to the Newtown shooting. For example, Cabela's said it would not sell the AR-15 at its Connecticut store and Dick's Sporting Goods said it removed all guns from its Newtown store and suspended sales of modern sporting rifles nationwide.