Shawn Moore said he was visited by the Division of Children and Families after he posted photos on Facebook of the birthday boy and his new semiautomatic weapon, which resembles an AR-15.
Moore quickly went on the Internet again to call the unannounced visit to his home in Salem County last week an attack on his constitutional rights.
"I got a text from my wife that the cops and (DCF) are at the house and they wanna check out my guns and needed me to open my safe," Moore wrote in a post on a gun-rights website. "The fight has officially been brought to my front door."
Police in Carneys Point, N.J. had no immediate comment on the incident but the Philadelphia Daily News said the story has been snapped up by arch-conservative commentators such as former Fox News host Glenn Beck.
The newspaper said the specific weapon wielded in the images by the camouflage-clad boy was a Smith-and-Wesson MP15-22., a .22-caliber version of the AR-15, which fires a larger bullet and was used in the bloody December shooting spree at Sandy Hook Elementary School in Connecticut.
New Jersey law says children under the age of 18 can possess a firearm only under the direct supervision of a parent. Moore is certified as a firearms instructor by the National Rifle Association and said he bought the gun so he and his son could go target shooting together, the Daily News said Wednesday.