Port Authority police spokesman Joe Pentangelo says Southwest Airlines notified officers of the discovery as the 23-year-old man tried to go through security in Terminal A on Thursday night.
Pentangelo says the Valparaiso resident told officers he had purchased the weapon, which was in a soft case, and 70 rounds of ammunition, for recreational purposes. But the Transportation Security Administration says he didn't properly pack or declare the weapon at the time of check-in.
Police arrested the traveler in the terminal on a state weapons charge after they said he failed to produce the required legal documents.
He's being held on $50,000 bail, charged with unlawful possession of an assault firearm and large-capacity ammunition magazines.