FORT PIERCE — A 39-year-old Vero Beach man was arrested Saturday after being found with a short-barrel assault rifle outside a gun show at the Havert L. Fenn Center in Fort Pierce, according to a recently released arrest affidavit and a sheriff’s official.

James Mathew Kennedy, of the 1000 block of 27th Avenue Southwest, took the AR15-type rifle into the gun show, where he went to buy parts, officials said. They gave this account: The rifle had a barrel length about three inches shorter than is allowed without first going through a lengthy federal approval process.

A federal Bureau of Alcohol, Tobacco, Firearms and Explosives official identified the weapon as being illegal.

A St. Lucie County Sheriff’s Office investigator arrested Kennedy, listed as self-employed, after he left the show and went to his vehicle. He faces a charge of possession of short barrel assault rifle.