Twenty-four-year-old Timothy Scott Murphy of Idaho Falls made the plea Thursday to a charge of possession of an unregistered firearm.
Murphy was indicted May 22 with possessing a 12-guage shotgun that was about 16 inches long. The minimum legal length is 18 inches.
He was taken into custody Feb. 21, 2012, following a four-hour standoff.
Murphy is scheduled to be sentenced June 17 by Chief U.S. District Judge B. Lynn Winmill.
Murphy faces up to 10 years in prison and a fine of up to $250,000.