He may have rechecked his bag in Minneapolis since he stayed overnight in Minneapolis. Now to send firearms, you need to have your ducks in a row: papers and firearm/bullets situation straight. So, I do not believe he would have picked up a firearm from another person while in Minneapolis (should that idea be brought up).
He was determined and sane enough to purchase a ticket and fill out paperwork for the gun and ammo. Both declared and was processed at a separate counter for the gun. He likely owned it the gun personally in Alaska.
He was methodical, patient to fly 5,000 miles and prepared to kill for ISIS>
Mid-Eastern keffiyeh, chin strap beard, ISIS one-finger salute and down-loading ISIS video.