Interesting question. If you get an ATF reading on that....get it in writing just in case.
I think you would be clean goin to the rifle length but what a field agent thinks would count too. That is why I reccomend gettin a reading in writing from ATF.
Title 18 USC, chapter 44, subchapter B, Sec. 5845 (d). Shotgun.--The term 'shotgun' means a weapon designed or redesigned, made or remade, and intended to be fired from the shoulder and designed or redesigned and made or remade to use the energy of the explosive in a fixed shotgun shell to fire through a smooth bore either a number of projectiles or a single projectile for each pull of the trigger, and shall include any such weapon which may be readily restored to fire a fixed shotgun shell.
The definition of Rifle is same wording except rifled bore instead of smooth bore.
The operative words here seem to be SMOOTH BORE.