Actually, Front Range IDPA does not
mandate an unloaded gun for shotgun stages. Most shotgun stages start just like the pistol stages, i.e. fully loaded mag, and you may top off at the line. Max magazine capacity is ten shells.
Go to http://idpa.virtualave.net/monthly.html
and click on the link for Rifle/Shotgun Rules for the straight scoop.
As for what gun to use, IDPA is all about training and practicing with your actual gear - whatever that is. So do you normally have the extention on or off?