Has anyone heard of using BPI's .500 12 gauge collet sabot to shoot a .505 Gibbs bullet from a rifled shotgun barrel?
It appears the collet is designed for a 325 grain pistol bullet but may have some room for a longer bullet from picture on site below. Sabot cups are near bottom of page above Foster slugs:

http://ballisticproducts.com/bpi/1products/components/slug/Slugs/slugs.htm