Mark, the shotgun shells do NOT fit in the Span-Am Mills belts. They are made for .30 caliber rounds (.30-40 Krag). I got around this by cutting the stitches of every other cartridge loop on the belt, then putting the shotgun shell in the double-wide loop. It will be too big and the shells can fall through, but you can glue them in place. Use Elmer's or similar white glue if you don't want it permanent.
Sorry, the photo I have of this is blocked on Photobucket at the moment. But it shows up in my thread:
Philippine Manstoppers, c.1899