The Magtech cases are for fibre wad only and for 12 bore you use an 11 bore wad and over powder card with an 10 bore over shot card.
http://claygame.co.uk/products/detai...-shotgun-sizes
http://claygame.co.uk/products/detai...ly-nitro-cards
http://claygame.co.uk/products/detai...ver-shot-cards