I've never come across such a thing here in England. The 20 has always been designed as a light weight gun made for light 13/16th. ounce loads as standard. It has only been in recent years that longer chambered 20's have appeared mainly to satisfy foreign trade. I would think that a 12, with its shorter shot column, would have patterned far better with 1 1/8th. ounce loads and been the same weight of gun. Lagopus.....