I built a 416 Ultra on a Olympic Arms BBK several years ago. Fed perfectly. Simply the 375 Ultra case necked up to 416. The rifle is now somewhere in Alaska. I have the reamer and about 500 each 375 Ultra cases, maybe I should build another, but have the 416 Rigby bug now.

PS, one of the better things I did way back when cases were available was to buy a lot of brass, including the 375 Ultra brass. It was cheap back then!