Friends and I have built several of these. The 338-416 Rigby Imp. has about the same capacity as the 338-378 Wby. A shortened chamber that uses 338 Lapua brass, blown out, has about the same capacity as the 338 U'mag, IIRC. Have not played with unimproved 338-416 cases.
Some were built on Geske actions, some on Rem 700s that were opened up and had Sako extractors installed.
They work, are fun and brass and bullets cost quite a bit. Everybody shoots 250 and 300 gr. Sierra MKs. They will kill elk.
Personally, I don't own any of them but have been involved with them.