Ray Ordorica wrote an excellent article in Gun Digest many years ago about building a 416 Rigby rifle on a standard Mauser action. He talked to the Rigby company (then owned by Paul Roberts) and was told that Rigby had performed the conversion but had then moved away from it as better choices were subsequently available.

I am no gunsmith myself as everyone here knows but I enjoyed reading the article. Anyone itnerested would do well to trace it - it is a lovely read.

Good hunting!