Interestingly enough, back in the good old days of the Winchester Custom Shop… they actually DID build ONE Winchester Model 70 Super Express in .416 Rigby in 1989.
Documented The One and Only Cased Winchester Model 70 Super Express Bolt Action Rifle Chambered in .416 Rigby with Factory Letters and Factory Caliber DieThe accompanying factory letters (three in all; dated: Oct. 1989, Dec. 1989 and Dec. 20 1989) confirm the Winchester Custom Shop grade of a...
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This is the world’s only .416 Rigby caliber Winchester Model 70 which was configured in said caliber from the Winchester factory itself.
Personally speaking, get yourself a Granite Mountain Arms African Magnum Mauser action. It’s properly designed for the gigantic .416 Rigby cartridge. If budget is a concern, then use a military surplus Pattern 14 Enfield action.
If you HAVE to purchase a Winchester Model 70, then get it in .416 Remington Magnum.