Mayfair Engineering in the UK makes different size Mauser actions for the trade. They claim to make the magazine assemblies to suit specific cartridges, including internally contouring the boxes to fit the cartridge shoulder, where required. They’re an expensive action but look to be very well thought out and built.
WC264 - thanks for mentioning your rifle build. Would it be possible see pics of your mag box and follower, along with the box dimensions? Thanks.
