Shoulder dia is .515" vs .490" with the Ruger shoulder further forward. Doesn't sound like much of a difference but there was not enough mag box thickness in the action I planned to use to open things up for proper function. Don't want to belabor the point so just let me say that anyone considering building a 375 or 416 Ruger on a MarkX action should do their research first. The old trick of rubber banding three cartridges together and using a dial caliper to measure for required mag box size would be a good start.

Cartridge Drawings-
http://www.stevespages.com/page8d.htm