Thumper 15, welcome aboard. Is there a specific thought or reason for wanting to find out about "big bores?" Do you have any thoughts or plans to hunt Africa? What do you consider a "big bore?" What species of game are you thinking about taking with it? What is the largest caliber you've ever fired? Do you reload? Your InterArms Mark X with a magnum bolt face may handle a 404 Jeffery. I'm not a gunsmith but others on this site can tell you for sure. Do you barrel actions yourself, or have someone do it for you? If someone else does, you might ask him what your options and associated costs are. Depending on your skill set and or the size of your wallet, you can do a lot with a K 98 action. You might redirect your question concerning the three actions you have to the Gunsmithing Forum on AH. One thing for sure, you'll get a lot of options.