The Mk 10 uses the same box for .473 case head cartridges as it does for .532 case head magnums, like 7RM and .300 Win mag. The difference is the width of the feed rails and the width of the feed ramp. I've been building Mauser '98s for 30yrs, both commercial and military. The length of the Mk10 box is 3.4, unless you get lucky enough to come across a .375 H&H action, which is 3.6". The same commercial Mk 10 action was used for .22-250 and .375 H&H. The difference is the length of the mag box. A block was spot welded into a .473 box for 'short' cartridges like .308, and the action was altered at the feed port and ramp for the H&H, the rails 'opened' for all magnums regardless of length. Then, there is the extractor that will need alteration if the action is converted to magnum. Even if NOT converted, many extractors need 'tuning' to work right and be reliable with .473 case head size rounds. JM2Cents, after building on these for 30+yrs. They ain't "drop-in", and can require 'real' gunsmithing skills.Good advice from adamjp - my guess is the Interarms X mag box is like the FN Mauser and is designed to work with rounds having the same diameter as the 7.92X57 that work in the M98. Much thinking/design effort went into that mag box width to work with .473 head diameter, non rebated cartridges. Enough cartridge base is needed for the bolt face to contact & receiver rails need to securely hold the round until it is stripped from the magazine. Good choices would be .25-06, .270W, .30-06 and the wildcat 6.5-06 (no over the counter ammo available). Been there & done that - I messed up a real nice FN Mauser with a 7mm RM conversion - unreliable/difficult feeding. I gave it to a relative. Stay with .473 diameter non rebated rounds.