The problem is to find something that will feed through the action, and I think that might not include a straight-walled, sharp-shoulder, big bore Ackley Improved.
The book "Bolt-Action Rifles" by Frank de Haas discusses the problems of converting the Krag to another caliber, even one based on the 30-40 case.
He mentions that he knew of one guy who had his barrel re-bored to a .35 caliber with good results. No mention if it was a blown-out Ackley Improved or not however.
He later went on to say that they can be converted successfully to .444 Marlin with only very minor magazine modification.
MacFarland, in "Modern Gunsmithing" says converting one to feed another caliber is fraught with problems and best not attempted.
Many years ago, there was a lot going on with the Krag when they were a dime a dozen. But those days are lone gone now, and I just don't have much info on converting them.
What you want to do, has probably been done & written up in the American Rifleman sixteen times back in the 20's & 30's. But I don't know where to tell you to start looking for information now.
BTW: I agree completely with Jim Watson on using just a necked up standard case.
After modern pressure testing equipment and chronographs became available, most of the "Improved" wildcats were found to be not much better then the parent cartridge they were based on when running equal & safe pressure.
And a Krag action is not one to play with high pressure in!
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