That's what I thought. Although I'm not a smith and there might be some things I'm not conisdering, but I know of some situations where folks have reamed a chamber with the barrel still mounted. I'm not sure how difficult it is compared to reaming on a removed barrel, but I know it is sometimes done. I can't remember which actions these were done on but I thought at least one was done on an Arisaka.

And regarding going from the Japanese round to the Swedish round ... I have been going over all the different re-ream options and decided that the Swede made the most sense after considering all the details of this particular rifle. The 6.5 X 257 crossed my mind, but the 6.5 X 55 seemed to be a better "clean out" proposition of a chamber I think is funky, plus loaded ammo and brass available makes more sense to me.

When it was given to me the guy handed it to me and said "heres the shells" whereby he handed me a box of 6.5 X 54 MS. I gave him a peek over my glasses and asked him if he had been shooting the gun with this ammo. He replied that he had not but the previous owner said it worked.

So, expecting problems I went ahead and did a hasty cast of the chamber and found that it was close but tight for the 6.5 X 54 and sloppy at the throat. I figured it was shot out but after looking at the bore it seemed usable but not pretty, but the throat looked burned.

Anyway I strapped the gun to a 2X6, propped it in an old tire, tied a 30' string to the trigger and let a round fly into a sand bank. I had taken the trouble to cover the action with a clean piece of paper to look for gas blowing out and covered all but the muzzle with a heavy rubber mat...then I hid behind my truck like a little kid and pulled the trigger string. Suprisingly there was little to nothing on the paper and the empty case looked ok but the shoulder seemed a little pushed forward when compared to a clean 6.5 X 54 case, ever so slightly. Also the case mouth looked pretty dirty and smudged.
After seven successive round in the tire and covered with the mat I held the gun at waist level and let a few more rounds go..Everything seem ok so with good protecton on I brought it up to a standing shot but I still didn't take a cheek weld. I was waiting to see if I felt any gas blowing. It all checked out and I eventually tried 5 rounds at 15 yards to try to print on paper. It seemd ok....but at 100 yards it was keyholing. after cleaning up the bore the keyholing slowed a bit, but I attributted the keyholing more to the 160 gr rounds and the short barrel that to anything else.
I still think that if I could get it reamed to 6.5 Swede, and handloaded 120 grain loads at modest velocities it would make an extremely handy little swamp carbine.