As I stated, it is a hypothetical question. I was wondering if it was something the old machine rest shooters may have tried. A barrel is subject to the most heat at the throat & the least at the muzzle. I don't know if any close examinations of the changes in the steel structure have been made along its length. With ends swapped would it perform as well as a brand new barrel?
From a practical stand point, benchrest shooters have some pretty chunky barrels of premium quality & cost that they deem to be throw-aways after 3,000 rounds. You would not swap ends but these could be re-contoured into AR barrels. Using the 6mm PPC as an example, the usual optimal length for that caliber is between 21" & 22" which allows plenty of barrel for a re-chamber. Not sure of a barrel extension's diameter but for a AR15 the whole chamber length would have to be removed to fit a smaller case like a 6x45. The normal twist would be 1/14".
I got a hold of one of these used-up benchrest barrels & put it on an old hunting rifle action. To say it shoots amazing is an understatement.
Just curious & thinking out load.