I'm not a gunwriter, but I believe there is a recent thread on this topic. Search for it.
The bottom line is that the improved reamer must be run further into the existing chamber in order to "clean it up" . The barrel must then be set back a bit in order to do this and still maintain correct headspace.
Some unscrupulous gunsmiths simply run the reamer into the chamber without removing the barrel from the action in order to save time. You end up with a chamber that is longer than specifications, and you have your own unique improved chambering. When this is done, the bolt will close on a no-go gauge, and headspace is too long. I have gone through this with two Ackley Improved chambers and such gunsmiths.
