Hey Chuck,
Re-read it again. The OP didn't ask if it could be done, he asked why it would be done.
Quote : "It is possible a plug of some kind was put into the chamber to re-configure the rifle for the shorter .308 round vs. a 30-06. (not sure about this) Can anyone give me any information on this rifle and why it may have been re-configured?" : Un-Quote.
From his description he seems to have a Greek return rifle and can't chamber a round. He didn't describe what it is or is not doing so a reason for it not chambering would have to wait for more information from the OP.
FWIW,
Emri