RE: Is this true?
There is a chamber insert that shortens a 06 chamber to fire 308 ammo .I think Gun Parts sold them . If I remember the directions You drop the insert into the chamber and tap tight with a dowel and then you can fire 308 , it is removed with a 30-06 broken case removal tool . I have never and will never do it . IMO you could mess up a rifle with or without it .
Another case of " Don't try this at home "