There HAVE been chamber inserts to switch from 30-06 to .308. The US Navy used them (w/ VERY poor success) for switching M1 garands to 7.62 NATO. The chamber insert kept getting ejected with a fired case, leaving the next round to go into an oversized chamber where it would not fire (bad ju-ju in combat). The case head is the same on both rounds, you would want to stick to heavier bullets (06 has diff rate of twist)
What we have here is... failure- to communicate.