In my experience , a .308 round will NOT chamber in over a dozen 30-06 rifles I own. Out of 5-1903s , 3-1917s , 4-M-1 Garands and 2-Rugers , a No.1 and M77 , I can only get a fresh 7.62 blank round to chamber in one of the 1917s , and only if I bang the bolt handle forward and down.

Though the case head dia is the same , and the .308 is shorter than the 30-06 , it also has less body taper. The Garands usually needed a chamber reamer with the Navy sleeve.