It is best to avoid this conversion. You have to ream the chamber for the adapter. Shoulder body junction of the 308 is larger then the 30-06 body at this point. You can never go back to 30-06 after converting it to 308. Plus the adapters are prone to ejecting with a fired case at times. The military tried this with the M1 and soon gave it up as unworkable due to the adapters being ejected. They stopped the practice after a very short time.
1+
They've even been known to eject after using permanent Lock-Tite. If you want a .308, buy one.
