Prefits- Chamber is usually cut too deep, and with a worn out, chipped, totally dull reamer.

May want to have .010" or so clearance between bolt head and barrel for gas to escape in case of a case head failure or pierced primer.

Your best solution for that is to remove barrel and lathe cut .010" off the breech end, it doesn't need to be set up to .00000001" of bore axis or anything fancy just cut reasonably perpendicular and then put a very slight polish radius on the chamber entry.