Yesterday a mate was sighting in a 300 H&H. The fired cases all had what seemed like an 'oily finish.'
I seem to remember reading that the Proof House oil chambers to increase the back thrust on the bolt by stopping the case gripping the chamber walls (??).
He said he pushed a plain patch thro' before shooting but didn't clean the cases after sizing (with lube).
I was always told to clean the cases before seating. Also to patch out the chamber with acetone /alcohol or similar degreaser before shooting, so have never give this any thought.
Should it make any practical difference ?