I've read about your "plugged case" method for finding throat deminsions, but not for slugging bbl. Are you asking about pouring lead down the bbl? Are you wanting to make a "lap" to lap the bore? or are you intrested in finding bore/groove deminsions? Just yesterday I slugged the bbl of my 6.5x55 Swedish Mauser; I used a bullet sinker (plastic worm "Carolina Rig"). I used some 3-in-one oil, started the sinker with a plastic mallet, used a brass punch to push the sinker a couple inches down the bore, finished pushing through with cleaning rod (with 8-32 brass screw in the end). .266". Piece of cake...
My Anchor is holding fast!