I like to mount the barrel verticley in the vise and place a catch pn under it to catch the oil/shavings.. Remove the reamer often to clean off chips, to avoid scratching the new chambers finish. Clean the chamber well before re-inserting the reamer. If using compressed air for either task be sure to remember eye protection as those flying chips can certainly ruin your eye. Since you're doing both rimmed cartridges simply cut until the rim recess that's already there is barely touched..headspace should be OK. I usually burnish/polish the newly cut chamber with steel wool on a rod spun with a drill motor to finish off such a project.
