denim
Sir:
An old gunsmith, Dave Taylor (deceased friend) showed me the denim trick.
You all know how drills etc., chatter when enlarging a hole - well if you cut some denim out of our old genes and fold it a few times and then place that over the point drill, countersink (etc) and the work and go slow you'll get NO chatter at all - sometimes two folds on small work or three or more for large work.
And, I never use a twist drill to remove some metal in chambering - use your reamer in about .030 at a time with air and good cutting oil - it can set up a chatter you can't get rid of.
Thank old Dave --Harry B.