I've used a couple of different methods now with graphite. For a number of years, I used a mechanical drafting pencil lead and because I had the sharpener for it, I have an easy way to produce the lead shavings.
Recently I picked up a small tube of powdered graphite from a locksmith shop and gave it a try with much the same end results.
Tiking - here's a pic of my mine roller that has been weathered with graphite.
The graphite really gives a "heavy metal" look to the finish IMO.
Happy Modelling and God Bless
Robert