Here is a good article with the play-by-play details of Dunmore's raid on the Williamsburg magazine. Interesting to note that not only did his troops steal the powder, but over several days before the raid they also were reported to have removed the locks from the militia's muskets which were stored in the Magazine.
The article is here: http://allthingsliberty.com/2015/04/prelude-to-rebellion-dunmores-raid-on-the-williamsburg-magazine-april-21-1775/ (http://allthingsliberty.com/2015/04/prelude-to-rebellion-dunmores-raid-on-the-williamsburg-magazine-april-21-1775/)