[email protected] is right, as usual. I had it backwards. Larry Pletcher's (not Pletch) article was published in the April 2009 issue of MuzzleBlasts. He concluded that priming powder banked up against the barrel resulted in the fastest lock times. He made a point of the fact that great pains were taken in his tests to not fill up the vent hole.
Sorry, Phil. I should have looked it up first.