The OP was about ".....a very fast system for loading black powder in the field." The Ruger Old Army is the
only BP revolver that I am aware of that can be loaded safely in this manner, due to its modern design.

While it may not be the most efficient use of powder, it has been the fastest method of loading that particular gun that I have used personally. Of course measured reduced charges will yield better results, but for quick down & dirty plinking......