A simple device that you can make. Weigh a powder charge. Dump into an empty case a little larger than the caliber you are using. Mark the empty and cut it off to that mark. Solder a piece of heavy copper wire onto the cut off case and VOILA you have a custom made powder measure. Dip it into the selected powder and fill your cases.
I've done this already - I have a little plastic vial that I have cut at just the right size for 48 grains of powder. Problem is, as you shake it around you get inconsistent results. Probably +/- 2 grains? I try to keep my charges +/- .2 grains.
I think a mechanical dispenser would be fairly repeatable once you set it up to dispense a certain charge. I know the smokeless dispenser on my Lee Pro 1000 is extremely repeatable. Probably within +/- .1 grains.