I'm not filling the drums all the way to the top - provides a little bit of control over where the coal lands, at least until I get a little more practice with the dumper.
I've dumped a couple of drums of dry coal, and I think it makes less dust than the wheelbarrow did. With the dumper I can get the lip of the drum near the coal in the center of the barrel, which seems to help. With the drums, it's also easy to sprinkle a little water on the next drum in line (i.e., rather than hose the front of the coal pile) if needed to make sure that the next load is damp when dumped.
Rob is correct, however, that I don't really worry much about dust because of the draft. As long as the window in the boiler room is open a crack, there is a prevailing wind from "behind" the dumper to the baro. Rob is joking, of course, about the 24" baro. It's only 18". But there are 2 of them (the granddaddy of the Mills boiler sits next to it), so Rob is pretty close on the square inches.