PB: Target shooters use every trick to get consistent powder charges. Some don't trust their cleaning methods to dry the land and grooves completely, and use the drop tube to avoid ANY powder sticking in the grooves at some wet spot.
Again, I think if they load an OP wad on top of the powder, the wad will scrape down any errant granules and add them to the main charge. But, this is one of those technidque debates that go on within the ranks of target shooters. I have used a chronograph to see what is the least complicated way to get consistent loads, and SDV, and it doesn't include using a drop tube in my flintlock rifle.
I have a percussion shotgun, right now, which is my ownly percussion gun. I use OP wads over the powder in my shotgun and never worry about velocities. I am going to chronograph my favorite loads( judged base on how they pattern) by putting the shot in a heavy paper tube closed at both ends. This " slug " will weigh whatever the shot charge weighs, but will stay together to go over the screen on my chronograph, without any danger of damaging the machine. It won't let me measure down range velocities of free flying shot, but MV is better than nothing at all. Lyman's Shotshell Reloading Manual and other reference materials in Dixie GW's catalogue, or in the articles by V.M. Starr and Bob Spenser on his " Black Powder Notebook " website, can be used to accurately estimate down range velocity, pellet energy, amount of drop of the shot at various yardage, and time of travel to various yardage, based on that MV.