The problem with the mounted measures on horns, or flasks, like the one shown is the crud that builds up and keeps the " gate" from closing completely after a number of shots are poured. Its particularly a bad problem in damp weather, or in high humidity. The powder residue gets on both sides of the gate, which is usually a thin piece of plated metal, and it also gets on the tubing and areas against which the gate moves and rests. Remember that the flask is held upside down to pour powder into the drop tube, and then the gate is allowed to close while all that powder is weighing down on top of it.
I have trouble with the gate on my Irish shot measure closing properly when I am measuring out shot, because of the weight of the shot behind it. I rarely carry a pound of shot in the snake in the field, so the problems with the two containers are similar. In my Irish measure, the lead is scraped off by the gate as it closes to cut off the shot behind it, and bits of lead load the edge of the gate, and surrounding structures, and eventually keep the shot from closing. I then get shot dripping out onto the ground, while I pour the scoop of shot into my barrel, and return the scoop to the snake. With powder flasks, the residue holds the gate open allowing any flame from a cook off in the barrel to travel right up into the main container of powder and set that off.
Such incidents have been reported over the past 30 years, with the lastest one a few years ago. My club rules will not allow these flasks to be used on the firing line to load charges in the barrel. A powder measure separate from the can, flask, or horn, MUST BE USED by the shooter, for the safety of all shooters. The powder measure can be fixed, home made, commercial, adjustable, with or without a funnel or cut off plate, but it has to be separate from the powder reservoir.
Be safe. Always. :thumbsup: