Somebody show the man a picture! Surely somebody is going to a range today!
I think the important thing to remember is that the main charge ignites from the incandescent flash of the priming. The vent should be open. For my guns, I use a little priming flask with a free-flowing valve tip. I try to keep the amount of powder just below the bottom of the flash channel. If powder fills the flash channel, you have effectively produced a fuse, rather than a flash, and you get a slight hang fire.
I’ve tried “priming from the horn” but apparently don’t have the dexterity needed to direct or regulate the flow, and I end up putting in too much powder and scattering the grains outside the pan. This makes for great spectator appeal, but is rather disconcerting if you are the shooter.
Notchy Bob