I think you could load it in any manner, but I always thought that the idea was to be able to just grab a handful of shot from a shot bag, drop it into the muzzle without spilling, (after dumping in some powder) and then putting a wad over that. All on a heaving deck, or in a coach or wagon going full speed ahead. The belled muzzle only acted as a funnel, and does not spread the shot pattern any more than a cylinder bore shotgun would.
Seems like that would require a ram-rod, so I'm not sure what the drill was for guns that didn't have ram rods. ? I suppose if you kept the muzzle pointed up, you could dispense with the wad, if you both brought it quickly on target, and pulled the trigger before any of the shot could roll out...!!??? I think Roger's Rangers had a method where they dumped powder and ball, or buck and ball down the barrel of a musket, slammed the but of the gun on the ground, then fired.