It's all because of repeating shotguns... when you try to load a double 12 with a 16 or 20, you know something's wrong, but when the shell is in the magazine... I knew repeaters were evil!

Also. Grandpa said he had loaded his "duck shells" into red hulls, and "hare shells" into green hulls, to help instant recognition and save the trouble of writing the shot # over them. When I inherited his shotgun, there were some red shells that came with it. I shot a few ducks with them, and never knew the difference, but leter it turned out that the shells were loaded with #1.

Russian Hunting Shotguns