Typically shotguns come in one of several calibers: 10, 12 & 20 gauge are the most common, but .410, 32, 28, 24, 16 & 10 gauge along with 9 mm and .22 rimfire c

Typically shotguns come in one of several calibers: 10, 12 & 20 gauge are the most common, but .410, 32, 28, 24, 16 & 10 gauge along with 9 mm and .22 rimfire calibers have been manufactured. He could have also been talking about what's referred to as an "over under" type shotgun. They are usually double barreled "over and under" rather than "side-by-side" and usually have a .410 on the bottom and a .22 long rifle on top. Either way, it would have been accurate, however I am not a rifle expert, nor can I see what rifle he used to produce the artwork.