For example, an unlined 16 bore Magtec brass case, easily holds a 13 bore bullet or ball with 14 bore wads taking up the space. The 13 bore ball (.705") is WAY too big for a normal sized 16 bore gun (.662"), but will chamber in a normal 16 bore chamber. To fire such a ctg. it would/could be dangerous.

A 16 bore paper or plastic case is the correct size inside for a 16 bore bullet or ball.

The 'same' differences/problems would be normal for 12 bore cases.