Why does Kibler's not recommend shooting round ball out of the 16 gauge fowler?
It is a matter of margin rather than outright danger. The fowler has a fairly thin wall barrel, around 50 thousandths or a bit less, which will whip more and be less accurate and stable with round ball, though pressure at the breech is much lower with shot than with ball so it is not considered a safety issue outright. Kibler's still prefers to encourage the 20 gauge for round ball shooting.