As CptCurl stated, you will probably have trouble with loose, in-the-case fit.
: Plastic works just fine, though.
: Some time ago,(30yrs or more) I read an article about lining oversize cases with card stock to reduce inner dimensions. this may be an option for your 16 bore brass, but there is a possiblity of some of it blocking the bore.
: When working loads up for my double 12 with slugs and round balls, I had good luck with WW AA cases.