Here's a simple and practical solution for vintage 16 bore guns made for 2-9/16" shells that are capable of being opened to 2-3/4" - as confirmed by barrel wall measurements forward of the chambers.
1930's 16 gauge chamber reamer that was made to period 2-3/4" specs. Note the relatively short forcing cone that's only about 3/8" long as measured on the diagonal.
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