Some early Belgian 16s are marked 16.8 = .661". Later up to 17.0 = .669"

This is from 1892 and note the 16g loads are 2 3/4 Dram Eq. and you need to add 10 - 14% for transducer numbers. The Belgian black powder standard was also 2 3/4 Dram but Smokeless loads were mostly 2 1/2 Dr. Eq.

[Linked Image from photos.smugmug.com]

1 oz. 2 1/2 Dr. Eq. Bulk Smokeless loads ran about 7,500 psi

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