The 20 in a circle means it has a standard 20 gauge chamber, which is 65mm( 2 1/2"). It is easy enough to measure, however. The 19 is the bore diameter ahead of the chamber, in gauge measurement. This works out to 15.90mm. BTW, the left barrel is choked( constriction not given), as shown by the crown W. The right barrel is not choked. The crown U was still used after 1912( view proof) to show it was closely inspected, including dimensions, after proof.
Mike
Last edited by Der Ami; 11/03/20 06:19 PM.
