Hello rigbymauser,

Thank for the reply.

As the picture shows in my opening post, I made a chamber cast which extends into the rifling. These barrels have 7-groove Henry-Style rifling which makes measuring across the lands difficult. Nearest can I measure is 0.507" groove dia.

Today I made a ball gauge to measure the bore diameter. It measured exactly 0.490" in both barrels. This would explain why the barrel flats are marked "40" and not "39". When gauging the bore a 39 gauge would not fit, but a 40 gauge would. (see chart)