John,
It seems to me he did say what he got from the data:
- Cannonballs from the Colonial Era tend to be a significant bit smaller in diameter than Mid-1800s cannonballs. Apparently, the increase in projectile diameter is due to advancements in metal refining and cannon-casting that resulted in "stronger" cannon metal, which could withstand the increased bore-pressure created by smaller windage.
And this makes sense to me. I would also think that it had a lot to do with machining capability and skills to be able to mass produce the cannon balls where they didn't have to discard a large percent because they wouldn't fit down the bore. I also wonder how accurately consistent the bores were to their intended caliber. Also, being French made, was a French mortar cannon actually 13 inches or was it close and we just call it a 13 inch. The equivalent French mortar might actually be intended to be 12.75 inches. I think this is the case.
I say this because I have a booklet entitled "British Artillery Ammunition 1780" by Adrian B. Caruana that I got from Rick Burton that lists a lot of information regarding English cannon and projectiles from the 18th century. There is a table that shows the weight of the various size shells and it does not list a 13-inch but does list a 12-3/4 inch. This was true of the other sizes as well. Most of the information reported in this book came from the Royal Artillery insitution, Woolwich.
In full disclosure, in the book there were a few tables regarding fuses that listed the ball at 13 inch but all tables that talked about the shell carcass itself listed them as 12.75.
