Steve-

I've kept my nose out of this one due to plain/simple ENVY!

*erhem - not that I've gotten that out of the way...*

Haven't you cast the chamber on this thing yet?
Do that right away - and post your findings here in the thread.
I've been curious about that.

Have you considered the possibility that this was built as a 14-bore roundball rifle for the paper case?
Might just be what you've got there.

I've gone through three different 16-bore double rifles, casting chambers to machining cartridge cases and moulds to developing loading tools and regulation loads.
I've had a great time doing so, and as you're qualified to guess, they're all different.

I want to see chamber casts complete with throat and with the first inch or two of rifling.
Post an image of your bore slug and the muzzle too if you could please.

In the meantime, convince yourself that when the time comes for you to sell this rifle you contact ME first so that I can prepare myself for the purchase...

By the way, once you have a chamber cast taken of this rifle (and have measured/photographed/posted to this thread...) you can send it to Rocky Mountain Cartridge where they'll make you a nice box of cartridge cases for it.
I think you should just make your own on that lathe of yours -- you'll be shooting it much sooner than otherwise!

Cheers
Tinker