Lancaster-

Thanks for your perspective.

On inside diameter of cases:
Something I notice with thick brass cases used in paper case guns is *poor gas seal*

I have worked around this in a couple of ways, one way is to make the 'belly' of the brass case thin wall, and the mouth thick wall to grasp the ball.
That is a bit of extra work on the lathe.
I also have run 'medium' wall thickness on the brass cases, and with special necking tool, re-size the mouth at each time after I fire the case.

Another way to go is as I pictured with the Westley Richards Explora case I picrured above -- here it is again

This WR special cartridge shows very thin brass case, like the cheap magtech brass we get today, with two-diameter paper tube liner.
See the step?
Internal 'Stop-Ring' accomplished in a different manner!

I have made a couple of these for my guns and tried them for function, they work.
I found that the brass expanded quite a bit to fit the chambers of my guns, and my plan to go forward with is to fire-form more thin magtech brass for my chambers, then make the mandrel for the special internal paper tube liner.

The paper tube liner can be made from high-quality paper found at art stores, I also will try Vellum. With such paper it is easy (as it is with the bullet paper jacket) to get precisely the thickness you desire. On a heated mandrel (hot enough to keep the bees wax melted) wrap the paper, wetting it with the liquid bees wax.
The finial tube is folded slightly at the base, held into the base of the magtech 'ballon-head' hulls with epoxy or other adhesive (I have not discovered the ideal solution here - epoxy may be the best)

Note, as you know - with the pinfire system this is quite the craft process down in the cartridge base. I have many things to consider down there including cap support and pin passage, also if fortunate and careful -- Gas-Seal!

I very much enjoy this conversation.
This forum site is a great old parlour for us to gather, with close access to so many workshops and reference books.
I feel fortunate to be a part of this, and to have the eclectic taste that I have for these fine old guns and rifles.

I am healing more every day, and I am getting closer to being able to get around my own parlour, grounds, and workshops (of which I have many here in this old place)
As soon as I can make it through the yard back into the machineshop and stand at the lathe, get up the stairs and the passage into my attic workshop, I will bring more of my parts and cartridges to this thread for us to see and discuss.

Until then, my thoughts are churning and my notepads are filling with thoughts, tasks, and images.

--Tinker