From what research I've done, I think that the 16/20 ga stuff is nonsense. I saw this in an early rolling block reference, and everybody has quoted it from there without verifying it. I think it was speculative at the time to try to explain the slightly oversize dimensions of these shotguns, and I've never been able to find any shell dimensional information that backed it up.
I've owned many of these rolling blocks, and every one of them has had a fairly standard 20 ga chamber. Bore is just a bit larger than a modern 20 ga, but the chamber is 20 ga for sure.
The bore diameter of a modern 20 ga is .615 or so and .681 shell diameter. I just went down and measured one of the 20 ga rolling blocks, it is .630 bore and .690 chamber. I've done this measurement numerous times, sometimes the bore is down to the .625 range, but the chamber has always measured standard 20 ga.
In comparison, a 16 ga is .662 bore and .735 chamber. Not even close to fitting.
Every piece of brass shotgun hull I have ever measured has been very close to the same size as the paper shells. That only makes sense, as the two were perfectly interchangeable for nearly 100 years. It would make absolutely no sense for a manufacturer to make their guns different sizes for different case materials! Far more sensible to vary the ammunition to match more or less standardized chambers, which is what they did.
I'd love to have some information that proves me wrong. Does anyone have old 16 ga cartridges that would fit in a .690" chamber?
Here's where you can get the proper Brass 20 ga to use in this gun:
(You need to Login or Register to view media files and links) The cases are available elsewhere, too, this has some dimensions is why I posted it.
Circle Fly Wads has the loading wads/cards/etc to allow you to properly load for this brass.
dave