Cupfire

I think I need a 28 cupfire for my collection
Can someone give me the correct diameter of 28 and 30 cupfire cartridges?
I have several that I think are 30s one measures .294 and one is .301.
I have found several on the internet listed as No 30 .28 cupfire with diameters of .297 and .301 This seems wrong or am I misunderstanding the numbering system? H. Logans “Cartridges” lists 28s dia. as .266 and municion.org lists .273
I would welcome any help on clearing up my confusion.
Thanks
Bob

From U.S. Cartridges And Their Handguns, C.R. Suydam. 1977

bdgreen
Thanks that is just what I was looking for.
Bob