Hmmmmnn - 8 bore, 10 bore, 12 bore, 16 bore, 20 bore, 28 bore.
But no 30 bore.
Boring!
I've never had the [ahem] pleasure of firing a rifle chambered for this round, but do think I'll pass...
Farther down the page from which I retrieved this pic, there's commentary claiming the Brits even went larger, to a "1" bore, which they then changed over to naming A bore/A gauge.