Rarest chambering I have in a double is the 20-bore/.577 Express, by Alexander Henry. Holland & Holland claimed this cartridge as their own in turn-of-the-century catalogues, but the true cogniscente know the truth! I think I have the only double so-chambered in Australia; only knew of one other and it went off to the US when we were flogging-off our double-rifle heritage a few years ago.

Sounds like my Tolley might be a rare one too, thanks for the enlightenment there Tinker!

In singles, I have a .500/.450 No.1 Carbine, one of Westley Richards' improved Martinis. Its a 'sleeper', very nice!

...but my favourite cartridge is the .577 x 2 3/4 Light Nitro! Older forum members will have already been treated to my opinion on this chambering!